Abstract
Some evidence suggests that the abundance of jellyfish is increasing in coastal areas around the world. Mass occurrences of jellyfish have been correlated with drivers such as introduction of new species, eutrophication, overfishing, and global warming. Furthermore, many jellyfish are known as voracious feeders on zooplankton (including fish larvae), to have high growth rates, and few predators. From this perspective it might be questioned why and how zooplanktivorous fish can compete with jellyfish anywhere in the oceans. In the first part I address this question by reviewing results from some intensively studied model systems: Fjords that host exceptionally high jellyfish abundances and other fjords that host large stocks of zooplanktivorous fishes. These results suggest that the two different predation modes of fish and jellyfish, visual versus tactile, is a key to understand their competitive relationship, and that the optical properties of their habitats are a major driver for the observed ecosystem structure. In the second part I apply an idealized steady state model to hypothesize how eutrophication and fishery affect the competitive relationship between fish and jellyfish with reference to observations in the Baltic Sea, the Black Sea, and Norwegian fjords.
Dag Lorents Aksnes, University of Bergen