Gonadal Asymmetry in Lake Trout: A Result of Climate Change?

Long-term population viability of coldwater salmonids in the face of climate change will depend on their ability to respond and adapt to changing thermal conditions and habitat loss.  Population viability will be affected by available habitat and population size.  As an archetypal coldwater salmonid, Inland populations of lake trout may be particularly vulnerable to climate change because of their sensitivity to increased temperatures, relatively small population sizes, and limited habitat availability.  Very little is known about the behaviour and responses of lake trout in already warming southern populations.  This work assessed the reproductive state of mature lake trout in a thermally stressed population in northwestern Ontario.  Lake trout in this previously unexploited, meromictic lake are abundant but habitat-limited due to high summer temperatures in the epilimnetic zone and anoxic conditions in the hypolimnion due to thermal stratification.  As a result, lake trout in this lake are confined to a narrow (1m) depth stratum for most of the year, although individuals have occasionally been captured in shallow waters as they feed on surface insects.  Examination of mature gonads showed strong asymmetry in gonad size and development in both male and female fish, suggesting that this population is under reproductive stress.  In addition, developing eggs within individuals varied considerably in their size and lipid content, further indicative of stressed fish.  Extensive sampling with netting and hydro-acoustic surveys detected very few juveniles in this lake.  Together, these results suggest that lake trout in this warming lake may have reduced reproductive success as a result of thermal stress, which may be an important finding to explain the observations of recruitment failures in lake trout populations along the southern edge of their range.