Evidence for Cogradient Growth and Survival Between Two Geographically Distinct Stocks of Muskellunge

Intraspecific genetic variation across the distribution of Muskellunge Esox masquinongy suggests the existence of divergent stocks among major river drainages. Often, stocks differentiate in response to latitudinal variation in thermal regime. Genetic variation and thermal adaptation may drive fish to either maximize growth during brief growing seasons at higher latitudes (counter-gradient growth) or experience longer periods of slower growth at lower latitudes (cogradient growth). The strength and direction of these genetic effects matter for fish stocked outside of their native drainage and for populations experiencing changes to regional thermal regimes. We used a replicated pond experiment with uniform initial fish size and environmental conditions to compare the survival and growth of stocked age-0 Ohio River drainage (OH) Muskellunge with those of the more northerly upper Mississippi River drainage (MISS) stock following their first winter and a year after introduction into a common environment. Both stocks had similarly high winter survival, though the MISS stock had a slower growth rate overwinter than the OH stock. Survival during the latter spring-to-fall period was greatly reduced for the MISS stock, however, with the differences in growth rate persisting. These findings suggest that cogradient adaptation to temperatures experienced during early life stages in native environments results in a disadvantage for high-latitude stocks of Muskellunge in thermal regimes warmer than those they have historically experienced in their native drainages.