"This study evaluates the life history responses of yellow perch to mass removal and the potential for population recovery. We removed approximately 94% of a perch population from Nepawin Lake, a 35 hectare oligotrophic lake in Algonquin Provincial Park, Ontario, as part of a study designed to enhance the recruitment success of brook trout. Several response variables were examined both before and after mass removal: (1) condition, which includes growth, diet and overall condition responses, and (2) reproduction, which includes size at maturity and fecundity. We examine the question of whether compensatory life history responses in the yellow perch will overcome brook trout predation leading to a reestablishment of a high density perch population. Results showed that prior to the manipulation, perch exhibited a narrow size distribution, high dietary overlap, and low condition, typifying a stunted population. After mass removal, the perch population remained in a narrow size distribution, exhibited decreased growth rates for older age classes, showed increased mean condition and increased consumption of zooplankton in all size classes. Perch also exhibited increased size at maturity and decreased fecundity immediately following the mass removal. A time lag is expected before compensatory recruitment is possible in the population, but it is likely that the perch will recover from the mass removal because of strong age 0+ and 1+ cohorts. However, stunting and bottlenecking may still occur in the population. Continued monitoring and management is necessary to observe further changes to the perch population dynamics in Nepawin Lake." --
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.
The genetic structure of a species encompasses the distribution of genetic diversity and composition among its component populations, providing important insight for conservation and management. This knowledge can be used to evaluate life history, gene flow, recruitment dynamics, and responses to exploitation and habitat changes. Discerning the changes or consistencies in population genetic patterns over time can provide important insights into the mechanisms that regulate genetic resiliency. Ultimately, analyses of spatial and temporal population genetic patterns may be used to conserve genetic diversity, unique variability, and adaptive potential. The yellow perch Perca flavescens (Percidae: Teleostei) provides an opportunity to investigate these patterns, as its population groups have experienced variable annual recruitment, high exploitation as a popular fishery in the Laurentian Great Lakes, and have not been evaluated previously for temporal consistency in genetic patterns. The objective of this thesis is to analyze the spatial and temporal genetic diversity and divergence of yellow perch spawning groups in order to better understand its life history responses and advance knowledge aiding its management. Population genetic patterns of yellow perch spawning groups are assessed across the Huron-Erie Corridor (HEC) and from locations in Lakes St. Clair, Erie, and Ontario using 15 nuclear DNA microsatellite loci. Results of this thesis research indicate that yellow perch spawning groups have appreciable genetic diversity and are distinguished from one another by considerable genetic differences. For example, the group spawning at the Belle Isle restoration site in the Detroit River has relatively high genetic diversity, with an appreciable number of alleles and private alleles. Yellow perch spawning at sites in Lakes St. Clair, Erie, and Ontario also show substantial genetic diversity whose levels are consistent over time. However, the genetic composition of yellow perch spawning at some given locations varied among different sampling years. Some age cohorts born in specific years who spawned together at Dunkirk NY (1980-2008) and Monroe MI (1997-2004), genetically varied across age groups. This pattern did not correspond to a pattern of isolation by time (i.e., there was not a consistent trend). The effective population size of yellow perch spawning at the Dunkirk, NY location is relatively modest and appears to have remained relatively consistent in size over the past 30 years. These spatial and temporal patterns likely are linked to life-history characters, such as kin-aggregation, natal site fidelity, and/or a sweepstakes model of reproduction. Genetic monitoring and development of long-time data sets like those assembled here are recommended to provide an important management assessment tool for monitoring and conserving fishery populations.
In wild fish populations, harvesting can reduce abundance, alter the distribution of phenotypes and drive changes in life history traits. Changes in maturation that influence adult fecundity are of concern when they reduce the productivity, yield and resilience of a population. Two hypotheses can account for earlier maturation of fish populations in response to harvest: individual compensatory growth responses to relaxed density-dependence and fisheries induced evolution (FIE). Plastic compensatory responses have received less attention in freshwater fish, while most prominent examples of FIE are for marine species. Independent of harvest, maturation may also respond to energy availability and mortality risk, and these may vary more in smaller freshwater compared to larger marine systems. I used 22 years of fisheries independent survey data from Lake Erie to assess the role of harvest on female age and length at maturation in yellow perch (Perca flavescens), walleye (Sander vitreus), white bass (Morone chrysops) and white perch (Morone americana). I asked whether, (i) changes in maturation were likely plastic; (ii) maturation schedule has evolved; and (iii) changes in maturation were more likely related to harvest or to other ecosystem factors. I tested a novel compensatory growth and life history model and found that adult yellow perch abundance was negatively related to harvest, but this had no effect on juvenile growth or on maturation. Nevertheless, the dynamic behaviour of maturation among cohorts strongly suggests plastic life history responses. I compared probabilistic maturation reaction norms for cohorts in the mid-1990s and late 2000s and found little evidence of evolutionary change in yellow perch maturation after accounting for plasticity in life history. Lastly, I evaluated the importance of harvest relative to the effects of large ecosystem scale factors on maturation dynamics for four fish species in Lake Erie. Analyses of compensatory growth and life history revealed no support for an influence of harvest on maturation within or among species. Instead, I found strong evidence of synchronous changes in maturity across species. My findings suggest that the maturation of Lake Erie's fishes is not strongly influenced by recent harvest, but is instead responding to changes in ecosystem conditions.
Yellow perch populations in Lake Erie exhibit large yearly fluctuations in year class strength (YCS), with most years showing relatively poor recruitment. For percids, no statistically significant relationship between stock and recruitment has been found. Most research has focussed on various environmental factors to explain the variability in YCS. Of the studies reviewed in the first chapter, variations in YCS could not be explained by variation in any single environmental factor. The second chapter revisited the theoretical equation that spawning stock size is related to recruitment. Yearly variation in the number of mature females in the spawning stock may explain the variability in the YCS of perch. We found that the proportion of fish at age varied annually from 1978-1990, suggesting that intermittent reproduction exists for Lake Erie yellow perch. The third chapter evaluated through a model whether variation in the proportion mature as well as interannual variations in age distributions, size and the associated size-related fecundity could explain the variability in YCS of the yellow perch populations. Variation in the proportion mature could explain a large proportion of the observed YCS, however we could not accurately predict recruitment from the variations in the life history parameters included in the model.
