European perch change their diet composition as a result of infection with eye flukes. | Photo: Solvin Zankl
Eye fluke infection is a common parasitic disease in wild fish populations. In Lake Müggelsee in Berlin, for example, European perch are affected at a rate of 99 percent. The researchers studied young European perch from Lake Müggelsee over a two-year period, counted eye flukes and used stomach content and stable isotope analyses to determine how the animals fed.
Normally, young perch eat zooplankton and benthic invertebrates. "More heavily infected European perch fed primarily on killer shrimp, while the less infected animals showed no preference for any particular prey species. The behavioral change is probably related to the reduced vision of the infected fish," explained Jenny Vivas Munoz.
Hidden effects on food webs and water clarity
Such hidden effects of parasites, leading to food specialization between individuals within a population, are underestimated influencing variables in food webs. They can significantly alter the energy flow within the ecosystem.
"In aquatic ecosystems, parasites can play an important role not only in the transfer of energy to higher trophic levels – for example by increasing the predation risk of infected animals – but also in the interaction of the host with lower trophic levels by changing its food choice," Klaus Knopf summarized. This top-down effect from the European perch on the zooplankton can also have cascading effects on the phytoplankton (algae) and thus affect the water clarity of a lake.
