The influence of spatial heterogeneity on the study of fish horizontal daily migration

We described the spatial and short term temporal heterogeneity of fish abundance in the perimeter of four lakes characterized by different levels of abundance in pelagic predators. Fish relative abundance was estimated by acoustically sampling 25 to 100% of the complete contour of the lakes at 4 h intervals during 24 h. During hydroacoustics, the transducer was installed on the side of a boat and directed towards the littoral such that the axis of the acoustic beam was set parallel to the surface of the lake. Maximum values of average fish relative abundance in the pelagic zone ranged from 6 to 209 targets/100 m(3). Fish relative abundance varied among segments by factors ranging from 3 to 84. Coefficient of variation of fish relative abundance ranged from 63 to 523% and supported the hypothesis that fish are more heterogeneously distributed in lakes containing pelagic piscivores. Our work suggests that the presence of pelagic piscivorous fish may affect the sampling design required to estimate fish abundance and distribution patterns in lakes. (C) 1998 Elsevier Science B.V.