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It appears that the majority of fish we eat are carnivorous, such as salmon and tuna. This got me wondering how common herbivores and carnivores are among fish species. I couldn't find an answer to this question through a cursory search of the internet.
I don't have an answer to this but there are a number of elements I think can be helpful for you in finding an answer.
First, a helpful word for searching here if you didn't know or hadn't thought of it is "trophic", as in "trophic level", as in "how high this organism is in the food chain".
Making searches related to fish and trophic levels I didn't find a single result for all fish; there are plenty of papers and graphs about the trophic distribution of fish but it's usually within a specific context or ecosystem. For example this graph gotten from Google Images, from a paper that looks at reef fish in Hawaii:
You can see in that graph, and plenty of other papers confirmed, that "herbivore/carnivore" may not always the most relevant distinction in terms of what a fish eats; unlike on land, where the base of the food chain is plants which are pretty big and visible and so are most of the organisms that eat them, in the ocean the base of the food chain is phytoplankton, which is eaten by zooplankton, both of which are what most of the fish we consider "at the bottom" of the fish food chain eat, so by that standard almost all fish are carnivores. But it doesn't seem to be how the word is used by everyone in the field; the graph in question being an example, distinguishing "Benthic carnivores" from "Planktivores" (Plankton-eaters) and "Piscivores" (fish eaters; presumably the "carnivores" eat non-fish animals).
See also this graph looking at reef fishes at various sites in the Caribbean:
Or this graph looking at the trophic structure in a stream
both of which similarly distinguish things more finely than "herbivore/carnivore".
And this, that I got from a search for "fish trophic histogram", ties the concepts to trophic levels (but on a local scale only again, this time the Mediterranean):
Otherwise the FishBase database seems to contain trophic pyramids but they need to be seen per ecosystem and there are a lot of available ecosystems, and given the shape of most pyramids they might include more than just fish.
As an aside, this paper might give a good explanation for why most of the fish we eat are carnivorous:
Trophic level scales positively with body size in fishes
Carnivores, Omnivores, and Herbivores: Their Differences and Roles in the Food Chain
Animals of all sorts live together in various ecosystems. Within these natural communities, the animals eat specific diets that connect them together in a food chain. The three diets of animals include creatures that eat only plants, those that eat only meat, and animals that eat both plants and meat. Animals that eat plants exclusively are herbivores, and animals that eat only meat are carnivores. When animals eat both plants and meat, they are called omnivores. The balance of an ecosystem depends on the presence of every type of animal. If one type of animal becomes too numerous or scarce, the entire balance of the ecosystem will change.
Carnivores will feed on herbivores, omnivores, and other carnivores in an ecosystem. A natural community depends on the presence of carnivores to control the populations of other animals. Large carnivores include wolves and mountain lions. A large carnivore might hunt down large herbivores such as elk and deer. Medium-sized carnivores include hawks and snakes, and these animals typically feed on rodents, birds, eggs, frogs, and insects. Examples of small carnivores include some smaller birds and toads. These carnivores may eat insects and worms. Carnivorous animals have strong jaws and sharp teeth to enable them to tear and rip prey. These animals often have long, sharp claws that they also use to tear prey. Carnivores depend on sufficient prey in the food chain to give them the food they need. If the herbivore population or the population of other carnivores declines in an ecosystem, carnivores may not survive.
With a diet comprised of only plants, herbivores can be surprisingly large animals. Examples of large herbivores include cows, elk, and buffalo. These animals eat grass, tree bark, aquatic vegetation, and shrubby growth. Herbivores can also be medium-sized animals such as sheep and goats, which eat shrubby vegetation and grasses. Small herbivores include rabbits, chipmunks, squirrels, and mice. These animals eat grass, shrubs, seeds, and nuts. An ecosystem must provide abundant plants to sustain herbivores, and many of them spend the majority of their lives eating to stay alive. If plant availability declines, herbivores may not have enough to eat. This could cause a decline in herbivore numbers, which would also impact carnivores. Herbivores usually have special biological systems to digest a variety of different plants. Their teeth also have special designs that enable them to rip off the plants and then grind them up with flat molars.
Omnivores have an advantage in an ecosystem because their diet is the most diverse. These animals can vary their diet depending on the food that is most plentiful, sometimes eating plants and other times eating meat. Herbivores have different digestive systems than omnivores, so omnivores usually cannot eat all of the plants that an herbivore can. Generally, omnivores eat fruits and vegetables freely, but they can’t eat grasses and some grains due to digestive limitations. Omnivores will also hunt both carnivores and herbivores for meat, including small mammals, reptiles, and insects. Large omnivores include bears and humans. Examples of medium-sized omnivores include raccoons and pigs. Small omnivores include some fish and insects such as flies. Omnivore teeth often resemble carnivore teeth because of the need for tearing meat. Omnivores also have flat molars for grinding up food.
Gut length and mass in herbivorous and carnivorous prickleback fishes (Teleostei: Stichaeidae): ontogenetic, dietary, and phylogenetic effects
Relative gut length, Zihler’s index, and relative gut mass were measured in four species of prickleback fishes and the effects of ontogeny, diet, and phylogeny on these gut dimensions were determined. Of the four species, Cebidichthys violaceus and Xiphister mucosus shift to herbivory with growth (>45 mm SL), whereas X. atropurpureus and Anoplarchus purpurescens remain carnivores. A. purpurescens belongs to a carnivorous clade, and the three other species belong to an adjacent, herbivorous clade. Gut dimensions were compared in three feeding categories of the four species: (1) small, wild-caught juveniles representing the carnivorous condition before two species shift to herbivory (2) larger, wild-caught juveniles representing the natural diet condition of the two carnivores and the two species that have shifted to herbivory and (3) larger, laboratory-raised juveniles produced by feeding a high-protein artificial diet to small juveniles until they have reached the size of the larger, wild-caught juveniles. Comparisons of gut dimensions in categories (1) versus (2) tested for an ontogenetic effect, in (2) versus (3) for a dietary effect, and within each category for a phylogenetic effect. C. violaceus and X. mucosus increased gut dimensions with increase in body size and did not change ontogenetic trajectory in gut dimensions on the high-protein artificial diet, suggesting that they are genetically programmed to develop relatively large guts associated with herbivory. X. atropurpureus increased its gut dimensions with increase in size similar to its sister taxon, X. mucosus, suggesting a phylogenetic influence, but decreased gut dimensions on the high-protein artificial diet, suggesting phenotypic plasticity. Nevertheless, X. atropurpureus displayed a larger gut than A. purpurescens, further evidence that it evolved in an herbivorous clade. A. purpurescens possessed a relatively small gut that was little affected by ontogeny or diet. Ontogeny and phylogeny more than diet appear to influence gut dimensions in the four species, thus favoring genetic adaptation over phenotypic plasticity as the major force acting on digestive system features in the two prickleback clades.
