Towards a future without stocking: harvest and river regulation determine long-term population viability of migratory salmonids

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.

Incorporating life history diversity in an integrated population model to inform viability analysis

“…Our population viability analysis suggests that management interventions may be necessary to meet recovery goals for abundance, and our model will be useful for simulating the outcomes of proposed actions in future work. For example, population trajectories may be simulated under alternative hatchery-control rules and with modified demographic parameters to represent habitat management actions (Honea et al 2009;Saunders et al 2018;Nater et al 2022;Sorel 2022). This will provide policy makers with estimates of expected population viability metrics (e.g., abundance, p QET , and PNI) across a range of different management scenarios to help inform decisions.…”

A paradoxical bias in knowledge about Norwegian freshwater fishes: research efforts during 1980-2020

“…Stocking has been used for different reasons, but most often to increase the potential yield in recreational inland fisheries. Such stocking is being discontinued, and studies on the potential population level effects of stopping stocking should be done (Nater et al 2022). One effect of stocking that should be evaluated further is the effect of introgression of stocked, non-native trout into wild populations (Wollebaek et al 2010).…”

A Spatial Scale Approach to Fish Habitat Ecology and Impacted Rivers

“…Recent reviews on fish habitat loss suggest 1) barrier removal, 2) fish passage construction, 3) habitat restoration, 4) restocking, and 5) fisheries management (Tickner et al 2020, Thieme et al 2021, Verhelst et al 2021). Other papers suggest to reduce stocking, to let "nature take its course" through rewilding, and to accept that some systems may be too unrealistic to restore (Rideout et al 2021, Nater et al 2022, Ward et al 2023. These seemingly contradictory recommendations stem from the rather unspoken view of whether these solutions are intended to restore a site versus restoring a process.…”