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Shark Declines Can Lead to Fish with Smaller Eyes and Tails

An analysis of two neighbouring coral reef systems off the coast of north-west Australia have linked body shape changes in fish with declining shark numbers due to overfishing.

The Rowley Shoals and the Scott Reefs are each comprised of multiple ring-shaped reefs and are 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 commercial shark fishing for more than a century.

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.

The research team collected 611 fish from seven species across multiple sites within the Rowley Shoals and the Scott Reefs. They then took photographs of each fish and digitally analysed photographs, measuring body length, body width, eye area and tail area of each fish.

At Scott Reefs the researchers found the eyes of fishes that are normally prey for sharks were on average up to 46% smaller compared with the same-sized fish of the same species on reefs at the Rowley Shoals. The same pattern was seen for fish tail sizes, with tails being up to 40% smaller at the Scott Reefs compared with the Rowley Shoals.

Dr Shanta Barley of The University of Western Australia, who conducted the fieldwork, said there was a need to understand the consequences of shark declines. “The differences in fish body shapes between the two coral reef systems could have consequences for energy flow throughout the ecosystem, ultimately impacting the food web.”

Lead author Dr Neil Hammerschlag of the University of Miami said the removal of sharks by humans had potentially caused a reduction in the size of fish body parts that are important for shark detection and evasion. “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,” he explained.

The results are significant as sharks are among the most threatened animals globally. The consequences of the removal of the “wolves-of-the-sea” are increasingly considered far-reaching.

The research has been published in the Marine Ecology Progress Series.