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What percentage of fish are herbivorous vs. carnivorous? - Biology
Fish farming in Marseille, France. Photo: marcovdz
The American Heart Association recommends that we eat fish at least twice a week, since fish are high in protein, low in saturated fats and rich in omega-3 fatty acids. Global per capita fish consumption has almost doubled from the 1960s to 2012. And today, about half of all the seafood destined for human consumption is produced through fish farming, also called aquaculture.
The Food and Agriculture Organization of the U.N. (FAO) projects that by 2030, fish farming, one of the fastest growing methods of producing food in the world, will be responsible for almost two-thirds of the fish we eat.
Aquaculture in Norway Photo: Ximonic
The most common type of aquaculture is farming in net pens or cages anchored to the sea floor in the ocean near the coast. There are also closed systems of tanks or ponds that float on water or operate on land. The FAO estimates that over 600 aquatic species are produced globally in a variety of aquaculture systems using freshwater, brackish water or salt water.
In the U.S., over 91 percent of the seafood we eat is imported. China is the largest exporter of fish globally, the third largest importer of fish and the biggest aquaculture producer. As early as 2500 B.C., the Chinese practiced fish farming, putting carp in rice paddies where they ate insects and weeds, fertilized the rice, and then were eaten. Today, 88 percent of the world’s aquaculture comes from Asia.
While fish are known for their omega-3 fatty acid benefits, they do not actually produce omega-3s themselves. Microscopic algae that live in fresh or salt water or sediments produce them. Herbivorous fish and forage fish like sardines, anchovies and herring obtain omega-3s by eating the microalgae. Larger carnivorous fish such as salmon or sea bass then eat the forage fish. Because salmon and other popular carnivorous fish need omega-3s to grow, 30 to 50 percent of the fish feed traditionally used for these species consists of fishmeal (ground fish) and fish oil. Over 50 percent of the world’s fish oil is used in feed for farmed salmon.
This is one reason that fish farming has a reputation as being unsustainable. In 1997, it took almost 3 tons of forage fish to produce one ton of salmon. A third of the global fish harvest still goes toward making fish meal and fish oil. As a result, forage fish are being overfished, and some populations have crashed, which has implications for the entire food web since larger fish depend on them for food.
Most fish farming methods are harmful to the ecosystem in other ways as well.
Graphic: Dr. George Pararas Carayannis
Fish waste and left over food spill out from nets into the ocean, causing nutrient pollution. This may lead to oxygen depletion in the water, which can stress or kill aquatic creatures. In addition, antibiotics or pesticides used on farmed fish can affect other marine life or human health. These nutrients and chemicals also sink to the ocean floor, where they may impact the biodiversity there.
Fish crowded together in nets or pens are more susceptible to stress, which can foster disease and parasites that may then spread to wild species.
Farmed fish sometimes escape into the ocean, breeding with wild species and affecting the population’s overall genetic diversity.
Fish farming in tanks Photo: Sue Waters
Land-based closed systems minimize the amount of waste and nutrients expelled to the environment, eliminate fish escapes and limit the spread of disease but pumping the water through them requires a great deal of energy, and the wastewater must still be disposed of properly.
Fish farming has also resulted in land conversion for feed and the destruction of ecosystems. To grow the soybeans used in herbivorous and other fish feed, vast areas of South America, almost 4 million hectares of forest, are razed yearly and converted to agricultural land. Growing the soybeans also depends on water availability.
Shrimp farming, which is usually done in salty coastal waters, has been responsible for the destruction of 38 percent of the world’s mangroves. Mangroves have critical ecological functions including providing food and habitat for many species, preventing erosion, sequestering carbon, and offering protection from storms. Moreover, many shrimp ponds accumulate shrimp waste, antibiotics and pesticides, and without mangroves to filter them, eventually become unusable.
But Pete Malinowski, former aquaculture director of the New York Harbor School and director of the Billion Oyster Project, which aims to restore one billion live oysters to New York Harbor, thinks aquaculture’s bad reputation is undeserved.
“Most aquaculture operations are far more sustainable than animal food production,” he said. “Aquaculture gets a bad rap because we see it as an alternative to natural environments like coasts that haven’t been developed, as opposed to comparing it with agriculture. Actually aquaculture is a much more sustainable way to get protein than agriculture.”
“A lot of damage is being caused by aquaculture, but it’s getting much better,” said Malinowski. “People are getting educated, and it’s becoming unacceptable to farm fish in a way that’s harmful to the environment.”
How Fish Farming is Becoming More Sustainable
One strategy involves moving aquaculture out into the open ocean where the water is pristine and currents are strong and steady enough to continually flush the farms of fish waste and pests such as sea lice. The open ocean also provides farmed fish with more consistent salinity and temperature. This means they are less stressed and less vulnerable to disease, which promotes better growth and minimizes the need for antibiotics or vaccines.
Open Blue Sea Farms grows cobia (related to remoras) in the largest open ocean farm in the world. After the fish develop in the hatchery, they spend 14 months in huge submersible pens deep below the surface in clean ocean waters seven miles off the coast of Panama.
Steve Page of Ocean Farm Technologies co-designed the Aquapod , a geodesic dome large enough to accommodate several hundred thousand fish. Kampachi Farms off the coast of Hawaii is using the Aquapod to grow kampachi (related to yellowtail), after successfully mounting the Velella Project. The project researched the viability of farming fish in an Aquapod tethered to a drifting boat in deep water. It had no measurable impact on the environment. Because satellite communication was not robust enough to handle the remote controls needed to manage the drifting Aquapod, however, Kampachi Farm is now using an Aquapod attached to a barge out in the ocean. In the future, Aquapods could potentially be equipped with propellers and a GPS system, and used to transport juvenile fish to arrive at their destination with the fish ready to harvest.
On land, some fish farms are using recirculation systems to recycle their water. Recirculation systems use 100 times less water per kilo of fish than traditional land-based systems. In addition, the water quality can be monitored continuously, which lessens the risk of disease and the need for antibiotics.
Denmark is a leader in recirculation system aquaculture. Hallenbaek Dambrug raises rainbow trout while recirculating over 96 percent of its water. The discharge wastewater is filtered, and the sludge used for biogas or fertilizer. The discarded water is treated to remove nitrate.
