ASSESSMENT OF LAKE TROUT SPAWNING - EVALUATION OF TRAPS FOR MEASUREMENT OF EGG ABUNDANCE

Biologists in the Great Lakes have used circular egg traps to study la ke trout (Salvelinus namaycush) reproduction for the past several year s; however, interpretation of egg trap data beyond the presence or abs ence of eggs has been speculative. The main objective of this study wa s to determine if trap data could be used to index egg abundance in sp awning substrate. In 1990 and 1992 the number of eggs captured in trap s on a reef in Lake Ontario was compared to the egg abundance in cobbl e substrate adjacent to each trap. Linear regression models indicated that the relationship between eggs captured in traps and egg abundance varied significantly, both among depth transects within years and bet ween years (P < 0.05). However, within a given year, egg capture in tr aps generally increased as egg abundance increased. Wind-induced waves and water currents probably affected the proportion of eggs that were dislodged from the substrate and drifted into traps or off the reef a nd caused much of the variation observed in the eggs . trap-1-egg abun dance relationship. Different substrate size and interstitial depth ma y also explain some of the within year variation among trap sites. Tra p data should not be used to compare egg abundance among different Gre at Lakes spawning areas unless data interpretation includes sources of variation such as water currents and substrate type. Comparison of tr ap data among years for the same spawning area should only be made amo ng years with similar water currents.