Ontogenetic diet shifts with potential ramifications for resource competition in a kokanee– Mysis diluviana system

Ontogenetic shifts represent important transitions that can influence how fish interact with their environment. However, ontogenetic shifts are rarely placed into a population context due to the difficulty of incorporating the vagaries of size-mediated interactions. As such, we evaluated the role of ontogenetic shifts in diet as they relate to potential competitive interactions between kokanee Oncorhynchus nerka and Opossum Shrimp Mysis diluviana (hereafter Mysis) in Lake Pend Oreille, Idaho. Contemporary data were used to understand diet patterns of Mysis and kokanee. Historical data were evaluated within the context of ontogenetic shifts to better understand the long-term, population-level ramifications of interactions between Mysis and kokanee. Diet analysis revealed age-specific divergences in diet whereby juvenile kokanee primarily consumed copepods and adult kokanee preferentially consumed cladocerans. When placed in a historical context, age-specific patterns in kokanee diet likely led to increases in adult growth following declines in Mysis abundance. Improved fitness of adult fish likely resulted in record high abundances of kokanee in Lake Pend Oreille thereby shifting the balance from inter- to intraspecific competition.