Anadramous fish like salmon and trout are born in fresh water then migrate to the ocean, returning to freshwater to spawn. Salmon and trout are typically raised in fresh water until they are mature enough to migrate to salt water, where they are farmed in sea cages. But some new recirculation systems allow these fish to spend their entire life on land by alternating fresh and salt water environments through controlling the water chemistry.
Scientists at the University of Maryland Department of Marine Biotechnology developed a recirculation system that facilitates predictable reproduction in farmed fish, one of the main challenges of aquaculture. The system recycles 99 percent of its water, filters waste through microbial communities and produces methane as a biofuel. By changing water temperature, lighting and salinity levels, and then feeding the fish a pellet that mimics a hormone prompting reproduction, the scientists have been able to get the fish to reproduce in predictable cycles.
RDM Aquaculture, an indoor saltwater shrimp farm in Indiana, has recycled the same water for five years, produces zero waste and uses no chemicals. Its “heterotrophic biofloc system” allows all organic matter—shrimp waste, bacteria, microalgae, shrimp shells and dead shrimp—to remain in the water. The shrimp eat what they need to eat and the bacteria feed on their waste.
An Integrated multi-trophic aquaculture site at Cooke Aquaculture Inc. in the Bay of Fundy, Canada: salmon cages (left), mussel raft (right foreground) and seaweed raft (right background).
Photo: Thierry Chopin
Thierry Chopin, professor of marine biology at the University of New Brunswick, combines species from different levels of the food web in a practice called “integrated multi-trophic aquaculture.” Working with Cooke Aquaculture in New Brunswick, Canada, Chopin places blue mussels and kelp downstream from salmon pens. The mussels feed on wastes from the salmon while the kelp take up inorganic nutrients. Sea urchins and sea cucumbers consume larger particles on the ocean floor. Cooke’s salmon and mussels are sold as food, and its seaweeds are used in restaurants and cosmetics manufacturing.
A Dutch and Vietnamese industry group along with universities and research organizations is designing a “nutritious-system” concept of aquaculture in Vietnam that utilizes the natural ecosystem of a pond to farm fish or shrimp and get rid of the waste. The project studies how omega-3 fatty acids are produced and will determine the right balance of algae and bacteria to ensure the best water quality, nutrition for fish and shrimp, and decomposition of waste.
The rising prices of fish feed and the environmental impacts of over-exploiting forage fish for feed and fish oil have led to an increase in the farming of herbivorous fish (such as carp and tilapia) and omnivorous fish (barramundi) that require much less fishmeal to produce protein. Meanwhile, research is also ongoing to find alternatives to fishmeal feed or ways to make it more sustainable.
New Kinds of Feed
- Kampachi Farm has experimented with fish diets supplemented with soybeans and plant waste, and replacing fish oil with microalgae and yeast products. In 2013, Kampachi tested three feeds containing no fishmeal at all, and found them all comparable to the standard diet. Researchers at the University of Maryland Center for Environmental Science developed fish feed made completely with corn, wheat and soy. The fish oil was replaced by fatty acids from algae, amino acids and soybean or canola oil. PCB and mercury levels in the fish were 100 times less than those in fish eating fishmeal feed. This is because fishmeal and fish oil in feed can transfer environmental pollutants to farmed fish, while feed made of ingredients from vegetation can reduce them.
- British scientists genetically modified camelina sativa, a plant known for its seed oil, with synthesized genes from algae, enabling the plant to produce omega-3s that successfully replaced fish oil in fish feed the salmon thrived.
- A Texas A&M University scientist is using distillers’ dried grains with solubles, nutrient-rich grains made in ethanol production, as a cheap source of protein in shrimp feed. He has successfully substituted sorghum and corn distillers’ dried grains for 10 percent of the protein in shrimp feed.
- Calysta, a California based company, is developing protein for feed using bacteria that are fermented and fed methane gas, in a process similar to making beer or bread. The product, called FeedKind, is a natural high-quality protein fishmeal replacement.
- Researchers from Wageningen University in the Netherlands are experimenting with insects as a new source of omega-3 fatty acids. The scientists extracted the naturally produced oil from a variety of insects and are researching the breeding, optimal diet and processing of the insects for oil. A 2014 FAO paper concluded that insect meal could replace between 25 and 100 percent of soymeal or fishmeal in fish diets with no adverse effects.
Aquaculture in Fuzhou, China
In 2010, only 36 percent of fishmeal came from the trimmings and waste (heads and innards) of fish fillets, which are usually discarded. China increasingly relies on wild-caught fish for fishmeal and fish oil. A Stanford University study found that using the waste from seafood processing plants, and adding algae or ethanol yeast to boost the protein content (since waste has less protein than wild-caught fish), could replace half to two-thirds of the current fishmeal used in Chinese aquaculture.
Finding the best formulas for fish feed also means trying to achieve the lowest feed conversion ratio—the amount of feed given in relation to the amount of weight gained by the fish. For example, Malinowski explained, tilapia can produce a pound of protein on less than a pound of food, while salmon require a pound and a half of food to produce a pound of protein.
A tilapia fish farm in Honduras. Photo: Brian Ross
“It depends on how you farm them—the right stock density, water quality, and nutrient-rich food translate into converting well to protein,” he said. “And the lower the food conversion ratio, the more profitable the fish farm, because food is so expensive.”
As the global population grows to 9 billion by 2050, more people enter the middle class, and fisheries are overexploited, fish farming will be critical to supplying the protein the world needs.
How Do We Make Sure Aquaculture is Sustainable?
Because there are a number of different international and national certification schemes for aquaculture, the FAO developed technical guidelines for aquaculture certification and an evaluation framework. But while environmental impact assessments and certification are required for many large fish farms, they are not required for small farms, many of which are unsustainable. Regulations governing responsible aquaculture development in many countries are weak.
- The Global Aquaculture Alliance developed the voluntary Best Aquaculture Practices Certification. The standards address environmental and social responsibility, animal welfare, food safety and traceability.
- The Aquaculture Stewardship Council, founded by the World Wildlife Fund and the Dutch Sustainable Trade Initiative, also aims to make fish farming environmentally sustainable and socially responsible. Its goal is to be the global leader in certifying responsibly farmed seafood and managing global standards for sustainable aquaculture.
- The World Wildlife Fund, the FAO, the World Bank, and others formed the Shrimp Aquaculture and the Environment Consortium to adopt international principles for responsible shrimp farming.
