This 2017 study demonstrates a high degree of spatial overlap between jellyfish and forage fish in the eastern Bering Sea, which is a sign of possible competition between these two groups that could harm commercial fisheries.
In this research, scientists used annual trawl surveys of the upper 30 meters of the ocean between 2004 and 2012 to calculate the spatial and temporal overlap between a dominant species of jellyfish and four types of forage fish: Pacific herring, capelin, juvenile Pacific cod, and juvenile walleye pollock. They found widespread overlap between jellyfish and forage fish in both space and time. However, the degree of overlap varied across years and species for multiple reasons, including climatic conditions and forage fish behavior. The overall pattern suggests that the two groups may be competing for food.
Although additional investigation is needed to confirm such competition in the Bering Sea, jellyfish populations in other regions have been shown to have detrimental effects on forage fish, reducing food available to commercially valuable fish species. In addition, in other regions where forage fish are heavily harvested, there is the potential for jellyfish to be released from competition with forage fish, resulting in more jellyfish. While forage fish represent a critical link between plankton and higher predators, by contrast, jellyfish have few predators; but they also eat plankton, and the eggs and larvae of many fish species. Thus, a shift toward high abundance of jellyfish could reduce forage fish abundance and reduce fisheries catch.
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Decker M.B., Robinson K.L., Dorji S., Cieciel K.D., Barceló C., Ruzicka J.J., Brodeur R.D. 2017. Jellyfish and forage fish spatial overlap on the eastern Bering Sea shelf during periods of high and low jellyfish biomass. Marine Ecology Progress Series. https://doi.org/10.3354/meps12273.