Influence of body condition on the population dynamics of Atlantic salmon with consideration of the potential impact of sea lice

Breeding stage impacts on chronic stress and physiological condition in northern gannets (Morus bassanus)

Physiological condition can affect survival and reproductive success in seabirds. However, seabirds rarely show outward signs of poor physiological condition, making it difficult to identify and address issues before they result in population level impacts. We investigate physiological condition of breeding northern gannets (Morus bassanus) between years and nesting stages. Blood smears were used to quantify blood cell profiles indicative of chronic stress, infection, disease, and immunocompetence. No blood parasites were observed, but elevated Heterophil to Lymphocyte (H:L) ratios, eosinophils, and monocytes suggest higher prevalence of infection in some years. Chronic stress levels, indicated by high H:L ratio, were elevated in incubation and early chick-rearing compared to late chick-rearing, which coincided with poorer body condition in breeding birds. This study highlights the value of haematology as a tool for identifying changing patterns of health that may serve as an early indicator of breeding failure, overwintering mortality, and population declines.

Body condition of returning Atlantic salmon Salmo salar L. correlates with scale δ13 C and δ15 N content deposited at the last marine foraging location

Patterns of feeding and growth of Atlantic salmon Salmo salar L. in the marine environment are critical to understanding how observed declines in recruitment may reflect warming or other oceanic drivers. The isotopic composition of scales can provide insight into differences in marine feeding location and possibly temperature regime. The authors used archived scale samples to measure δ13 C and δ15 N deposited in the scales of one sea-winter (1SW) salmon during their last season of growth at sea before they returned to five Irish rivers. δ13 C values were related statistically to observed salmon body condition (Fulton's K), and fish with higher δ13 C values tended to show significantly better condition. In contrast, δ15 N values were negatively related to body condition. There was no important effect on condition of length at smolt migration, and the effect of duration of marine residence varied among rivers. It is likely that δ13 C values partly reflected ambient ocean temperature and recent marine feeding environment before return migration, such that the observed relationship between higher δ13 C values and increased body condition may express an advantage for adult fish feeding in warmer, potentially closer, waters. If greater body condition influences fitness, then a changing temperature regime in the Northeast Atlantic may drive shifts in salmon survival and reproduction. This study provides evidence that there is spatial and trophic variation at sea between salmon from rivers of origin that are located relatively close to each other, with potential consequences for body condition and, consequently, fitness and life history; this suggests that salmon populations from geographically proximate rivers within regions may exhibit differential responses to ocean-scale climatic changes across the Northeast Atlantic.

Delayed maturity does not offset negative impact afflicted by ectoparasitism in salmon

The negative effects of parasitism on host population dynamics may be mediated by plastic compensatory life-history changes in hosts. Theory predicts that hosts should shift their life-history towards early reproduction in response to virulent pathogens to maximize reproduction before death. However, for sublethal infections that affect growth, hosts whose fecundity is correlated with body size are predicted to shift towards delayed reproduction associated with larger body size and higher fecundity. This has been observed in Atlantic salmon and parasitic sea lice, via mark-recapture studies that recover mature fish from paired groups of control and parasiticide-treated smolts. We investigated whether such louse-induced changes to age at maturity can offset some of the negative effect of mortality on population growth rate in salmon using a structured population matrix model. Model results show that delayed maturity can partially compensate for reduced survival. However, this only occurs when marine survival is moderate to poor and growth conditions at sea are good. Also, the impact of delayed maturity on population growth when parameterizing the model with empirical data is negligible compared with effects of direct mortality. Our model thus suggests that management that works on minimizing the effect of sea lice from fish farms on wild salmon should focus mainly on correctly quantifying the effect of parasite-induced mortality during the smolt stage if the goal is to maximize population growth rate.

Effects of laboratory salmon louse infection on osmoregulation, growth and survival in Atlantic salmon

Anadromous Atlantic salmon (Salmo salar) rely on long ocean migrations to build energy stores for maturation and spawning. In seawater, wild Atlantic salmon are threatened by high salmon lice (Lepeophtheirus salmonis) infestation levels resulting from intensive salmonid sea-cage aquaculture. Salmon lice infection can cause a stress response and an osmotic imbalance in the host. The lice infection intensity threshold values for these responses, however, remain to be identified in Atlantic salmon. In order to define this under laboratory conditions, individually tagged F1 wild origin Atlantic post-smolts (40 g) were infected with salmon lice copepodids or left as uninfected controls. Twenty-eight days post infection, infected post-smolts had a mean of 0.38 (range of 0.07-0.9) mobile lice g^-1 fish weight. During this period, specific growth rates (SGRs) were lower in infected than control fish (0.4 vs 1.0% day^-1). Higher plasma Na⁺, Cl^- and osmolality in infected fish also indicate osmoregulatory impairment. SGR correlated negatively with plasma Na⁺, Cl^-, osmolality and cortisol in the infected, but not in the control group. Infection intensity (lice g^-1 fish) correlated positively with mortality rate and plasma Na⁺, Cl^-, osmolality and cortisol and correlated negatively with SGR and condition factor. Calculated lice intensity threshold values for changes in plasma ions were 0.18 lice g^-1 for plasma Cl^-, and 0.22 lice g^-1 for plasma Na⁺. Moribund infected fish occurred at infection intensities above 0.2 lice g^-1, and these fish had extreme plasma Cl^-, Na⁺, osmolality and cortisol levels. There was a positive correlation between plasma cortisol and plasma Na⁺, Cl^- and osmolality in infected fish. This study provides vital information that can be used to define thresholds in the monitoring and conservation of wild Atlantic salmon populations affected by aquaculture-driven salmon lice infestations.

Parasite induced mortality is context dependent in Atlantic salmon: insights from an individual-based model

An individual-based model was parameterized to explore the impact of a crustacean ectoparasite (sea louse, Lepeophtheirus salmonis & Caligus spp.) on migrating Atlantic salmon smolt. The model explores how environmental and intrinsic factors can modulate the effect of sea lice on survival, growth and maturation of Atlantic salmon at sea. Relative to other effects, the parasite infestation pressure from fish farms and the encounter process emerge as the most important parameters. Although small variations in parasite-induced mortality may be masked by variable environmental effects, episodes of high infestation pressure from fish farms should be observable in wild populations of Atlantic salmon if laboratory studies accurately reflect the physiological effects of sea lice. Increases in temperature in the model negatively influenced fish survival by affecting the development time of the parasite at a rate that was not compensated for by the growth of the host. Discharge from rivers was parameterized to increase migration speed and influenced parasite induced mortality by decreasing time spent in areas with increased infestation pressure. Initial size and growth of the host was inversely related to the impact of the parasite because of size-dependent parasite-induced mortality in the early phase of migration. Overall, the model illustrates how environmental factors modulate effects on the host population by impacting either the parasite load or the relative effect of the parasite. The results suggest that linking population-level effects to parasite infestation pressure across climatic and environmental gradients may be challenging without correctly accounting for these effects.