1.5 The impact of gelatinous plankton on fish populations

Summarizing, the impact of gelatinous zooplankton on fish populations can be: i) positive, due to a keystone effect that prevents the monopolization of overly successful fish species at the expenses of others, so maintaining fish biodiversity high.

This effect occurs when fish and jellyfish coevolved in the same environmental context and if the jellyfish are abundant just for short periods; ii) negative, due to predation on and competition with fish larvae and juveniles (predation occurs also on fish eggs) if the jellyfish are not coevolved with the resident fish or if the fish populations are not “healthy”, due to overfishing, and the jellyfish blooms are abnormally large and long-lasting. A different kind of competition might be exerted by thaliaceans, since they overexploit the phytoplankton and deplete resources for the crustacean grazers that are fed upon by fish larvae and juveniles.