Extreme discharge influences on freshwater habitat of early life stages of Atlantic salmon (Salmo salar) and associated recruitment disturbances

Abstract

Climate change-mediated precipitation patterns are rapidly increasing the frequency and intensity of storm events and resultant discharge regimes within Nova Scotian rivers. Extreme discharge regimes re-distribute streambed substrates by erosional processes during higher discharges and depositional processes of previously suspended particles during low-flow conditions. Due to their need for various sized substrates during freshwater early life-stages, climate-mediated extreme discharge regimes can exert detrimental impacts on early life-stages of Atlantic salmon (Salmo salar)(Finstad et al., 2011). The objective of this thesis was to evaluate the effects of discharge extremes on the habitat of early life-stage of Atlantic salmon within Clyburn Brook in Cape Breton Highlands National Park. Potential impacts were evaluated by re-assessing habitats over various periods of time and using hydraulic computational software (HEC-RAS) to predict the discharge required to cause the observed morphological alterations. Biota density (Atlantic salmon, EPT (Ephemeroptera, Plecoptera, and Trichoptera) benthic invertebrates) were correlated with changes in habitat variables to infer changes in habitat quality over time. The results provide evidence that high discharge regimes influence early life stage Atlantic salmon recruitment directly (via mass displacement) and indirectly (via habitat losses through bed load transport and loss of habitat heterogeneity) as biota abundance negatively correlated with a decline in streambed heterogeneity over time. However, the effects of low flow regimes were inconclusive due to anomalously wet summer 2023 conditions. Hydraulic computational models provided evidence that bed stability decreased from 2008-2021, indicating that in-stream morphological alterations may become more substantial as climate-mediated precipitation patterns continue to escalate, which may decrease habitat availability for juvenile Atlantic salmon in Clyburn Brook.