Daytime Exposure to Fish Predation for Littoral Benthic Organisms in Unproductive Lakes

Abstract

Fish activity near the bottom was monitored in two central Ontario lakes using time-lapse video equipment. Predation risk for benthic invertebrates was measured as the frequency of fish entries into a hypothetical cylinder with dimensions such that an imaginary 2–3 mm invertebrate at the bottom-center of the cylinder would be within the detection radius of any fish entering the space. By this criterion, risk of predation is high, even in lakes with low fish densities. For 7 d of data spread over different lakes, sites, and months, the median rate of exposure of the hypothetical invertebrate to passing fish was 14.2 times/h; the median rate of exposure to fish that stopped or turned as if foraging was 3 times/h. Exposure to benthivorous fish varied markedly between two lakes in June and within a lake between June and August. Changes in the fish species, sizes, and level of activity all contributed to the variance. Exposure to fish also varied markedly among days, within a day, and among sites within a lake. Differences in predation risk between two neighboring sites were consistent for several days, suggesting that some of the patchiness in benthic invertebrate composition might be generated by patchy fish exploitation. The high median levels of exposure to fish suggest that benthic organisms should spend a high proportion of their time in risk-reducing positions or behaviors, so that competition among fish caused by resource depression will be important.