Relationships Among Fish Traits, Demographics and Flow and Thermal Patterns

Abstract

The complex nature of riverine ecosystems creates both fine- and coarse scaled physicochemical conditions that can influence the selective pressures on certain morphologies and life histories, altering occupancy and population demographics. Our research objectives were to 1) determine the physicochemical conditions related to assemblage structure of 60 species, and 2) assess growth, mortality and recruitment of Bluegill Lepomis macrochirus, Alabama Bass Micropterus henshalli, Largemouth Bass Micropterus nigricans, and Freshwater Drum Aplodinotus grunnniens as related to hydrology and temperature conditions. We used an occupancy model framework in combination with morphological trait-based clustering to assess the physicochemical factors related to species occupancy. We found that the daily rate of flow changes, depth, sinuosity, the timing of flow pulses, and median water temperature were related to species occupancy. Small-bodied species appeared sensitive to increased rate of flow changes, but the strength of the relationship depended on cluster membership. Among small bodied species, morphological clusters associated with benthic dwelling individuals, had decreased occupancy probabilities with on average deeper reaches. Whereas pelagic dwelling species had decreased occupancy probability with more sinuous segments. The occupancy of many larger-bodied species (e.g., compressed with deep bodies and sagittiforms) was positively related to the increased time between flow pulses. We also determined migratory species were more likely to occupy on average cooler reaches creating possible confounding issues due to the inability to pass the dam. For our selected species we built linear mixed models and multiple regressions respectively, to relate environmental conditions to growth and recruitment. Growth in all species was positively related to the number of changes in flow direction (i.e., reversals). Alternatively, all species recruitment models were negatively related to the frequency of high flows. Beyond this metric, recruitment in Alabama Bass appeared to benefit from highly altered flow and thermal conditions whereas Largemouth Bass did not. My results indicate that flow and thermal alteration is influencing the assemblage structure and growth and recruitment of certain species. If the goal is to conserve diversity and improve population demographics, an adaptive management would allow mitigation and monitoring beyond dam relicensing events. Bluegill could be an informative candidate from monitoring as recruitment failures are more detrimental over shorter live spans. Moreover, consideration of both the timing and frequency of high flows and the reduction of rapid flow changes through ramping and draw down rates could contribute to improving biological conditions.