Fisheries restoration potential for a large lake ecosystem: using ecosystem models to examine dynamic relationships between walleye, cormorant, and perch

Abstract

ABSTRACT. Increased population sizes of Double-crested Cormorants (Phalacrocorax auritus) and small-bodied (<15 cm total length) yellow perch (Perca flavescens) have occurred at Lac la Biche, Alberta, Canada, since fisheries collapsed the walleye (Sander vitreus) population. A walleye restoration program was introduced in 2005, but uncertainty around the ecosystem's response to management made it difficult to evaluate program success. This study used 40 variations of Ecopath with Ecosim models representing ecosystem conditions over 200 years to test the potential for multiple attractors, i.e., possible ecosytem states, in a large lake ecosystem. Results suggest that alternate stable states, defined by walleye-dominated and cormorant-dominated equilibriums, existed in historical models (1800, 1900), whereas contemporary models (1965, 2005) had a single cormorant-dominated attractor. Alternate stable states were triggered by smaller perturbations in 1900 than in 1800, and model responses were more intense in 1900, suggesting a decline in system resilience between model periods. Total prey biomass consumed by walleye was up to four times greater than the biomass consumed by cormorants in historical models, but dropped to 10% of cormorant consumption in 2005 models. Differential size-selection pressures of cormorants and walleye on yellow perch provided strong feedback that stabilized each state. These results provide important theoretical support for alternate stable states as well as practical insights for restoration of large lake ecosystems affected by human induced overharvest of top-level fish predators.

Synthesizing Professional Opinion and Published Science to Build a Conceptual Model of Walleye Recruitment

“…This disparity is largely due to episodic or disaster events (e.g., mass mortality events; Olsen et al 2019) that, while difficult to predict, have a large, singular impact on recruitment (Houde 1994;Roseman et al 1999). Recruitment patterns and the relationships among drivers can also shift over time due to ecosystem change (Ludsin et al 2014;McGregor et al 2015;Marcek et al 2021). For example, the nature and influence of many secondary mortality causes that we discuss may be influenced by climate change (Jones et al 2006).…”

Fisheries Volume 48 Number 4 April 2023

“…This disparity is largely due to episodic or disaster events (e.g., mass mortality events; Olsen et al 2019 ) that, while diffi cult to predict, have a large, singular impact on recruitment (Houde 1994 ;Roseman et al 1999 ). Recruitment patterns and the relationships among drivers can also shift over time due to ecosystem change (Ludsin et al 2014 ;McGregor et al 2015 ;Marcek et al 2021 ). For example, the nature and infl uence of many secondary mortality causes that we discuss may be infl uenced by climate change (Jones et al 2006 ).…”