Relationship of diet breadth to population genetic patterns in juvenile coho salmon (Oncorhychus kisutch) along a stream-age gradient in Glacier Bay, Alaska

Abstract

Generalist predators are thought to attain greater population stability and size in locations with greater available prey diversity. Along the well studied stream-age gradient in Glacier Bay National Park (USA), the most common generalist predators are juveniles of coho salmon (Onchorhynchus kisutch), and invertebrate prey diversity reaches a peak in intermediate-aged streams with greater habitat heterogeneity. We characterized genetic diversity and genetically inferred demographic stability of coho populations collected from five streams along the age gradient and one additional stream just outside Glacier Bay (presumed ice-free since the Pleistocene). In the same streams, we measured stable isotope ratios of juvenile coho and representatives of common invertebrate prey taxa to describe diet breadth in δ13C and δ15N space. We used these data to address the relationship between population demographic parameters and diet breadth. Genetic diversity was high and similar in all populations (He range 0.81–0.85), but diet breadth clearly peaked in in intermediate-aged streams. A genetic signal of demographic instability (presumably due to recent founder events) was strong only in youngest and most physically dynamic streams and was undetectable in the stream outside Glacier Bay. We conclude that rapid colonization of newly formed streams by coho from large source populations in the greater region likely drive high genetic diversity even in youngest streams, but expanded diet breadth and decreased physical habitat dynamism in streams> 150 years old combine to increase demographic stability as populations become more established. Once established, however, coho populations appear to maintain demographic stability even with strongly decreased diet breadth in the less heterogeneous older streams.