Spatial and Temporal Variation in Lake Huron Yellow Perch (Perca flavescens) Life History Traits
Knowledge of fish population structure allows fisheries managers to account for potentially different responses of discrete groups to external stressors. Life history traits are very useful indicators of population structure because they provide information about fish populations that incorporates elements of genetics, environment, and resource use simultaneously. In Lake Huron, the yellow perch (Perca flavescens) is managed based on 17 geographic management units; however, it is unknown if management units accurately represent discrete perch groups. Furthermore, it is unclear whether yellow perch population structure changed temporally over the course of the major ecosystem shift in the early 2000s, where invasive mussels decreased zooplankton and benthic invertebrate abundance, altered nutrient and energy distribution, and reduced lake productivity. Here, I used data from the Ontario Ministry of Natural Resources and Forestry’s Lake Huron Index Netting Program to derive sex-specific life history traits for yellow perch including size at maturity, age at maturity, maximum size, lifespan, and growth at age 2 from contemporary (2009–2018) and historical (1990–1999) timeframes. In the first part of my study, I examined how yellow perch were spatially structured in Lake Huron. Generalized linear mixed models showed that yellow perch life history traits varied with location and depth, but primarily with latitude. Male maximum size was 1.1-fold greater at southernmost sites (276.3 ± 4.6 mm) compared to northernmost sites (247.7 ± 3.2 mm), while female maximum size was 1.1-fold greater at southernmost sites (318.5 ± 1.3 mm) compared to northernmost sites (293.9 ± 8.1 mm). Longitudinal and depth-based variation existed in fewer life history traits. Female maximum size was 1.2-fold greater at westernmost sites (293.9 ± 8.1 mm) compared to easternmost sites (244.2 ± 12.4 mm). Male growth at age 2 was 1.2-fold greater at deeper sites (160.0 ± 11.4 mm) compared to shallower sites (131.1 ± 0.3 mm), while female growth at age 2 was 1.2-fold greater at deeper sites (166.1 ± 16.2 mm) compared to shallower sites (139.6 ± 4.0 mm). I found 6 discrete clusters of yellow perch in Lake Huron based on variation in life history trait values, encompassing fish in the (1) South Basin, which were superior in growth, maturity, and lifespan; (2) Main Basin, which grew fast, but died fast; (3) North Channel, which had average growth and maturity, and lived long; (4) northeast Georgian Bay, which were short lived, slow growers; (5) central Georgian Bay, which had slow growth and fast maturity, but died quickly; and (6) south Georgian Bay, which had average growth and maturity, but died quickly. In the second part of my study, I found that yellow perch life history trait values showed no significant temporal variation. The only life history trait that was different before and after the major ecosystem shift was male maximum size, which increased on average 5% from 232.9 ± 23.3 mm to 244.6 ± 30.6 mm. The influence of location and depth varied across timeframes depending on the life history trait analyzed, but did not follow any specific pattern. Clusters of perch identified based on combinations of life history traits were similar in the contemporary and historical datasets. Current management units appear to adequately represent yellow perch population structure in Lake Huron, which suggests that no major change to the spatial arrangement of these management units is necessary. The discovery of no change in life history values over time despite the major ecosystem shift is surprising, and suggests that recent population declines are not via major shifts in the parameters I assessed.