The impact of density-dependant growth on whitefish production in re-oligotrophic lakes : a bioenergetics simulation study

Abstract

Following a period of eutrophication in the 1960s and 1970s, the productivity of many European lakes was restored to near-pristine levels at the end of the 20th century, a process termed re-oligotrophication. Lower phosphorus concentrations, however, lead to slower growth of fish due to a reduced food base, and eventually to smaller harvests. This problem is particularly serious in perialpine lakes whose fish communities are dominated by whitefish. In these lakes fishermen benefitted from enhanced lake productivity under eutrophic conditions but are now confronted with smaller whitefish yields under oligotrophic conditions. As growth of lake whitefish is densitydependent, manipulation of standing stock biomass has the potential of increasing growth rate and eventually harvests. This possibility is evaluated through bioenergetics modelling, simulating growth and yield of whitefish under various population densities and zooplankton food concentrations. These simulations clearly indicate that not only individual growth but total biomass production increase with decreasing population size, and the increase in total biomass production is even more pronounced at low ration levels. Decreasing whitefish population density therefore has the potential of increasing fishery yields. The unresolved problem however persists, how whitefish population density can be manipulated in the desired way.