Samantha Bui

Understanding how stocking density influences welfare, growth, and infestation susceptibility can significantly contribute to optimising Atlantic salmon Salmo salar production practices. We investigated the effects of 3 stocking densities—low (6.8-12.9 kg m ^-3 ), medium (12.1-21.9 kg m ^-3 ), and high (22.7-43.6 kg m ^-3 )—on post-smolt size, condition factor, welfare, anti-parasite behaviour, and susceptibility to salmon lice Lepeophtheirus salmonis infestation under controlled flow-through tank conditions. Fish were monitored over 54 d, including a 27 d infestation period. Stocking density had negligible effects on growth, and condition factor ( K ) remained above 1.1, though fish at higher densities displayed lower K by the end of the trial. External welfare indicators showed very low frequency of severe scores across groups, with the exception of poor fin condition early in the experiment. All tanks received equal copepodid numbers (~2300 per tank), and louse abundances differed significantly between density treatments. This difference was greater than could be explained by the resulting differences in per-fish exposure, demonstrating a clear encounter-dilution effect: fish in high-density tanks harboured lower mean louse abundance than those in low or medium densities. Jumping and acceleration behaviours in response to infestation were elevated when lice were introduced to the fish, but were attenuated at higher densities, suggesting space limitation may restrict anti-parasite behaviours. These results demonstrate that moderately elevated stocking densities can reduce individual louse burdens without measurable short-term detriment to growth or welfare under tank conditions. The findings support the encounter-dilution hypothesis and suggest that managing early sea phase stocking densities may provide a viable complementary strategy for reducing lice infestation levels in salmon aquaculture.

The marine ectoparasite Lepeophtheirus salmonis (salmon louse) constitutes a significant threat to wild and farmed Salmo salar (Atlantic salmon) in intensive aquaculture regions of the North Atlantic. The effects of L. salmonis on wild salmon remain poorly understood, as field studies, the predominant research approach for wild salmon, often fail to account for individuals that die from infection. We investigated the combined effects of L. salmonis infection intensity and water temperature (13, 10, 7°C) on 321 post-smolt S. salar (17.3 ± 5.0 g; condition factor: 0.9 ± 0.1) captured in the wild and brought into tank facilities. Infection intensity was negatively associated with survival probability, whereas temperature had a comparatively smaller effect. The bootstrapped 50% mortality risk threshold was estimated at 0.54 lice g-1 (95% CI: 0.44–0.62), based on a generalized linear mixed-effects model (marginal R² = 0.875). Final weight, specific growth rate (SGR), and condition factor varied across temperatures, with fish at lower temperatures showing higher growth and better condition, possibly due to the longer duration of the trial at lower temperatures, which allowed for more feeding opportunities, as all temperature treatments were run until lice reached the adult stage. The onset of mortality aligned with the development of female preadult lice, which was strongly temperature-dependent. SGR was significantly reduced for the Mortalities compared to the Survivors across all temperatures (t-test and Wilcoxon rank-sum; p < 0.01). Condition was most severely affected at 13°C, with Mortalities showing significantly reduced values compared to Survivors (Wilcoxon; p < 0.01). Osmoregulatory impairment in moribund fish was generally high and greater at higher temperatures. Across temperatures, infection intensity had inconsistent effects on CSI, HSI, and %DWL, calling into question their reliability as fitness indicators for fish of the size used in this study. As the salmon used in this study were sourced from a single river population, population-specific immune and microbiome differences may limit the generality of these findings across wild Atlantic salmon populations. Collectively, this study highlights L. salmonis as a major driver of increased mortality, reduced growth potential, and osmoregulatory stress in wild post-smolt salmon across the studied temperature range.

Wild salmonid fish are faced with the challenge of infections from the ectoparasitic salmon louse, which thus dictates the management of salmon aquaculture in areas where farms and wild populations co‐exist. The mortality risk of wild Atlantic salmon is used as an indicator of threat levels when monitoring wild populations, and therefore the aim of this study was to robustly investigate the effect of infection intensity on probability of mortality in salmon of wild origin. To assess critical infection levels, recently‐smoltified Atlantic salmon were infected with intensities ranging from 0 to 2.5 lice g ⁻¹ and exposed to a 48‐h simulated migration. The first mortalities occurred when preadults appeared and ceased as lice became adults. Infection intensity negatively affected mortality probability and survival time, whereas the simulated migration had no influence on the probability of mortality. Individual‐level effects of infection on host mortality is difficult to quantify in the field; however, the correlation between louse load and mortality probability obtained in this study can facilitate indirect estimates of mortality risk of wild salmon populations and contribute to conservation efforts associated with this parasite.

Since salmon lice have developed resistance to pesticides, non-medicinal delousing treatments have become increasingly used to remove the parasite from fish on salmon farms in Norwegian waters. These novel techniques are an important means of controlling epidemics of lice at farms for maintaining the health of farmed fish and for preventing the spread of lice to wild salmonid populations. However, some treatments are associated with higher mortality rates and negative welfare impacts on the host fish. Furthermore, it is unclear how effective each treatment is in removing lice compared to alternatives. Here, in a controlled laboratory environment, we tested the efficacy of mechanical, warm water (28 °C and 34 °C), and freshwater treatments alone and in combination, and examined their welfare impact on host fish. Regardless of treatment group or control, the handling of fish through the experiment led to a reduction in lice load and decline in fish welfare. Among the treatments examined, the freshwater bath alone and in combination with other treatments had the greatest delousing efficacy. The 34 °C warm water baths also significantly reduced lice loads but led to worse welfare outcomes with fish having a higher prevalence of injuries and reduced growth and condition factor. Delousing treatments were however not associated with long-term effects on neuroendocrine parameters or stress coping ability, suggesting that immediate welfare impacts from these procedures are generally reversible. It was also found that sedating fish prior to treatment was shown to mitigate the welfare impact. These findings are useful for the needed optimization of delousing strategies for greater delousing efficacy and reduced welfare impact on fish.