- The SNV Netherlands Development Organization and the International Union for Conservation of Nature launched the Mangroves and Markets project in Cà Mau, Vietnam, to promote sustainable shrimp farming. The project provides training to farmers on breeding and marketing sustainably certified shrimp, promotes the replanting of mangrove forests, and helps shrimp farmers get certified in carbon markets. These shrimp farms must have 50 percent mangrove growth or more.
In a sign of the ongoing growth of fish farming, NOAA Fisheries recently published a new rule, opening up federal waters from three to 200 miles offshore in the Gulf of Mexico to aquaculture.
“There is no longer enough seafood in the ocean to meet our demand,” Malinowski said. “Every important commercial fishery is in a state of decline or collapse. So, if we want to keep eating seafood, aquaculture is the only way, and we have to do it in a more sustainable way.”
To make sure you are choosing sustainably farmed seafood, visit Seafood Watch.
A new report, The State of Sustainability Initiatives: Standards and the Blue Economy analyzes the market and performance characteristics of international sustainability standards operating across both the wild catch and aquaculture sectors.
Coral-reef Restoration and Fish Population Restocking
One suggested countermeasure to the fast degradation of coral reefs is that of ecological restoration intervention, which at present consists mainly of coral transplantation (“reef gardening” e.g., Edwards, 2010 Rinkevich, 2014). This restoration solution has been widely criticized for its high costs (and therefore scalability problems e.g., Adger et al., 2005 Mumby and Steneck, 2008), and for treating the symptoms rather than the causes of degradation (Stone, 2007 Mumby and Steneck, 2008). These drawbacks may be attributable to the limitations of available tools and practices (Abelson et al., 2016b), scarce and poorly-directed funding, and the common goal of restoration projects to achieve an “item-based success” (i.e., survival of planted transplants, seedlings, or spats sensu Bayraktarov et al., 2016).
Here, we propose 𠇏ish restocking” practices as a coral-reef restoration tool, as well as a fishery management aid in degraded reefs. The fish population restocking should be applied in addition to forms of protection (e.g., MPAs) or fishery management. It can also be applied in addition to other restoration interventions, where applicable (e.g., Abelson et al., 2016b), and requires the compliance and active support of local stakeholders (Ferse et al., 2010). The restocking of herbivorous fish populations could serve as a potential solution to fish depletion and its adverse consequences in localities of high dependency of the coastal communities on the reef as a source of livelihood. Fish restocking and “stock enhancement” are used as generic terms referring to all forms of hatchery-based fishery enhancement tools in aquatic and marine ecosystems (e.g., Blankenship and Leber, 1995 Lorenzen et al., 2013). The proposed approach is based on the re-introduction of cultured fish (i.e., reproduced and reared to young stages by aquaculture methods) into the reef as a restoration tool that may be able to address both the ecological and the social challenges of degraded reefs and depleted reef fish populations.
Fish restocking, or stock enhancement, can have several effects (Figure 1): (1) Shortening the recovery time of depleted fish populations in the target reef. (2) Supporting fishery by increasing fishing yields. (3) Enhancing the ecosystem functions essential for reef recovery (notably the removal of benthic algae—macroalgae and/or algal turfs) and, therefore accelerating recovery and 𠇋ouncing” the reefs back to their coral-dominated state. (4) Enhancing the resilience of the target reefs to future perturbations.
Target Reefs and Applicable Conditions
Fish population restocking is not suggested as an ultimate solution for all degraded reefs. It could, however, serve as an additional restoration tool in areas in which MPAs, or existing fishery management, are not likely to be effective as sole measures, due to the social–political circumstances or the progressively-depleted reef state. Moreover, fish restocking is not expected to fulfill its goal in conditions of low structural complexity, low coral recruitment, and/or high macroalgal cover reefs without artificial complexity enhancement, coral transplantation, and algal eradication, respectively (see Table 1 Abelson, 2006 Rogers et al., 2015 Abelson et al., 2016b).
Table 1. Categories of degraded states of coral reefs, their main live cover, and potential restoration tools.
One category of high-priority target reefs for restocking-based restoration is that of the degraded reefs that have undergone a phase shift from a coral-dominated state to domination by benthic algae (e.g., Bellwood et al., 2006 Bahartan et al., 2010), either macroalgae (Type I Table 1 Figure 2A), or algal turfs (Type II Table 1 Figure 2B), mainly due to over-fishing. Reefs dominated by macroalgae or algal turfs may suffer from limited recruitment, due to either adverse effects on corals and fish recruits, or a negative preference of potential recruits (Kuffner et al., 2006 Birrell et al., 2008 Dixson et al., 2014 Kelly et al., 2014). In such cases, the common reef restoration tool of coral transplantation is not likely to be effective in counteracting the potential damaging effects of algae on adult corals (Birrell et al., 2008 Dixson et al., 2014). On the other hand, intensive grazing and browsing activities following augmented populations of herbivorous fish may help in re-shifting the reef back to a coral dominated state through the removal of algae. An example of such rapid removal of algae was demonstrated in exclosures (exclusion experiments) in which fish herbivory reduced about half of a 3-year old macroalgal cover within 5 days (Bellwood et al., 2006).
Figure 2. Six major types of degraded coral reef states. A common denominator of all six types is poor fish community indicators, i.e., low biomass (π.25 B0 see text), low species richness and small average size of predators and large herbivores. Type I: High cover of macroalgae (A) Type II: High live cover of algal turfs (B) Type III: high live coral cover with depauperate fish community(C) Type IV: High fraction of exposed rock (or crustose coralline algae CCA) of high structural complexity (D) or low structural complexity (E) Type V: Structurally destroyed reef (i.e., high cover of rubble/debris F).
Another category of target reefs for fish restocking implementation comprises degraded reefs of Type III (Table 1 Figure 2C), where the coral live-cover is high, but the reefs suffer from severely depleted fish communities, notably of herbivorous fish. Type III reefs, despite their high coral cover, may be of low resilience and under high risk of deterioration if exposed to mass coral mortality following extensive disturbances, such as crown-of-thorn starfish outbreaks and mass bleaching (e.g., Folke et al., 2004). In such reefs, fish restocking can be applied as a “proactive measure” to augment reef resilience and the livelihood of local fishing communities.
A third category of candidate reefs for fish restocking includes degraded reefs of Type IV (Table 1 Figures 2D,E), which are reefs of low coral cover and a high fraction of exposed rock, or crustose coralline algae (CCA). As in Type III, fish restocking can be applied as a “proactive measure” to augment reef resilience and the livelihood of local fishing communities. However, prior to implementing the restocking, artificial complexity enhancement and coral transplantation interventions should be considered (Table 1) to enable adequate shelter for the restocked fish.
