Variability in Diadromous Behavior of a Northern Coregonid Based on Scanning Proton Microprobe Analysis of Otolith Strontium

Several coregonid species in northern latitudes exhibit diadromous behavior and may share river/lake systems with conspecifics who are freshwater residents. Although both life history strategies have been reported in a variety of systems, little information on the extent and variability in diadromous behavior both within and among Populations exists. We addressed this issue using inconnu Stenodus leucichthys from the Mackenzie River drainage (Northwest Territories, Canada) as a model. At the extremes of their distribution Within this system, inconnu have been characterized by alternative life history strategies. Those in the north exhibit diadromous behavior, while those in the south demonstrate life history closure within the upper reaches of the system (i.e., freshwater residency). We examined the otolith strontium profiles of inconnu collected from 12 regions throughout the Mackenzie watershed to (1) determine the geographical range over which diadromy occurs in this system, (2) characterize the variability in diadromous behavior within and among Populations, and (3) identify physical/environmental factors that may influence choice between a diadromous or freshwater life history. Our results demonstrate that the degree of diadromy within populations of inconnu tends to gradually decrease with distance from marine waters, rather than showing an abrupt transition from diadromy to freshwater residency. We conclude that distance from a marine food source plays an important role in determining whether fish remain in freshwater or migrate to the sea to feed. Within diadromous populations of inconnu, there is a substantial level of flexibility in the extent and frequency of marine migration by individuals.