A future without stocking? The importance of harvest and river regulation for long-term population viability of migratory salmonids

1. Humans are influencing animal and plant populations both directly (e.g.through harvest) and indirectly by altering environments. For many exploitedspecies, stocking with captive-bred individuals is a common strategy tomitigate negative human impacts and sustain populations over time. However,accumulating knowledge of negative side effects of stocking calls for quantificationof consequences and exploration of sustainable alternatives.2. Evaluating alternative management strategies using quantitative models iscentral to conservation. Here, we investigate the effects of several managementstrategies on a population of landlocked, migratory brown trout (Salmotrutta) inhabiting a large lake and spawning in a dammed river. We assess thepopulation level consequences of terminating a long-term stocking programmeand evaluate whether the loss of artificial recruitment may be compensatedby changes in harvest regulations and/or river habitat improvement.3. We build an integral projection model (IPM) classifying individuals bybody size, life history stage, and location relative to the hydropower damand parameterised it with 50 years of individual-based data supplementedwith literature values. We first analyse the model to assess size, structure,and relative importance of different mortality components across life stagesand locations in trout populations with and without stocking. We theninvestigate potential responses of an unstocked population to managementactions involving different sets of harvest rules, reductions in dam passagemortality, and improvements of spawning habitat below the dam.4. Our model predicts a strong population decline of 12–21% per year in theabsence of stocking. This decline is largely attributed to high harvest mortality,and drastic reductions in fishing pressure thus necessary to ensure populationviability without stocking. Reducing mortality associated with passage of thehydropower dam and restoring spawning areas has only small positive effectson population growth. Nonetheless, these mitigation measures can contributeto population viability when combined with changes in harvest regulations.5. Intensely harvested populations may rely heavily on the addition of captive-bredindividuals, and our results indicate that premature termination of stockingprogrammes can be detrimental without compensatory mitigation measuressuch as harvest reductions and habitat improvements. It is therefore crucialto collect necessary data and assess the impacts of alternative managementstrategies using quantitative models prior to making decisions.