Movements of Adult Lake Trout tagged in Northwestern Lake Michigan

Abstract

Movement plays an important role in animal life history, evolution, and population dynamics. Knowledge of movements can provide valuable information to aid in the management of animal populations. This is especially true in the management of many of the world's fisheries. Management of many marine and freshwater species relies on understanding movement patterns, especially in relation to jurisdictional boundaries and for highly migratory species. Lake trout management and restoration in the Great Lakes is multi-jurisdictional and knowledge of movements could aid in management decisions as well as restoration efforts. My objectives were to quantify lake trout movements in northwestern Lake Michigan in both distance and direction, examine temporal patterns, and test factors that may affect movement. Lake trout were tagged in fall gill net assessments during 1983-1993 and during spring pound net assessments in 1984-1996 in the Clay Banks area of northwestern Lake Michigan. Lake trout were recaptured in angling, assessment, and commercial fisheries during 1983-1997. Recapture rates were calculated for fish recaptured within one year after tagging. Angling recaptures were standardized by salmonid angling effort and the cumulative proportion of recaptures per unit effort (RPE) as a function of distance was fit to an exponential sigmoid model. Dispersal radius (analogous to home range) and straying rate (beyond 50 miles) was calculated using parameters estimated from the model fitting procedure. Separate models were fit to recaptures by tagging season and recapture season. Directional movement was calculated using vector analysis. Mean direction of travel and movement rate for the tagged lake trout populations were calculated. Linear regression was used to test for the effects of time at large ( days between mark and recapture) and population density on lake trout movement. Recapture rates were significantly higher for spring-tagged lake trout than for fall-tagged lake trout and, for both tagging seasons, recapture rates were significantly higher in the 1980s than the 1990s. Lake trout in northwestern Lake Michigan occupied an area within 35 to 43 miles of the tagging location, and strayed beyond 50 miles 1-6% of the time. Spring-tagged lake trout occupied a significantly higher area and strayed beyond 50 miles at a significantly higher rate than fall-tagged lake trout. Movement distance and straying did not differ among recapture seasons. Directional preference for lake trout tagged in northwestern Lake Michigan was always southward along the western shoreline of Lake Michigan regardless of tagging or recapture season. Movement rate, however, was significantly higher for spring-tagged fish. The distance traveled by spring-tagged lake trout increased with increased time at large, but distance traveled by fall-tagged lake trout decreased with increased time at large. Straying rate and dispersal radius increased with increasing population density for spring-tagged lake trout but not for fall-tagged lake trout. Recapture rates seemed to be affected by rewards offered during the early years of the study. The area occupied by most lake trout was consistent with other studies of lake trout movement. Results of this study may be helpful in determining spatial scales for modeling efforts, to help set stocking locations and regulations for harvest management based on fish stocks rather than jurisdictional boundaries alone.