Diversity of life history characteristics of Lake Superior migratory rainbow trout

Abstract

Rainbow Trout (Oncorhynchus mykiss) have been present in the Great Lakes since their introduction in the late 1800’s (Clarkson et al. 1997). These fish have naturalized many streams and have established naturally reproducing populations in many of the Great Lakes tributaries. Localized adaptations have allowed for each population to thrive, given the natural conditions with which they are presented. When faced with ecological change, a population may alter life history strategies to better adapt to new conditions. We compared local adaptations in three of Lake Superior’s North Shore streams, and evaluated how life history strategies have changed in a population that has experienced significant ecological changes to better understand why certain life histories strategies are selected for given a particular set of ecological conditions. Rainbow trout were angled during their annual spawning migration (April-June) on McVicar Creek (Thunder Bay), Portage Creek (Black Bay), and the Cypress River (Nipigon River) over multiple spawning seasons. Scales were taken and, length, and sex recorded from the captured Rainbow Trout. Growth annuli from the scales were analyzed to determine the number of years spent in the stream, number of years spent in the lake prior to first spawning, age at maturity, total age, and number of spawning events. Individual Rainbow Trout were then categorized based on similar life history traits (number of years spent in the stream and number of years spent in the lake prior to first spawning). Portage Creek life histories shifted with the closure of the fishery in 1994 from an older smolting life history with later maturity to a younger smolting age with faster maturity; they then reverted to the older smolting age with slower maturity since following a massive population decline in 2007 McVicar Creek and the Cypress River life histories have stayed fairly consistent while their populations have also remained stable. The Portage Creek population collapse, and subsequent shift in life histories, is likely a result of a change in the Black Bay fish community which has led to the predation of young Rainbow Trout smolts and a reduction in the food availability in the Lake Superior once the smolts migrate to the lake.