Coho salmon population genetics and food-web complexity along a stream-age gradient in Glacier Bay National Park, Alaska

Background/Question/Methods

Extensive glacial retreat creates new freshwater habitats available for colonization by aquatic organisms. Glacier Bay National Park, USA (‘GBNP’) was predominantly ice-covered ~250 y ago, but glaciers subsequently have receded over 100 km up-bay, sequentially revealing many new streams.  We evaluated population-genetic patterns of a generalist top predator (juvenile coho salmon, Oncorhynchus kisutch) and food-web complexity across five GBNP streams ranging in age from ~40-200 years plus one “control” stream (ice-free since the Pleistocene) just outside GBNP.  Initially, given earlier studies that have indicated maximum habitat complexity in intermediate-aged GBNP streams, we hypothesized that food-web complexity (determined with C and N stable isotope analysis as both food-chain length, FCL, and coho trophic niche width, TNW) also would peak in intermediate ages.  We then tested two alternative hypotheses regarding O. kisutch population genetics: 1) genetic diversity and measures of population stability (estimated from 25 individuals/stream and 12 microsatellite loci) are correlated with food-web complexity (i.e.: both peak in intermediate-aged streams); 2) genetic diversity and population stability are inversely correlated with stream age, as predicted by general range expansion models.

Results/Conclusions

Food-web complexity, as both FCL and juvenile coho TNW, peaked strongly in intermediate-aged streams.  Interestingly, populations in all six streams were in Hardy-Weinberg equilibrium, and there was no evidence of the recent founder events that have occurred within GBNP streams.  Furthermore, all pairs of streams showed significant F_ST; that is, coho populations in each stream had a distinct genetic signature.  Genetic diversity, however, was negatively correlated with stream age (and not correlated with food-web complexity), partially supporting hypothesis 2.  We conclude that spawning coho salmon, although recognized to have a relatively high degree of site fidelity to their natal streams, are able both to colonize new habitat and to achieve population demographic stability within a few generations of colonization.  As climate change continues to cause poleward glacial retreat, coho and other anadromous salmonids should be able to expand their ranges quickly and successfully into newly created stream habitat, as long as overfishing, detrimental habitat modification, and other negative influences on fish populations are minimized.