AN EVALUATION OF LIGHT-MEDIATED VERTICAL MIGRATION OF FISH BASED ON HYDROACOUSTIC ANALYSIS OF THE DIEL VERTICAL MOVEMENTS OF RAINBOW SMELT (OSMERUS-MORDAX)

We used hydroacoustics to examine diel changes in the vertical distributions of rainbow smelt, Osmerus mordax, in Lake Memphremagog, Quebec/Vermont. Our objective was to evaluate hypotheses linking diel vertical movements of fish with light levels. Smelt distributions were also monitored from June through October (1988 and 1990) to evaluate seasonal changes in their behavior. A strong relationship (r(2) = 0.83) between ambient light intensities and the upper fish layer in the water column was observed. Fish depth was also related to the depth of the thermocline during the night and when surface water temperatures were >18 degrees C. The most characteristic feature was the strong avoidance of light levels >0.1 mu W/cm(2). However, we found considerable variation in lower light levels experienced by the whole fish population. The results suggest that existing models of anti-predation behavior relating light and fish depth are consistent, with some limitations, with patterns of diel vertical migration in rainbow smelt.