Rates of Food Exploitation by Littoral Fishes in a Mesotrophic North-Temperate Lake

Abstract

The relationship between the biomass of food consumed by the littoral zone fish community of Lake Memphremagog, Quebec, and the total biomass of littoral zone food present was estimated on a daily and annual basis. The dominant food consumed were benthic invertebrates followed by forage fishes and plankton. The daily geometric mean exploitation rate was 0.55% of the total dry weight food biomass in the littoral zone. The corresponding annual rate was 72%. Energy equivalents were 0.81% (daily) and 105% (annual). When benthic invertebrate biomass values were adjusted to more accurately reflect availability (on the basis of depth distribution in the sediments and size) the geometric mean dry weight exploitation rates were 0.80% (daily) and 104% (annual). Corresponding energy rates were 1.15% (daily) and 150% (annual). These rates are significantly lower than reported benthic P/B ratios for 21 north-temperate lakes. These data indicate that littoral zone fishes crop only a very small fraction of the food biomass in the littoral zone. We conclude that the behaviors of both predator and prey act to limit the available fraction of the total food biomass and predict that the ratio of available to total food biomass in the littoral zone of north-temperate lakes will approximate 1%. This is consistent with existing empirical models linking fish production/biomass with biomass at lower trophic levels.