Changes in growth and migration patterns of sea trout before and after the introduction of Atlantic salmon farming

Abstract

Marine growth has strong implications for reproductive potential and ultimate fitness of sea trout. Hence, the effects of anthropogenic factors on marine growth are important when evaluating population responses and implementing management measures. Temporal changes in growth patterns of sea trout from three Norwegian and two Irish watercourses were examined, covering time spans of 25–65 years. Elemental chemistry Ba:Ca profiles and visual reading of fish scales were used to estimate smolt length and lifetime growth after first sea entry. Reduced growth after the first sea entry coincided with periods of nearby (<14 km) salmon-farming activity in impacted watersheds in both countries. Increased Ba:Ca levels were also recorded during these periods, likely indicating reduced residency in marine habitats caused by premature return to freshwater and estuaries. An increase in estimated length at first sea entry coinciding with salmon-farming activity, for groups of fish sampled after sea migration, suggests a size-selective marine mortality, with the smallest individuals experiencing a larger mortality.

Development and field validation of RPA‐CRISPR‐Cas environmental DNA assays for the detection of brown trout (Salmo trutta) and Arctic char (Salvelinus alpinus)

“…They also tend to reside in coastal areas during their migration meaning they are threatened by anthropogenic changes such as Atlantic salmon aquaculture, habitat alterations and fishing (Nevoux et al, 2019). The negative impact of Atlantic salmon aquaculture is of particular interest, primarily due to the abundance of Lepeophtheirus salmonis (sea lice) (Thorstad et al, 2015) in open net pen farms which negatively affect wild salmonids in the area (Eldøy et al, 2020). These infestations can cause increased mortality and affect the growth of surviving individuals by reducing feeding activity and causing negative stress responses (Eldøy et al, 2020).…”

“…The negative impact of Atlantic salmon aquaculture is of particular interest, primarily due to the abundance of Lepeophtheirus salmonis (sea lice) (Thorstad et al, 2015) in open net pen farms which negatively affect wild salmonids in the area (Eldøy et al, 2020). These infestations can cause increased mortality and affect the growth of surviving individuals by reducing feeding activity and causing negative stress responses (Eldøy et al, 2020). This has been seen in the Burrishoole catchment with high marine mortality of smolts paralleled by poor growth of surviving fish (de Eyto et al, 2022).…”

Factors influencing return rate and marine residence duration in sea trout populations in Central Norway

“…For instance, the relatively large, shallow, highly productive lakes of the Fremstad catchment may provide profitable foraging resources enabling trout to grow larger and/or delay out‐migration. Alternatively, the presence of larger out‐migrating sea trout from this catchment may reflect a prolonged size‐related selection pressure driven by salmon lice infestations (Eldøy et al ., 2020) and/or predatory fish and mammals associated with the relatively open coastal waters of the outer Trondheimsfjord. In contrast, the presence of two size cohorts of out‐migrating Vatne sea trout demonstrates that a range of out‐migration sizes remains a viable life history strategy for sea trout from this catchment.…”

“…In contrast, the presence of two size cohorts of out‐migrating Vatne sea trout demonstrates that a range of out‐migration sizes remains a viable life history strategy for sea trout from this catchment. Whether selection pressures from increased salmon lice infestations will eventually cause a shift in out‐migration size to a single large size class (Eldøy et al ., 2020) or shorter marine migrations to avoid being unable to escape rising salmon lice infestations which coincide with summertime low river level periods is yet to be seen.…”

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

“…We expected lower lice infestation at lower salinity, but the data showed a weak opposite result, although the parameter estimate was not significantly different from zero. That many sea trout with high salmon lice infestations were found in areas where the salinity was low (<20) suggests that the fish had been infested elsewhere and were seeking brackish or fresh water, where they could reestablish their salt balance and reduce or get rid of their lice burdens (Bjørn et al, 2001;Eldøy et al, 2020;Gjelland et al, 2014). Such behavioural response to lice has been shown to significantly reduce the time sea trout spend for feeding and growing in the marine environment.…”