Day-to-day variation in fish horizontal migration and its potential consequence on estimates of trophic interactions in lakes

We assessed the day-to-day variation in fish horizontal migration by acoustically sampling a small oligotrophic lake at 4-h intervals during 15 consecutive days. Hydroacoustics was performed by directing a 200-kHz transducer from a floating platform situated in the middle of the lake towards the littoral zone such that the longitudinal axis of the acoustic beam was parallel to the surface of the lake. Fish horizontal migration displayed a qualitatively regular daily pattern. Fish relative abundance in the pelagic zone increased at dusk, peaked during the night (up to an average of 62 targets/100 m(3)), and decreased at dawn. However, fish relative abundance in the pelagic zone varied by up to 12-fold at peak of daily migration. On specific days, fish stayed in the pelagic zone for a time period that ranged from 4 to 24 h, and ventured in the pelagic zone at distances from the hydroacoustic shore that ranged from 7 to 16 m. Simulations indicated that one strategy to minimize the influence of among-day variation of the characteristics of fish horizontal migration at the temporal scale we studied would be to sample the lakes at 4-day intervals. (C) 1998 Elsevier Science B.V.