Kootenay Lake kokanee (<i>Oncorhynchus nerka</i>) collapse into a predator pit
Will G. Warnock, Joe Thorley, Steven K. A. Arndt, Tyler Weir, Matthew D. Neufeld, Jeff A. Burrows, Greg Andrusak
Abstract
Kootenay Lake is a large, oligotrophic waterbody in southern British Columbia renowned for recreational fisheries for piscivorous rainbow trout (Oncorhynchus mykiss) and bull trout (Salvelinus confluentus). Long-term datasets showed an increase in large-bodied (>2 kg) piscivore abundance followed by a collapse of the kokanee (Oncorhynchus nerka) prey population in 2013 and subsequent decline of large-bodied piscivores. An unprecedented post-collapse state formed in 2015–2018, characterized by low kokanee spawner abundance and biomass and high catch rates for small-bodied (<2 kg), slow-growing piscivores. Bioenergetics model estimates of average historical (1961–2008) piscivore consumption was 29.3% of the average historical (1993–2008) kokanee prey supply (biomass and production), but increased to 78.7% in 2011, immediately preceding kokanee collapse. From 2015–2018, kokanee did not recover due to persistently poor juvenile survival; estimated piscivore consumption relative to prey supply remained high (73.0%), suggesting that kokanee were trapped in a predator pit. Although the ultimate and interacting causes of the predator build up remain uncertain, overcoming current depensatory dynamics may be aided by kokanee stocking or increasing harvest on still-abundant, unsatiated piscivores.