Sounds kinda weird.. but studying nearly identical coral reef systems off Australia, my collaborators and I discovered something unusual on the reefs subjected to nearly exclusive fishing of sharks—fish with significantly smaller eyes and tails. This provides evidence of body shape changes in fish due to human-driven shark declines from overfishing.
For these reef fish, eye size is critical for detecting sharks, especially under low-light conditions, and tail size is important for escaping sharks with burst speed. Graphic: Hiram Henriquez and Alberto Cairo, University of Miami
In the study, our research team analyzed seven different fish species from two neighboring coral reef systems off the coast of northwestern Australia. The coral reef systems, known as the Rowley Shoals and the Scott Reefs, are each comprised of multiple atoll-like reefs, are nearly identical biologically and physically in all but one way— the coral reefs in Rowley Shoals are protected from fishing, while the coral reefs in the Scott Reefs have been subjected to nearly exclusive commercial shark fishing for centuries. Targeted shark fishing has intensified in the region in recent decades to fuel the demand of shark fin soup. As a result, shark populations have been decimated at the Scott Reefs, but remain healthy at the Rowley Shoals.
Our team collected 611 fish of seven different species across multiple sites from different coral reefs within the Rowley Shoals and the Scott Reefs. We then took photographs of each fish and digitally analyzed photographs, measuring body length, body width, eye area and tail area of each fish.
We found that at Scott Reefs, where shark populations have declined, the eyes of fishes that are normally prey for sharks were on average up to 46 percent smaller compared to the same sized fish of the same species on reefs at the Rowley Shoals where shark populations are healthy. The same pattern was true for fish tail sizes, with the overall size of fish tails being on average up to 40 percent smaller at the Scott Reefs compared to the Rowley Shoals. Interestingly, these patterns were consistent across seven fish species that vary in behavior, diet and trophic-guild.
Eye size is critical for detecting predators, especially under low-light conditions when many sharks usually hunt, and tail shape enables burst speed and rapid escape from sharks. So we believe that the removals of sharks by humans have potentially caused a reduction in the size of fish body parts that are otherwise normally important for detecting and avoiding sharks. It’s possible that the removal of sharks have lead to an evolutionary change in the fish.
The differences in fish body shapes measured between the two coral reef systems could also have consequences for energy flow throughout the ecosystem, ultimately impacting the food web. These results are particularly important since sharks are among the most threatened marine animals and the consequences of their global removals due to fishing is not well understood and has been a topic of significant speculation, debate and concern. However, these finding shed new light on our understanding of the potential cascading effects the loss of sharks on marine ecosystems.
Sharks have been cruising the world’s oceans for millions of years. We know them as ferocious hunters, built for the kill. And some are. However, most shark and ray species have somewhat less aggressive feeding behaviour and, of course, many end up as food themselves.
These magnificent creatures have adapted to an incredible diversity of habitats, from the open ocean to deep ocean trenches, volcanic seamounts, coral reefs, mangroves, estuaries and rivers. There are even some that have evolved to live exclusively—or almost exclusively—in freshwater environments like the freshwater stingrays in South America and the river sharks in Southeast Asia and Australia.
In the waters off East Africa, in the Western Indian Ocean, nearly 230 shark and ray species have been identified, making this one of a handful of global shark and ray biodiversity hotspots. Because of this high diversity, including several species that are found only in this region, and because shark and ray species are millions of years old, the Western Indian Ocean is considered globally important for shark and ray conservation.
A shark is eviscerated and its fins are removed on the streets of Zanzibar. Photo credit: Rhett Bennett/WCS.
At the same time, in East Africa there are 70 million people living within 100 km of the coastline, many of whom are dependent on fishing and marine resources as their primary form of protein or income. Artisanal and traditional fishers use a range of fishing gears, such as handlines, longlines, spears, mosquito nets, beach seine nets, gill nets and even baited gill nets to target fishes, and many also target sharks and rays.
There are also small-scale commercial, commercial and industrial fishing vessels using deepwater trawl nets, shrimp trawl nets, deepset gill nets, longlines and purse seine nets. The result is that many of these either target—or result in considerable bycatch of—shark and ray species.
This heavy fishing pressure creates a major threat to sharks and rays. Most species grow much slower than other fish species and become sexually mature much later in life. They also tend to have very few offspring. Most will have just five or ten well-developed young per year, compared to some fishes that may release several million eggs in a single spawning event.
While sharks and rays may have evolved the perfect biology to capitalise on all aquatic habitats, their reproductive design cannot support extensive fisheries. Approximately one quarter of the species found in the Western Indian Ocean face a high risk of extinction in the wild due to overfishing. Sawfishes have not been seen in East Africa in several decades.
Fishing of sharks and rays has increased exponentially in recent years, driven largely by the global trade in fins—especially guitarfishes and wedgefishes—to supply the demand for shark fin soup.
It is not only direct fishing pressure that has a negative impact on sharks and rays, but also the destruction of critical habitat. Examples include the burning of mangrove trees to create coal (mangroves provide nursery areas for many species of coastal sharks and rays), destruction of coral reefs for coastal development, and overfishing of fish species that provide food for sharks and some ray species.
WCS staff Dr Rhett Bennett and Katya Kalashnikova prepare to deploy a baited underwater video camera off Pemba Island, Zanzibar. Photo courtesy of Katya Kalashnikova.
A recent status report led by WCS (Wildlife Conservation Society) in collaboration with several other organisations highlighted the key threats to sharks and rays in the Western Indian Ocean. These include directed and incidental mortality in several fisheries, a lack of ecological knowledge and information on catches in the different fisheries, as well as poor controls on trade and a lack of legislation specifically for sharks and rays.
Nevertheless, it is not all bad news for these animals. There has been a new wave of focus on shark and ray conservation in recent years, both globally and within the Western Indian Ocean. Seychelles and South Africa have developed national plans of action, for the conservation and management of sharks and rays in their waters.
In addition, WCS is supporting the Ministry of the Environment, Ecology and Forests of Madagascar and the Kenya Fisheries Service to produce national plans of action for shark and ray conservation and management in Madagascar and Kenya, respectively, while plans are underway to develop a guiding roadmap for shark and ray management in Mozambique, and hopefully Tanzania will follow soon.
A cowshark investigates an underwater camera in a South African marine protected area. Photo credit: Michael Markovina.
WCS has several conservation initiatives in the Western Indian Ocean aimed at addressing the threats to sharks and rays, through collection of ecological and fishery data, supporting governments to develop and implement regulations and legislation specifically for shark and ray species, and through engagement with fishing communities to raise awareness of the poor status of most shark and ray species, and the need for their conservation.
While sharks may kill four or five humans per year, the annual number of sharks and rays killed by humans exceeds 100 million! And most shark attacks are effected by just a few species. Ultimately, sharks and rays support many human activities and contribute essential ecological services, and they have very few negative impacts on humans.
Shark Week reminds us that it is time to improve our knowledge of sharks and rays, and support initiatives to protect these prehistoric species and their habitats, rather than persecute them.
——————————————— Dr. Rhett Bennettis Shark and Ray Conservation Officer for Madagascar and the Western Indian Ocean at WCS (Wildlife Conservation Society).