2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Green moray eel (Gymnothorax funebris)
On the Caribbean reefs, located close to major fish markets, the abundance and species diversity of moray eels is especially high. This is the conclusion reached by a team of ichthyologists after analyzing video recordings of autonomous cameras and samples of exogenous DNA. As noted in an article for iScience magazine, predatory fish are usually rapidly disappearing from human-prone coral reefs. However, moray eels are rarely hunted by humans, so these eels thrive in the absence of human-fished sharks and other large predators and competitors.
Coral reefs are not only one of the most diverse ecosystems on the planet, but also a source of fish and seafood for millions of people around the world. Unfortunately, fishing here is often overly intensive and disrupts the populations of many marine organisms. For example, predatory fish, from groupers to sharks, suffer greatly from overfishing. Scientists have even found an interesting pattern: the shorter the distance to the nearest fish market, the less predatory fish on the reef. This rule does not work only in protected areas where fishing is strictly prohibited.
However, not all predatory fish are equally interesting to fishermen. For example, moray eels (Muraenidae) are rarely caught because their meat is tasteless and often contains dangerous concentrations of ciguatoxin. It is logical to assume that the abundance of these eels is approximately the same both in marine reserves and national parks, and in fishing areas. However, due to the secretive lifestyle of moray eels, assessing the size of their populations and how they are affected by human activity is not easy.
A team of specialists led by Demian D. Chapman from Florida International University decided to understand the relationship between moray eels and people. As material, the scientists used videos that were made in 2015-2019 during the census of the number of predatory fish on 67 reefs in 12 countries and dependent territories of the Caribbean, from the Bahamas and the United States to Colombia. On each reef, researchers set up a cage with edible bait in front of the camera and recorded the activity of fish around it for an hour. On each reef, such sessions were carried out several times and in different areas.
Chapman and his colleagues counted the number of moray eels on each record and estimated their abundance on each of the studied reefs. These figures were compared with the location and size of the nearest fish market. In addition, the authors took into account the conservation status and complexity of the structure of each reef, as well as the number of sharks - enemies and food competitors of moray eels.
It turned out that Caribbean moray eels are most numerous on reefs with a complex structure, where it is easier for them to find shelter for hunting and protection from enemies. Moreover, the shorter the distance to the nearest fish market and the larger it is, the higher the number of moray eels on the reef (p <0, 0001). But the conservation status of the site is not reflected in their populations. For comparison, shark numbers, as in previous similar studies, were lower on reefs that are closer to large fish markets. The analysis also showed that the numbers of sharks and moray eels are negatively correlated at the level of individual reefs and entire countries.
The authors hypothesized that sharks are becoming extinct due to overfishing on reefs close to major fish markets. As a result, moray eels lose their enemies and food competitors and increase their numbers, because people themselves hardly hunt moray eels. Nevertheless, it was impossible to exclude the possibility that on reefs where there are many sharks, moray eels are simply more fearful, less willing to approach the bait and less often come across the camera lens.
To choose one of the alternative hypotheses, Chapman and his colleagues decided to estimate the moray eel population using a different method. To do this, they collected samples of exogenous DNA from six reefs, subject to various human influences, and isolated about twenty thousand sequences from them. As a result, it turned out that in the areas located near large fish markets, the species diversity and number of moray eels is higher.
Thus, moray eels do appear to benefit from intensive coral reef fishing. As noted by the authors, this is the only known exception among all carnivorous reef fish, which usually suffer greatly from overfishing. At the same time, the absence of sharks may not be the only reason why moray eels are so fond of reefs located near human settlements: it is possible that overfishing of competitors from among bony fish or the ability of moray eels to feed on small fish discarded by fishermen also play a role.
In conclusion, Chapman and his co-authors acknowledge that the ability of moray eels to adapt to their neighborhood with people is not limitless. Even these resilient creatures can be negatively affected by the simplification of coral reef structures and declining prey numbers.
Inhabitants of reefs where fishing is prohibited are not only numerous and varied, but also achieve record longevity. For example, ichthyologists recently discovered a macolor macularis (Macolor macularis) in a protected area off the west coast of Australia, which lived to be 81 years old. This is a record for reef fish.