The effect of artificial light on escapement beneath a survey trawl

An underwater video system is under investigation by the National Marine Fisheries Service for use in studying the capture efficiency of a survey bottom trawl’s footrope in diverse habitats commonly sampled during surveys. This system utilizes a 50-watt quartz halogen light since most survey stations are at depths greater than natural light can penetrate for low-light camera use. One potential shortcoming to this approach is that the light emitted might alter fish behaviour and thereby bias estimates of escapement. The light effect on escapement was examined by attaching the video system onto a trawl fitted with an auxiliary capture bag to retain those fish which would have otherwise escaped beneath. A paired towing experiment (unlit vs. lit) was conducted to test the hypothesis that there is no effect on capture proportion due to the addition of artificial light. ANOVA was used to test for the effect both fish length and water depth had on escapement. Since neither length nor depth were significant, a paired t test (α=0.05) was used to test the null hypothesis. For five of the six species examined, we found no effect of the light on fish escapement. Only flathead sole (Hippoglossoides elassodon) showed a statistically significant difference in capture proportion due to artificial light, the proportion decreasing by 0.0698.