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How can large jellyfish populations reduce fish populations in enclosed seas?

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Diagram showing ocean zonation by depth and region, with labeled layers: littoral, sublittoral, bathyal, abyssal, and hadal along the seafloor, and epipelagic, mesopelagic, bathypelagic, and abyssalpelagic in open water. The littoral zone is at the high-tide mark. Vertical scale marks photic and aphotic zones from surface to 10,000 meters. — Marine ecosystem Zonation of the ocean. Note that in the littoral zone the water is at the high-tide mark.

In enclosed seas, large jellyfish populations can significantly reduce fish populations primarily through intense predatory activity. When jellyfish occur in high concentrations, they consume such vast numbers of fish larvae that they effectively suppress the recruitment and growth of fish populations.1

This impact is often exacerbated by environmental changes. For example, nutrient enrichment in enclosed bodies of water—such as the Black Sea—has been linked to massive increases in invasive species like the comb jelly Mnemiopsis leidyi. These population explosions can lead to the decline of native species and the collapse of local fisheries. Because jellyfish are highly effective predators that secure prey using specialized stinging cells called nematocysts or sticky cells, they can exert significant pressure on the marine ecosystem when their numbers are unchecked.21