Abstract
Freshwater species are particularly vulnerable to emerging threats linked to climate change because they are often already heavily impacted by habitat destruction, pollution, and exploitation. For many harvested populations of freshwater fish, these combined impacts have been mitigated for decades through stocking with captive-bred individuals. However, stocking may lead to loss of genetic variation, which may be crucial for adaptation under climate change. Exploration of sustainable alternatives is therefore paramount. We used a female-based integral projection model (IPM) to assess the consequences of terminating a long-term stocking programme for a population of landlocked, migratory brown trout Salmo trutta, and to evaluate relative effectiveness of alternative management strategies involving harvest regulations and river habitat improvement. The IPM classified individuals by body size, life history stage, and location relative to a hydropower dam, and was parameterised with 50 yr of individual-based data, supplemented with literature values. Model simulations indicated a strong population decline of 22-29% per year without stocking, much of which was attributed to high harvest mortality. Consequently, drastic reductions in fishing pressure were predicted to be necessary to ensure population viability without stocking. Mitigation measures reducing mortality associated with the hydropower dam or restoring spawning areas could further contribute to population viability when combined with changes in harvest regulations. Our results thus emphasise that large changes in management strategies, such as termination of long-term stocking programmes, require a thorough assessment of potential consequences and alternative mitigation strategies using data and models, or, at the very least, a precautionary approach under consideration of on-going climate change.
Highlights
Freshwaters are amongst the most threatened ecosystems in the Anthropocene; rivers and lakes that have already been strongly impacted by centuries of Publisher: Inter-Research · www.int-res.comClim Res · Advance View able management and conservation of freshwater species hinge on a thorough understanding and joint mitigation of several interacting anthropogenic stressors.Freshwater megafauna are intrinsically vulnerable to the interplay of anthropogenic stressors (He et al 2019), and for economically valuable fish species, such as salmonids, large-scale stocking programmes have been used as a common mitigation measure for decades (Laikre et al 2010, Aas et al 2018)
Under the strict assumptions of the current model, we found that the total harvest mortality of fish of all sizes would have to be reduced by 76−100% to sustain a population in the long run without stocking and in the absence of other mitigation measures (Fig. 5; Fig. S1.5 in Supplement 1)
The effects of stocking and harvest on the Hunder trout population are intricately linked to hydropower production in the river, and we investigated potential synergistic effects with 2 consequences of river regulation: compromised spawning and rearing habitat below the dam and additional mortality of smolts and upriver spawners associated with passing the dam on the downriver migration
Summary
Freshwaters are amongst the most threatened ecosystems in the Anthropocene; rivers and lakes that have already been strongly impacted by centuries of Publisher: Inter-Research · www.int-res.comClim Res · Advance View able management and conservation of freshwater species hinge on a thorough understanding and joint mitigation of several interacting anthropogenic stressors.Freshwater megafauna are intrinsically vulnerable to the interplay of anthropogenic stressors (He et al 2019), and for economically valuable fish species, such as salmonids, large-scale stocking programmes have been used as a common mitigation measure for decades (Laikre et al 2010, Aas et al 2018). Stocking programmes may have considerable adverse side effects on the genetic integrity of wild populations, potentially leading to loss of genetic variation through increased levels of inbreeding and breakdown of local adaptations over time (Laikre et al 2010, Allendorf et al 2013). This can lead to increased vulnerability to climate change (McGinnity et al 2009) by reducing the potential for adaptation, making consideration of alternative and more sustainable management strategies paramount. The relative effectiveness of different management strategies for any wild population depends on its reproductive capacity, the state of its habitat, and the total extent of harvest (Laikre et al 2010, Rogers et al 2010, Arlinghaus et al 2016)
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