Degraded reefs of Type V, which are structurally destroyed (Table 1 Figure 2F), require substrate stabilization interventions, in addition to the other available restoration methods. At present, substrate stabilization interventions are not considered to be effective on large spatial scales (Fox and Caldwell, 2006). Moreover, the combined restoration projects of such destroyed reefs are expected to entail a high cost.
Pros and Cons of Fish Restocking
There are diverse technical, ecological, and socio-economic aspects that may impede the potential success of restocking interventions in coral reefs, the majority of which are also relevant to other restoration interventions. These include, among others: High-cost production expenses, insufficient funding, lack of compliance of local stakeholders, below-threshold survival rates of the restocked fish, fish movement out of the target site, non-restricted illegal fishing, inadequate aquaculture technologies of relevant species, and adverse effects of the restocked fish on the target reef community. Additionally, there are potential adverse effects of aquaculture practices on coastal ecosystems, related to food sources (mainly of carnivorous fish), eutrophication (by mass culture to adult size) and the culturing of non-indigenous species, or genotypes. The 𠇏ish restocking” concept refers to indigenous genotypes (i.e., local broodstocks) of herbivorous fish cultured to juvenile stages, thus minimizing the abovementioned adverse effects. Moreover, if “Integrated Multi-Trophic Aquaculture” (IMTA) is applied (e.g., Soto, 2009), it is expected to further reduce the environmental impact and enhance the income of local stakeholders via aquaculture as an alternative source of livelihood.
These potential impediments and drawbacks can be assessed in the early stages by means of pre-launch experiments and analyses, to evaluate the site-specific success chances and cost-benefit of the proposed fish-restocking projects. These pre-launch steps (see Section Proposed implementation steps of restoration by restocking) can help in determining whether the proposed approach of restocking is applicable, and where, which reef types, and under what socio-economic circumstances.
In contrast, there are several arguments that may support the feasibility of fish restocking when appropriately planned and applied. These include:
Economic feasibility. The results of a recent model-based study suggest that restocking may be a financially beneficial restoration tool (Obolski et al., 2016), due to the high economic value of coral-reef services (Costanza et al., 2014) and the potentially low cost of restocking (Lorenzen et al., 2013 Obolski et al., 2016). When compared with coral transplantation (which is currently the major restoration tool Edwards, 2010 Rinkevich, 2014), fish restocking has significantly lower costs and carries potential socio-economic benefits to coastal communities. These advantages should incentivize its implementation, alongside other tools, or as a major intervention, if applicable (Obolski et al., 2016).
Fish restocking experience in aquatic and marine environments. Fish population restocking has been commonly used as a restoration, or biomanipulation, tool in the management of aquatic ecosystems, aimed at restoring water quality and vegetation characteristics (e.g., Cowx, 1999 Angeler, 2010). Furthermore, recent attempts of fish restocking have been carried out in coastal marine ecosystems, mainly as a fishery management tool of target commercial fish populations (Lindegren et al., 2010 Lorenzen et al., 2013). In coral reefs, however, to the best of our knowledge, there are no reports on fish restocking intervention attempts other than experimental stock enhancement with two species of rabbitfish (Siganus lineatus and Siganus fuscescens Bowling, 2014).
Technical breeding (culture) feasibility. At present, most herbivorous reef-fish species are not cultured. However, there is some knowledge of breeding technologies of several herbivorous species (e.g., Hara et al., 1986 Estudillo et al., 1998), most of which have been reported in non-peer reviewed literature (e.g., Hirai et al., 2013 Ayson et al., 2014 Bowling, 2014). Based on the progress achieved in breeding technologies, in spite of poor funding, the possibility of acquiring additional knowledge seems feasible if adequate resources are allocated to reef-fish aquaculture. Moreover, there are diverse ongoing initiatives around the world that are examining the restocking of exploitable fish species either for human consumption, or as ornamental species (MASNA, 2016).
Proposed Implementation Steps of Restoration by Restocking
The fish restocking management tool should be viewed as part of a comprehensive coral-reef management approach, which includes protection and fishing regulations, and requires the compliance and support of the local communities for its success (Ferse et al., 2010). To maximize the chances of success and minimize risks, it is proposed to consider a series of steps that would allow assessing and implementing critical parameters:
1. The reef degradation state and the appropriateness of the restocking tool. The reef state should fit one of the relevant degradation types (Table 1 Figure 2). In degraded reefs of type III, the restocking tool can be considered as a sole restoration intervention. However, in most degradation types further interventions, such as algal eradication, artificial complexity enhancement, coral transplantation, and substrate stabilization, may be required (Table 1 e.g., Rogers et al., 2015 Abelson et al., 2016a). The costs of these interventions may raise, however, questions regarding the scalability and implementation circumstances of combined restoration interventions.
2. The target herbivorous species. In addition to general functional groups (i.e., based on trophic level), fish species should be categorized into grazers, scrapers, and browsers (Green and Bellwood, 2009) where “grazers” are species that feed on algal turfs, preventing the establishment and growth of macroalgae “scrapers” feed on turfs but erode some component of the reef substratum, which clears areas for coral recruitment and 𠇋rowsers” graze on macroalgae (Green and Bellwood, 2009). Therefore, the latter (browsers), or generalist herbivore fishes (Bellwood et al., 2006 Green and Bellwood, 2009) may be the key species for the reversal of macroalgal-dominated reefs. Moreover, recent studies indicate that only a small subset of taxa may be necessary in order to remove dominant macroalgae once these become established (e.g., Vergés et al., 2012). For example, despite marked differences in the diversity, biomass, and community composition of resident herbivorous fishes of different reefs, Sargassum consumption was found to be dominated by only four species, with two species (i.e., Naso unicornis and Kyphosus vaigiensis) consistently emerging as dominant feeders of the macroalgae (Hoey and Bellwood, 2009 Vergés et al., 2012), where the target species seem to present regional and site-specific features. Thus, selecting an appropriate set of species is crucial for the success of the restocking intervention and has to be determined by pre-launch studies. Following the selection of candidate species, exploratory enclosure/exclosure experiments (e.g., Bellwood et al., 2006 Burkepile and Hay, 2011) should be conducted to examine the compatibility of the candidate herbivorous fish with the target reef. Factors to be considered should include, among others, the site-specific survival of released fish, their actual functional role and efficiency, and their potential effects on the target reef community.