Disease and parasitism cause major welfare, environmental and economic concerns for global aquaculture. In this review, we examine the status and potential of technologies that exploit genetic variation in host resistance to tackle this problem. We argue that there is an urgent need to improve understanding of the genetic mechanisms involved, leading to the development of tools that can be applied to boost host resistance and reduce the disease burden. We draw on two pressing global disease problems as case studies—sea lice infestations in salmonids and white spot syndrome in shrimp. We review how the latest genetic technologies can be capitalised upon to determine the mechanisms underlying inter‐ and intra‐species variation in pathogen/parasite resistance, and how the derived knowledge could be applied to boost disease resistance using selective breeding, gene editing and/or with targeted feed treatments and vaccines. Gene editing brings novel opportunities, but also implementation and dissemination challenges, and necessitates new protocols to integrate the technology into aquaculture breeding programmes. There is also an ongoing need to minimise risks of disease agents evolving to overcome genetic improvements to host resistance, and insights from epidemiological and evolutionary models of pathogen infestation in wild and cultured host populations are explored. Ethical issues around the different approaches for achieving genetic resistance are discussed. Application of genetic technologies and approaches has potential to improve fundamental knowledge of mechanisms affecting genetic resistance and provide effective pathways for implementation that could lead to more resistant aquaculture stocks, transforming global aquaculture.

Elevated salmon louse infection pressure generated by salmon farming represents a major threat for wild Atlantic salmon (Salmo salar). This study explored the effect of salmon lice on mortality, and body and gonad growth in F1 wild Atlantic salmon. Mature males (jacks) and immature fish were either infected with two different louse infection intensities (LIIs, 0.08 or 0.35 lice g⁻¹) or kept as uninfected controls. Fish maturation was thereafter environmentally stimulated in seawater, followed by transfer to freshwater for 38 d to simulate river ascendance. No females matured, while 99% of the initially immature males started puberty. Jacks had high, and immature and maturing fish low, seawater mortality, independent of lice. The parasites had an LII-dependent negative effect on growth in length, weight, and condition factor in seawater. In freshwater, after the lice had detached, fish that were previously infected in seawater had reduced growth in length but not weight when compared to the uninfected control. The parasites did not affect relative gonad size in any fish phenotypes. The present results show that Atlantic salmon has a complex, and unexplored, regulation of growth when recovering from lice infection under laboratory settings. Further, the results suggest that possible negative effects of salmon louse on reproductive success in Atlantic salmon is most likely governed by the reduced body size and condition. However, further work on possible effects of salmon louse on semen quality is encouraged.

Parasites are widespread in nature, where they affect the energy budget of hosts, and depending on the imposed pathogenic severity, this may reduce host fitness. However, the energetic costs of parasite infections are rarely quantified. In this study, we measured metabolic rates in recently seawater adapted Atlantic salmon (Salmo salar) infected with the ectoparasitic copepod Lepeophtheirus salmonis and used an aerobic scope framework to assess the potential ecological impact of this parasite–host interaction. The early chalimus stages of L. salmonis did not affect either standard or maximum metabolic rates. However, the later mobile pre-adult stages caused an increase in both standard and maximum metabolic rate yielding a preserved aerobic scope. Notably, standard metabolic rates were elevated by 26%, presumably caused by increased osmoregulatory burdens and costs of mobilizing immune responses. The positive impact on maximum metabolic rates was unexpected and suggests that fish are able to transiently overcompensate energy production to endure the burden of parasites and thus allow for continuation of normal activities. However, infected fish are known to suffer reduced growth, and this suggests that a trade-off exists in acquisition and assimilation of resources despite an uncompromised aerobic scope. As such, when assessing impacts of environmental or biotic factors, we suggest that elevated routine costs may be a stronger predictor of reduced fitness than the available aerobic scope. Furthermore, studying the effects on parasitized fish in an ecophysiological context deserves more attention, especially considering interacting effects of other stressors in the Anthropocene.

Aquaculture is providing an increasingly larger proportion of the world’s protein for human consumption; however, its environmental impact is a bottleneck for sustainable expansion. In Norway, the government has enacted a framework where salmon lice-induced mortality in wild salmonid populations is used for assessing the environmental sustainability in production zones. Direct measurements of the level of lice-induced mortality on wild salmonids are difficult to acquire, thus comprehensive sustainability assessments are based on a set of evidence-based proxies. One such proxy is the infestation pressure from a bio-hydrodynamic model, from which we develop an index that summarize the sustainability of aquaculture in terms of lice infestation. This index is based on the proportion of areas with elevated lice loads, and is a novel approach used to investigate how sustainability could be achieved through scenario testing of different management strategies. The analyses identified a mismatch between legal and sustainable lice levels, but also a beneficial effect of reducing lice levels on farms. This study’s approach demonstrated how bio-hydrodynamic models might be used to assess sustainability and to predict the necessary reduction of lice larvae from farms to classify the entire Norwegian aquaculture industry as environmentally sustainable.