A fish introduction and its impact on the plankton community

Abstract

This thesis is about how one species can invade an ecosystem and change the whole biological community within a few years. The question is whether such changes can be foreseen or are more or less unpredictable? Biological communities in lakes are vulnerable to fish predation and the zooplankton is strongly affected by the fish types present. Of particular importance is whether they feed on zooplankton (planktivore), benthic animals (benthivore) or are fish eaters (piscivore). Lake Myravatn in Bergen had for a long time a very rare fish community consisting of only piscivore fish due to the introduction of Northern pike (Esox lucius) about 200 years ago. No other fish survived there except some eel (Anguilla anguilla). The consequence of this was a zooplankton community with large species that are usually eliminated by planktivorous fish. Predation on the zooplankton was caused by an invertebrate, larvae of the phantom midge (Chaoborus flavicans), feeding on small zooplankton individuals. This situation was completely changed when Eurasian perch (Perca fluviatilis) was illegally introduced by unknown person(s) and for unknown reasons, most likely in autumn 2006. Since studies on the zooplankton were ongoing, the consequences for the zooplankton could be recorded. The monitoring of the lake included both quantitative and qualitative zooplankton sampling and fishing with multi-mesh gill nets and echo-sounding able to discriminate between fish and larvae of phantom midge. Contemporary theory predicted that the perch would encounter very good conditions in the lake with a superabundance of invertebrate prey of optimal size. Both growth and reproduction would be exceptional and a dense perch population would quickly establish. This would reduce the large zooplankton species in the lake, i.e. Daphnia pulex, D. longispina and the phantom midge larvae. Disappearance of the invertebrate predator, the phantom midge, and the competitors, the daphnids, would give an opportunity for smaller species to flourish. The large zooplankton would try to escape fish predation by undergoing diel vertical migration whereby they hide in the dark at great depths during day time and feed on small zooplankton in the upper water layers only during night. The predictions were mostly correct. The large zooplankton species more or less disappeared and other species took over. A marked change occurred when the small Bosmina longirostris took over as the most abundant species in 2010. Over time, also rotifers increased in number of species. This was supposed to be an indirect response to changes in the invertebrate predation regime. Diel vertical migration started for the phantom midge larvae coincidentally with the appearance of the perch. This reduced the time the phantom midge could feed and it changed its life history to one generation per year while previously it had two generations per year. Although the Chaoborus started diel vertical migration, their density became very much reduced, and their predator avoidance behaviour was not sufficient for survival. Possibly, over the 200 years without fish predation, they might have lost their ability to go deep enough to avoid the fish. It was unexpected that it took such long time to change the zooplankton community in Lake Myravatn in spite of the fast development of the perch population. It took three years before B. longirostris appeared, and more changes seem to have occurred after that. The pike in the lake that for long had survived without fish prey, suddenly had a superabundance of new prey when the perch came, but although individual pike increased their growth, a numerical response was not noticeable. This study showed that contemporary theory concerning food-web structure in lakes is mainly correct. However, it is necessary to better quantify the processes involved to foresee the time it takes to cause changes and for predicting the effects on other parts of the freshwater ecosystem than just zooplankton and fish.