Temporal variations in the pelagic fish community of Lake Winnipeg from 2002 to 2019

Lake Winnipeg has undergone extensive changes in environmental conditions and fish community structure during recent decades. The presence of cyanobacteria and invasive species as well as eutrophication, flow regulation, fishing pressure, and climate change all have potential impacts on the native fish community. Since 2002, pelagic forage fish species and early life stages of large-bodied fish species in the pelagic zone in Lake Winnipeg (Manitoba, Canada) have been monitored. Catch data were used to study the temporal variation in species-specific occurrence and catch-per-unit-effort (CPUE) and to analyse the effects of lake condition on species occurrence and CPUE estimates. To account for the temporal variation observed for commonly caught prey species in Lake Winnipeg, we used a Bayesian approach, Integrated Nested Laplace Approximation (INLA), to compare the effects of environmental variables on occurrence and CPUE. The pelagic fish community composition varied among years, and CPUE declined by 2.2-fold between 2005–2008 and 2015–2018. Emerald shiner was the most dominate prey fish species but decreased in the catches since the early 2000’s, while rainbow smelt have almost entirely disappeared from the catches in recent years. Overall, we observed a decrease in CPUE in four of six most caught, pelagic species. Important environmental variables explaining occurrence and CPUE of the six most caught species in the survey were temperature and Secchi, trawl, and water depths. The trawl survey serves as a valuable long-term monitoring tool to study trends in responses of both native and non-native species to ecosystem changes in Lake Winnipeg.