3. Capacity building and public awareness of local stakeholders prior to any “restocking project” implementation. This is an essential step to ensure the collaboration of the relevant communities, sound exploitation based on a compatible fishery management, and to avoid abuse of the new, enriched fishing conditions.
4. Technical knowledge of the relevant species' culture. Establishment of appropriate aquaculture systems, including hatchery, nursery, and grow-out systems to produce the fish stock, from eggs to released size/age. Since the “restocking” tool is applied to enhance depleted natural fish populations by releasing cultured fish into the wild, the brood-stock sources should be fish from local populations and, ideally, the brood populations should be large enough to ensure high variability and avoid the founder's effect and population bottlenecks (e.g., Champagnon et al., 2012).
5. Feasibility examination. The proposed approach should be compared with costs of conservation, different fishery management options, and other coral-reef restoration tools (e.g., coral transplantation) in order to evaluate the most effective approach—or combination of approaches—in terms of recovery time, scalability, and socio-economic benefits. There are diverse social and cost-related considerations that may determine the feasibility of any restocking program, based on its location, such as accessibility, available infrastructure, and local expertise. These aspects emphasize the importance of site-specific feasibility examination.
To our knowledge, this is the first study examining habitat associations, distribution, and foraging patterns of seagrass and algivorous, detritivorous, and microphagous fishes from varying life stages across several shallow-water habitat types (coral reefs, macroalgae beds, and seagrass meadows) within a tropical seascape. As hypothesized, densities of focal functional groups of fishes and foraging patterns were not equally distributed within the seascape, but varied with habitats. Likewise, species composition of nominally herbivorous and detritivorous fishes also changed significantly with habitat, suggesting that fish communities in this study are subjected to, and to a certain extent, driven by bottom-up processes, which was most obvious for the microphagous parrotfish.
Life stages of microphagous parrotfish also varied with habitat smaller individuals (< 5 cm) were more abundant in macroalgal habitats and larger individuals more common in coral reef and seagrass habitats. This was mirrored in the inventory of grazing scars, which were significantly smaller in macroalgal habitats compared to coral reef sites, likely reflecting the distribution patterns of ontogenetic stages of this functional group. Lower numbers of grazing scars in the macroalgal habitats might be the result of fishes feeding more intensely on epiphytes than on hard substrate surfaces. However, that would not explain the significant differences in sizes of grazing scars. Whether or not it was the same species using different habitats during distinct life stages was impossible to tell, as a majority of the juveniles were seldom identified to species level. Tano et al. (2017) also observed large numbers of juvenile parrotfish in macroalgal areas in Zanzibar, Tanzania, compared to neighboring habitats, suggesting that shallow areas in the Western Indian Ocean (WIO) dominated by canopy-forming macroalgae may serve as nurseries for this particular group of fishes.
Habitat features benefiting certain functional groups of fishes in one habitat might not be the same in another since different environmental variables explained fish abundances in different habitat types. “Live coral cover”, “CCA cover” and “CCA cover” in combination with “EAM cover”, had a positive effect on the abundance of fish (all functional groups pooled) in coral reef habitats, which is similar to results from other studies (Friedlander et al. 2003 Osuka et al. 2018). In the macroalgal beds, habitat quality variables such as “macrophyte height”, “macrophyte cover” and “rugosity”, had a positive effect, patterns that are consistent with studies of microhabitat selection of labrid fishes in both tropical (Lim et al. 2016) and temperate macroalgal areas (Fulton et al. 2016 van Lier et al. 2017). “Calcareous rubble”, which has been shown to have a strong positive effect on abundances of acanthurids and parrotfish (Russ et al. 2015, 2018), did not have any effects on abundance in the present study. This is probably due to the rather high cover of calcareous rubble in coral and macroalgal habitats (50.3 and 52.8%, respectively), implying that this is currently not a food-limiting resource on Mafia Island.
The positive relationship between “live coral cover” and nominally herbivorous fish abundances is probably due to the numerous feeding surfaces in the form of dead hard substrata this benthic category is associated with. Note that “live coral cover” was almost never the dominating substrate in UVCs (48.465 ± 5.26%), not even in the coral reef habitats, which all had a high degree of calcareous rubble. Although not a food source for the majority of the fishes in the study, CCA cover might indicate areas with low sedimentation, which are more attractive feeding grounds for many herbivorous fishes (Bellwood and Fulton 2008).
The lack of significant explanatory variables in the seagrass habitats is likely a consequence of the limited sample size and small value range within measured variables. Factors such as spatial arrangement within the seascape and distance to other habitats or deeper waters might be more important for structuring fish assemblages in these habitats (Gullström et al. 2008 Henderson et al. 2017).
Fish likely respond differently to habitat characteristics and environmental variables depending on ontogenetic stage (Macpherson 1998 Almany 2004). The differing bottom–up variables explaining fish abundance in different habitat types in the present study are probably reflecting shifts in habitat preferences for different life stages of fishes. Abundance of microphagous parrotfish was negatively associated with macroalgal cover in coral reef habitats, while in macroalgal areas the relationship was reversed and there was instead a positive effect of macroalgal cover and canopy height on parrotfish abundance. This is likely an effect of smaller/younger fish seeking shelter from predators in the form of structural complexity, provided by height and cover of macroalgae, and by hard underlying structure (rugosity) (van Lier et al. 2018). The negative relationship with macroalgal cover is in line with results from previous studies from the Great Barrier Reef, Australia, where dense macroalgal stands of Sargassum spp. in coral reef environments were found to induce avoidance behavior of roving herbivores (Hoey and Bellwood 2011). There is likely a trade-off between food and shelter changing throughout ontogeny in these fishes, but also between different types of shelter, provided by live coral or by microhabitat topography in the macroalgal areas. Shelter from predators is an important resource in coral reef environments (Kerry and Bellwood 2016), and can be size-specific, as inter-structural spaces which are smaller than the potential predator width will act as shelter for a fish that fit within the space provided (Bartholomew et al. 2000 Gullström et al. 2011). Macroalgal areas with both soft and hard complexity in the form of fronds and coral rubble might therefore provide sufficient shelter for smaller individuals, while it is advantageous for larger fish to keep to areas with larger structures, as provided by live coral. Consequently, macroalgal areas in shallow areas with tall canopies and high macrophyte cover might be advantageous for the survival and recruitment of parrotfish and hence contribute to the replenishment of important herbivores and microphages on coral reefs. This is strengthened by recent studies showing that tropical macroalgal habitats host high numbers of juvenile and subadult parrotfish, suggesting that they are important nursery grounds for these species and that linkages between habitats should be highlighted in coral reef management (Tano et al. 2017 Eggertsen et al. 2017). However, we would like to stress that deteriorated coral reefs with high cover of fleshy macroalgae is not what we define as “natural macroalgal areas”, and that nursery habitat qualities might depend on where in the seascape a habitat is located. This is, to some extent, illustrated in the present study as high macroalgal cover in the coral reef habitat had a negative effect on the abundance of parrotfish while it was positive in another habitat type.
