Abstract
The regulation of aquaculture production in Norway considers the potential impact of salmon lice on wild fish. However, most attention has been focused on impacts on wild Atlantic salmon, despite the fact that anadromous brown trout spend the majority of their marine phase in coastal waters, where salmon lice have the highest impact. In the present study, we first suggest changes in marine living area and marine feeding time as sustainability indicators for first-time migrant sea trout, as high salmon lice densities may exclude sea trout from otherwise usable habitat and force them to return early to freshwater. Further, a method based on a bio-hydrodynamic model was developed to serve as a proxy for these indicators. The method accounted for the size, migration timing and spatial extent of sea trout and was demonstrated in 2 Norwegian salmon aquaculture production areas, Hardangerfjord (PO3) and Romsdalsfjord (PO5), and 2 focal rivers from within each fjord. Based on these comparisons, we exemplify how the change in marine living area and marine feeding time differed between PO3 and PO5 and within the areas. Sea trout migrating to sea late (June 5) were always more affected by lice than those migrating early (April 24) or at intermediate dates (May 15). Our estimates revealed dramatic potential impacts of salmon lice on sea trout populations, which were greatly influenced by spatial and temporal aspects. Considering the negative impacts of salmon lice on sea trout, a holistic view of environmental interactions between aquaculture and wild species that depend on habitats exploited for production is necessary.
Highlights
Aquaculture of fish is broadly viewed as an industry contributing to economic development in rural areas (Bostock et al 2010)
We conclude that the proposed indices (RML and reduced marine feeding time (RMT)) are well suited for estimating the impact of salmon lice on sea trout populations, and that the model proxies for Reduced marine living area (RML) and RMT work as intended
The examples show that the method works and can be implemented for all production areas and in all rivers with known sea trout populations
Summary
Aquaculture of fish is broadly viewed as an industry contributing to economic development in rural areas (Bostock et al 2010). Most of the attention has been focused on negative impacts of aquaculture on wild Atlantic salmon, with less consideration given to potential effects on anadromous brown trout Salmo trutta, known as sea trout (Thorstad et al 2015). Sea trout may be especially vulnerable to lice given that they spend so much time in coastal environments where planktonic salmon lice are distributed, and the potential habitat available to them may be reduced if they must avoid high risk areas where lice are abundant (Finstad & Bjørn 2011, Thorstad & Finstad 2018). We demonstrate how results from biophysical models can be used to develop proxies for risk assessment by predicting consequences of salmon lice densities on marine feeding space or marine residence time of sea trout. The method can be used in other countries (e.g. Gargan et al 2016a,b, Shephard et al 2016, Eldøy et al 2020) where similar biophysical models (Adams et al 2016, Rabe et al 2020) have been implemented to determine density of lice larvae in the water masses
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