Climate warming is expected to positively affect cool-water, temperate fish populations by lengthening the growing season and expanding thermal habitats suitable for positive growth. Yet, little is known about how a corresponding shortened winter might affect temperate fish populations, especially for species that require a prolonged period of cold temperature during the winter prior to spawning for proper ovary development. Additionally, events such as hypolimnetic hypoxia (O2 < 2 mg/L), are expected to increase with continue warming. We hypothesized that climate change would negatively affect temperate fish populations by 1) increasing bottom hypoxia during summer, which can reduce energy reserves (fish condition) prior to winter, when ovaries develop for many species, and 2) increasing winter water temperature, which could increase basal metabolic rates during winter (i.e., reduce energy available for ovary development) and disrupt thermal requirements necessary for proper ovary development.
To test these hypotheses, we investigated the effects of winter temperature and female condition on Lake Erie yellow perch Perca flavescens reproductive development, egg and larval quality, and ultimately, fall juvenile abundance (a strong predictor of future recruitment to the yellow perch fishery in Lake Erie). Towards this end, we conducted laboratory experiments, a multi-year field study, and historical analyses. In our laboratory experiments, female yellow perch exposed to a long winter produced higher quality eggs (i.e., in terms of size, energetic, and lipid content) that both hatched at higher rates and produced larger larvae than lower quality eggs from females exposed to a short winter (Chapters 2 and 3). Counter to our hypotheses, reduced female condition entering winter did not adversely affect reproductive success (Chapter 3). Additionally, field and laboratory studies found that when spring warming happened extremely early, yellow perch spawning did not fully adjust, increasing the possibility of a mis-match between first-feeding larvae and their zooplankton prey (Chapter 2). Finally, we show through historical analyses that the negative effect of warm winters on Lake Erie yellow perch juvenile abundance appears to be consistent over 42 years (i.e., 1969-2010), and has persisted throughout a large-scale, nutrient-driven regime shift and restructuring of the food-web due to numerous introductions of invasive species (Chapter 4).
Our research offers a previously unrecognized mechanism by which climate change can threaten temperate fish populations, through reductions in reproductive success. Our results also may have relevance to fisheries managers seeking to better anticipate the responses of fish populations to climate change. Specifically, given that our study has identified mechanisms that appear to be responsible for long-term population dynamics, our findings may allow for managers to monitor the appropriate variables (i.e., winter thermal regime) necessary to predict annual recruitment to the fishery for Lake Erie yellow perch. Additionally, because our study species has similar life-history and physiological requirements not unlike many other cool-water, temperate fishes, our findings may have relevance to fish populations in many ecosystems.