Harvesting of Lake Trout Induces Alternative States in Lake Opeongo

Cisco was introduced to Lake Opeongo as a forage fish for Lake Trout over 60 years ago. While the introduction had an initial positive effect on both Cisco and Lake Trout growth and abundance, over time the system changed resulting in the currently lower body and population sizes observed for both species. We use time-series data and life-history theory to disentangle a set of hypotheses on why this later change occurred. We find a strong correspondence with an Emergent Allee effect, which is driven by the predator’s ability to create more profitable prey by feeding on it. This conclusion is supported by a drastic decrease in Lake Trout abundance in the late 1970s due to high fishing mortality. Prior to this event, Lake Trout foraging thinned the Cisco population, which reduced intraspecific competition that led to larger Cisco sizes. After the event, increased competition between Cisco slowed their growth rate. As the Lake Trout population recovered in size over the last few decades, there was no transition back to the previous state. Cisco appears to be locked in a state with small size at maturation and slow growth.