Chloé Rebecca Nater, Marlene Wæge Stubberud, Øystein Langangen, Atle Rustadbakken, S. Jannicke Moe, Torbjørn Ergon, Asbjørn Vøllestad, Yngvild Vindenes

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