Salmon louse infestation levels on sea trout can be predicted from a hydrodynamic lice dispersal model

Abstract

1. The abundance of the parasitic salmon louse has increased with the growth in aquaculture of salmonids in open net pens. This represents a threat to wild salmonid populations as well as a key limiting factor for salmon farming. The Norwegian 'traffic light' management system for salmon farming aims to increase aquaculture production while securing sustainable wild salmonid populations. However, this system is at present solely focusing on mortality in wild Atlantic salmon, while the responses of sea trout with different ecological characteristics are not included.2. We analyse lice counts on sea trout from surveillance data and use Bayesian statistical models to relate observed lice infestations to the environmental lice infestation pressure, salinity and current speed. These models can be used in risk assessment to predict when and where lice numbers surpass threshold levels for expected serious health effects in wild sea trout.3. We find that in production areas with the highest density of salmon farms (West coast), more than 50% of the sea trout experienced lice infestations above the levels of expected serious health effects. 4. We also observed high lice infestations on sea trout in areas with salinities below louse tolerance levels, indicating that fish had been infested elsewhere but were returning to low-saline waters to delouse. This behavioural response may over time disrupt anadromy in sea trout.5. The observed infestations on sea trout can be explained by the hydrodynamic lice dispersal model, which provides continuous estimates of lice exposure along the whole Norwegian coast. These estimates, which are used in Atlantic salmon research and management, can also be used for sea trout.6. Synthesis and applications. Wild sea trout, spending its entire feeding migration in fjords and coastal areas, is at higher risk than wild Atlantic salmon to lice infestations from industrial salmon farming. The high levels of lice infestation we observed on sea trout question the environmental sustainability of the current This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Novel large‐scale mapping highlights poor state of sea trout populations

“…Salmon lice from aquaculture can considerably increase mortality and reduce growth both in Atlantic salmon and sea trout in farm‐intensive areas (Johnsen et al, 2020; Skaala et al, 2014; Vollset et al, 2016). Sea trout are likely to be more seriously affected by salmon lice than Atlantic salmon because most sea trout remain feeding and growing in coastal waters, where salmon farms are situated, during their entire marine migration (Bøhn et al, 2021; Fjørtoft et al, 2014). Atlantic salmon, in contrast, migrate through coastal areas on the way to ocean feeding grounds or during return to their native rivers and are hence exposed to salmon lice over shorter time periods than sea trout.…”

“…An alternative multinominal logistic regression with mortality classes as factors produced similar results (significant effects of lice index and salinity) and an overall correct classification at 62%. The approach taken is supported by studies showing significant relationships between predicted salmon lice infestation intensity from models and the observed infestation of sea trout sampled in the monitoring programme (Bøhn et al, 2021; Myksvoll et al, 2018).…”

Balancing risks and rewards of alternate strategies in the seaward extent, duration and timing of fjord use in contemporary anadromy of brown trout (Salmo trutta)

“…Anadromy comprises a successful life-cycle adaptation for salmonids migrating between adjacent low-risk, nutrient-poor habitats (streams, rivers and lakes) and high-risk, nutrient-rich habitats (marine waters) [ 4 , 5 ]. Natural selection for, or against expression of anadromy has occurred since the last ice-age, for some 10,000 years [ 6 ], yet in relative terms, recent human activities have altered the planet’s environment to an extent worth coining a new geological epoch: the Anthropocene [ 7 ]. Anadromous salmonids are greatly impacted by these anthropogenic environmental changes, due to their dependence on several different habitats and connectivity between them [ 8 ].…”

“…Overfishing, habitat degradation (e.g., hydropower development, riverbed regulations), climate change and water pollution are listed amongst the main disturbances affecting brown trout populations [ 21 ]. Increasing salmonid aquaculture activity has compounded densities of the parasitic salmon-lice ( Lepeophtheirus salmonis ) in many fjords [ 6 , 44 , 45 ], with anadromous brown trout prone to infection by this parasite since they mainly utilise marine areas in close vicinity to aquaculture facilities and during periods of high salmon-lice densities [ 44 ]. Therefore, anadromous brown trout experience increased marine mortality and reduced individual growth in fjords with extensive salmon farming activity [ 46 – 48 ].…”

Direct evidence of increased natural mortality of a wild fish caused by parasite spillback from domestic conspecifics

“…Atlantic salmon (Salmo salar) infested with lice incur damage to the skin, which may lead to pathological responses in the host when numbers are unnaturally high, ultimately resulting in death if untreated [3][4][5]. The presence of fish farming has repeatedly been linked to epidemic outbreaks of salmon lice on wild salmonids [6,7]. Consequently, salmon lice parasitism of wild Atlantic salmon, sea trout (Salmo trutta), and sea-run Arctic char (Salvelinus alpinus) post-smolts is one of the major environmental concerns for wild salmonids in countries with intensive salmon farming [8][9][10].…”