Scientists studying nearly identical coral reef systems off Australia discovered something unusual on the reefs subjected to nearly exclusive fishing of sharks—fish with significantly smaller eyes and tails. The study is the first field evidence of body shape changes in fish due to human-driven shark declines from overfishing. These findings shed new light on the cascading effects the loss of the ocean’s top predators is having on marine ecosystems.
The collaborative research team analyzed seven different fish species from two neighboring coral reef systems off the coast of northwestern Australia to uncover this unusual effect. 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.
The researchers 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 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.
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. These results suggest that removals of sharks by humans have potentially caused a reduction in the size of fish body parts that are important for shark detection and evasion. These patterns were consistent across seven fish species that vary in behavior, diet and trophic-guild. The differences in fish body shapes measured between the two coral reef systems could have consequences for energy flow throughout the ecosystem, ultimately impacting the food web.
The study, titled “Predator declines and morphological changes in prey: evidence from coral reefs depleted of sharks,” was published n the journal Marine Ecology Progress Series. DOI: doi.org/10.3354/meps12426
The study’s authors include: Neil Hammerschlag and Emily Rose Nelson from the University of Miami Rosenstiel School, Shanta Barley and Jessica Meeuwig from the University of Western Australia, Mark Meekan from the Australian Institute of Marine Science and Duncan Irschick from University of Massachusetts at Amherst.
Citation: Hammerschlag N, Barley SC, Irshick DJ, Meeuwig JJ, Nelson ER, Meekan MG. (2018) Predator declines and morphological changes in prey: evidence from coral reefs depleted of sharks. Marine Ecology Progress Series; DOI: doi.org/10.3354/meps12426
Video summary was created by Savannah Geary and Alison Enchelmaier