Relationship between fish and jellyfish as a function of eutrophication and water clarity

There is a concern that blooms of cnidarians and ctenophores, often referred to as jellyfish, are increasing in frequency and intensity worldwide and that there is a shift from fishto jellyfish-dominated systems. We present an idealized analysis of the competitive relationship between zooplanktivorous jellyfish that is based on a generic model, termed ‘Killing the Winner’ (KtW), for the coexistence of 2 groups utilizing the same resource. Tactile predation by jellyfish makes them less dependent on water optics than fish using vision, and we modified the KtW model to account for this particular trait difference. Expectations of the model are illustrated by use of observations from the Baltic Sea. The model predicts a general succession on how mass of the system distributes when going from an oligotrophic to a eutrophic system. Initially the mass of the system accumulates at the level of the common resource (zooplankton) and planktivorous fish (sprat/herring). At one point, with increased eutrophication, mass starts to accumulate at the level of the top predator (cod) and at a later point, at the level of the jellyfish. For those organisms utilizing vision (fishes) an optimal degree of eutrophication and water clarity is predicted due to a 2-sided effect of eutrophication.