Algal consumption, measured as the removal of macroalgae from assays, was the highest within coral reef sites, followed by macroalgal beds, and seagrass meadows. However, it does not necessarily mean that grazing intensity per se was the highest in coral reef habitats in the present study. Since biomass loss of tethered macroalgae was negatively related to macroalgal cover in the surrounding habitat, it suggests a density-dependent process, where grazing pressure might be “smoothed out” in areas with high macrophyte cover. Macroalgal feeding fish might therefore not be able to eradicate dense macroalgal stands or “blooms” on coral reefs, if they are already established. Solitary fronds may, however, be targeted and limited by browsing fish since the survival and growth of newly settled Sargassum spp. have been shown to be strongly suppressed by top–down control (Diaz-Pulido and McCook 2003).
Understanding how habitats structure fish assemblages in spatially heterogeneous environments is imperative because it will improve habitat-based approaches of management and conservation. As tropical natural macroalgal habitats have been shown to host high numbers of juvenile and subadult scarine labrids, these linkages between reefs and macroalgal beds should be considered in seascape management (Tano et al. 2017 Eggertsen et al. 2017 Fulton et al. 2019), even though these habitats have only been recently acknowledged. Furthermore, the availability of nursery grounds and connectivity to coral reefs by nominally herbivorous fishes have been found to enhance herbivory (Adam et al. 2011 Olds et al. 2012, Harborne et al. 2016). Therefore, the protection of important ecological functions urges the preservation of seascapes comprising multiple habitats with a high degree of connectedness. This is particularly important in the WIO area, where a large part of the population is living within the coastal region and is heavily dependent on the local marine resources.
In their natural settings, herbivorous fish trophic-step 15 N/ 14 N fractionation was significantly higher than 3·4. This contrasts with the studies that show herbivore trophic fractionation to be lower than the commonly cited ΔN of 3·4 ( Owens 1987 McCutchan et al. 2003 Vanderklift et al. 2003). Previous studies have attributed high ΔN exhibited by animals consuming low-quality diets (high C : N ratio) to unknown animal material in the diet (assuming the diet contained more protein than was observed Pinnegar & Polunin 2001 ) or assumed that the animal was undergoing nutritional stress ( Adams & Sterner 2000 ). The wild animals in this study were evidently in good condition, and visual inspection revealed no additional protein among materials in the diet other than from the algae. However, by using a dynamic model, incorporating absorption, feeding rate and excretion, we repeatedly predicted higher ΔN values for herbivorous fish. Previous fractionation models have used bioenergetics to determine δfood with ΔN based on literature values ( Harvey et al. 2002 ). This is the first model to our knowledge to calculate both δfood and ΔN.
Herbivorous fish consume around 20% of their body weight per day compared to only 3%–4% for carnivorous fish ( Horn 1989 ). In this model, food consumption rate was incorporated into the term Ωin, along with N content of the diet and the absorption efficiency, q. The increased Rd in herbivores would contribute to a greater ΔN if the %N of the diet and q were not much smaller than those of carnivores. However, these factors are related such that to meet the bioenergetic needs of an animal feeding on a low-N food, it is necessary to have a high Rd ( Fris & Horn 1993 Choat, Clements & Robbins 2002 Choat, Robbins & Clements 2004 ). Conversely, an organism with an N-rich diet will not feed as much Ωin of herbivorous and carnivorous species may thus be broadly similar, corresponding with the dietary requirements of herbivores and carnivores being significantly different ( Pandian & Marian 1985 ). The Rd values used in the model were taken, where possible, from species of the same genus with similar feeding habits because a slight change in Rd can result in a significantly different prediction for δfood (Fig. 2). Rd may also be affected by a change in temperature as fish can alter their metabolic rates to suit their environment ( Klumpp & McKinnon 1989 ), which may lead to seasonal variations in the observed ΔN. NAE calculations for A. sohal and Z. xanthurum were similar to those of other herbivorous fish from the literature. The ash marker method (Montgomery & Gerking 1980) used to determine NAE may, however, lead to some inaccuracies as it is based on the assumption that ash is indigestible and that all organic matter is absorbed by the fish. Herbivorous species with gizzard-like stomachs (e.g. Z. xanthurum) have previously been found to have negative assimilation efficiencies for some macronutrients, thought to be a result of high levels of inorganic materials retained in their guts ( Crossman, Choat & Clements 2005 ). The gut material of Z. xanthurum used for ash analysis in this study was taken from the immediate anterior and posterior of the intestine to minimise excess inorganics not present in the diet material. Sediment or inorganic matter was present in the diet however, the mean NAE obtained from ash analysis was within the published range. NAE has been positively correlated with the N content of food (Pandian & Marian 1985) hence, in the model, if N content of the food were to decrease, a decrease in NAE would be expected. Body weight, food ration and temperature significantly influence absorption (Pandian et al. 1985), yet, NAE may vary with the size of the fish ( Lassuy 1984 ) this was not the case in this study. When q in the model was below the value of 0·5 (50% efficiency), the predicted value for δfood would decrease significantly. The q parameter was found to influence the difference between δa and δfood in a way opposite to Olive et al. (2003 ) whereby when q < 1, the isotopic ratio for the animal would be depleted relative to the isotopic ratio of the food ( Olive et al. 2003 ). Fish in this study had a q value of > 0·5.
Stomach contents analysis showed Feldmannia, Phylophora and Enteromorpha to be among the most dominant algal genera, but these were not analysed for δ 15 N and N% content due to cost constraints. Similarly, it was not possible to obtain δ 15 N value for the detritus fraction of the P. arabicus diet. The omission of these dietary components may have led to errors in empirical estimations of ΔN. However, macroalgae δ 15 N varied little across genera, and since their nitrogen source was the same, it is unlikely the seven genera used would have significantly biased the mean value. The accuracy of these estimates could be improved by further analysis of all genera and weighting the contribution of each by their relative importance in the diet.
