Seasonal patterns in growth, diet, and prey availability of Brown Trout Salmo Trutta in groundwater-dominated streams in the Driftless Ecoregion of southeastern Minnesota

Abstract

Groundwater-dominated streams of southeastern Minnesota support highly productive populations of Brown Trout and aquatic macroinvertebrate fauna, but past research has shown that fish growth varies on spatial and temporal scales. As the diet of Brown Trout depends on the availability and abundance of aquatic macroinvertebrates, it is possible that spatial and temporal variations in macroinvertebrate assemblages (e.g. abundance, diversity, distribution) shape patterns in the growth, distribution, fitness, and abundance of Brown Trout populations. In particular, there is evidence that suggests spatial and seasonal variation in winter conditions (including water temperature and prey assemblages) may influence broader patterns of Brown Trout growth in this region. Although studies related to Brown Trout growth, diet, prey availability, and selectivity have been conducted in southeastern Minnesota, no research has addressed all of these factors simultaneously over multiple streams, seasons, and years. The overall goal of this dissertation was to examine seasonal and spatial patterns in Brown Trout growth, diet, and prey selection, and in the availability of macroinvertebrate prey in six streams that demonstrate a gradient in fish growth. Chapter 1 examines temporal and spatial differences in the growth and diet of Brown Trout, and identifies associations between diet and physical stream characteristics, including groundwater input, stream drainage area, and channel slope. Although stomach contents were dominated by relatively few taxa, including Chironomidae and Brachycentrus, there were significant differences in diet composition among streams and seasons. Overall, fish in most streams demonstrated positive growth during summer, winter, and spring; and winter growth was significantly associated with groundwater input. Chapter 2 addresses seasonal patterns in Brown Trout prey assemblages, including the abundance, diversity, and overlap of macroinvertebrate benthos and drift. Benthic and drift assemblages differed among streams and seasons, with most pronounced differences among streams. Abundances of drift and benthos varied significantly, with highest availability in spring and summer, and lowest availability during winter in most streams. Non-metric multidimensional scaling indicated associations between assemblage composition and groundwater input. The goal of the third and final chapter was to identify seasonal and spatial patterns in the availability and selection of macroinvertebrate prey by Brown Trout. Overall electivity of benthos and drift varied spatially and temporally with a negative relationship between the total proportion of prey available and prey electivity. In general, Brown Trout consistently favored one or two taxa, including Physella and Gammarus while displaying negative selection for all other prey types. Foraging patterns suggested a preference of benthic feeding. Ultimately, the results of this dissertation will help managers take effective actions in mitigating the effects of climate change on stream communities. In addition to a better understanding of the influence of groundwater control on patterns in Brown Trout growth, foraging, and prey communities, this dissertation identifies key prey taxa associated with the growth of Brown Trout on seasonal and spatial scales. With this knowledge, managers can more accurately predict how climate-induced changes to water temperature and prey communities will alter fish productivity on a stream-by-stream basis. Considering the substantial economic and cultural importance of groundwater-dominated streams in southeastern Minnesota, conserving the integrity and health of stream communities through the development and implementation of science-based management plans should be a priority among natural resource managers, scientists, and policymakers.