If accurate values are to be applied to trophic fractionation, controlling processes must be well understood. Our sensitivity analysis has highlighted that the value of Z is an important factor in determining the level trophic fractionation however, Z has not yet been measured directly. Z has been estimated from diet switch experiments where the study animals were not in equilibrium with their diet. All the diet switch species were carnivorous hence, the mean Z value used in the model would be appropriate for carnivores. Whether herbivores would be more accurately described with a significantly different Z value remains to be tested. There may be differences in Z between herbivores and carnivores as much more ingested N appears to be released as dissolved waste in carnivores than herbivores ( Polunin & Koike 1987 ), and Z is therefore likely to be higher in herbivores. High nitrogen-use efficiency, whereby only a small portion of the ingested N is excreted, is an adaptation in herbivores to deal with low N intake and has been suggested to contribute to low ΔN values (Vanderklift et al. 2003). This may be the case for aphids and certain detritivores (Vanderklift et al. 2003) but does not seem to be the case for herbivorous fish as they are known to exhibit high fractionation values (Table 1). Differential nitrogen excretion has previously been suggested as a factor contributing to variance in ΔN ( Minagawa & Wada 1984 Ponsard & Averbuch 1999 Vanderklift et al. 2003), but so far, researchers have failed to reach a consensus view. Excretion rate measurements in a range of species of differing trophic groups would potentially further our understanding of how these processes affect fractionation.
A single mean ΔN may seem useful and convenient in application to food web studies, especially to determine trophic level. However, by applying one value to determine δfood simply reflects the consumer signatures offset by 3·4. This approach may lead to the misinterpretation of the relative importance of potential food sources of a consumer. This study has emphasized the importance of determining fractionation values for consumers on a case-by-case basis. As the Olive model takes into account nutritional functionality, it has the potential to be used for a range of consumers in a food web, giving species-specific fractionation values. The model output – the value for δfood– could be used within isotope ‘mixing models’ (e.g. Phillips 2001 ) to determine the different proportions that contribute to the diet mixture (Koch & Phillips 2002 Lubetkin & Simenstad 2004 ).
Herbivores, Omnivores, and Carnivores
Herbivores are animals whose primary food source is plant-based. Examples of herbivores include vertebrates like deer, koalas, and some bird species, as well as invertebrates such as crickets and caterpillars. These animals have evolved digestive systems capable of digesting large amounts of plant material. The plants are high in fiber and starch, which provide the main energy source in their diet. Since some parts of plant materials, such as cellulose, are hard to digest, the digestive tract of herbivores is adapted so that food may be digested properly. Many large herbivores have symbiotic bacteria within their guts to assist with the breakdown of cellulose. They have long and complex digestive tracts to allow enough space and time for microbial fermentation to occur. Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).
Figure (PageIndex<1>): Examples of herbivores: Herbivores, such as this (a) mule deer and (b) monarch caterpillar, eat primarily plant material. Some herbivores contain symbiotic bacteria within their intestines to aid with the digestion of the cellulose found in plant cell walls.
Omnivores are animals that eat both plant- and animal- derived food. Although the Latin term omnivore literally means &ldquoeater of everything&rdquo, omnivores cannot really eat everything that other animals eat. They can only eat things that are moderately easy to acquire while being moderately nutritious. For example, most omnivores cannot live by grazing, nor are they able to eat some hard-shelled animals or successfully hunt large or fast prey. Humans, bears, and chickens are examples of vertebrate omnivores invertebrate omnivores include cockroaches and crayfish.
Figure (PageIndex<1>): Examples of omnivores: Omnivores such as the (a) bear and (b) crayfish eat both plant- and animal-based food. While their food options are greater than those of herbivores or carnivores, they are still limited by what they can find to eat, or what they can catch.
Carnivores are animals that eat other animals. The word carnivore is derived from Latin and means &ldquomeat eater.&rdquo Wild cats, such as lions and tigers, are examples of vertebrate carnivores, as are snakes and sharks, while invertebrate carnivores include sea stars, spiders, and ladybugs. Obligate carnivores are those that rely entirely on animal flesh to obtain their nutrients examples of obligate carnivores are members of the cat family. Facultative carnivores are those that also eat non-animal food in addition to animal food. Note that there is no clear line that differentiates facultative carnivores from omnivores dogs would be considered facultative carnivores.
Figure (PageIndex<1>): Examples of carnivores: Carnivores such as the (a) lion eat primarily meat. The (b) ladybug is also a carnivore that consumes small insects called aphids.
Biology Centre of the Czech Academy of Sciences, Institute of Hydrobiology, Na Sádkách 7, České Budějovice, 37005, Czech Republic
Ivana Vejříková, Lukáš Vejřík, Martin Čech, Petr Blabolil, Mojmír Vašek, Zuzana Sajdlová, Son Hoang The Chung, Marek Šmejkal, Jaroslava Frouzová & Jiří Peterka
Faculty of Science, University of South Bohemia in České Budějovice, Branišovská 31, České Budějovice, 37005, Czech Republic
Department of Bioscience, Aarhus University, Vejlsøvej 25, Silkeborg, 8600, Denmark
6.8. FishBase, archaeology and shifting baselines
Existing frescos, such as the 'Little Fisherman from Thera (Santorini)' (Figure 6.1), and other paintings have, apart from their historic, cultural and artistic value, an untold ecological one (Stergiou 2005b). Because of their bright colors and fine, detailed representations, it is possible for the specialist to identify, at the species level, many of the marine organisms (e.g., echinoderms, cephalopods, fishes, dolphins) depicted in the frescoes (Economidis 2000 Eleftheriou 2004).
In addition, there are many descriptions of various aspects of marine life and biodiversity, and fishing methods in the writings of many 'classic' writers (e.g. the Homer 'rhapsode' Aristotle who wrote that larger fishes prey upon smaller ones, implies that trophic level increases with size the poet Oppianos who refered to many fishing gears) (Stergiou 2005b). The evaluation of written, pictorial, and archaelogical information is critical for establishing 'baselines' (Pauly 1995) and reconstructing the history of marine animal populations (Stergiou 2005b).
Identify the species depicted in the fresco 'The Little Fisherman' (Figure 6.1). Estimate the size of each individual fish. [Hint: make the assumption that the height of the boy is about 1,6 m.] Construct the length frequency of the 'sample'. Compare the maxiumum size with that reported in FishBase.
Read Aristotle's book 'The history of Animals', Book VIII. Find at least 5 quotes that are related to fish and can be included in FishBase. [Hint: Aristotle's books are available free on the Internet.]
Figure 6.1. The 'Little Fisherman from Thera (Santorini)'.