Using simulated escape events to assess the annual numbers and destinies of escaped farmed Atlantic salmon of different life stages from farm sites in Norway

To improve assessments of the environmental risks of aquaculture, a series of simulated escapes of farmed Atlantic salmon (Salmo salar L.) from seawater netpens were performed. Individually tagged post-smolts and adult Atlantic salmon were released from various locations at different times of the year. Post-smolts that escaped during their first summer were capable of rapid migration towards the open sea. A small fraction returned to spawn and were recaptured after 1.3 years at sea (0.4%, range 0.0.1.1%). A total of 13% of the post-smolts that escaped during autumn were reported in nearby fisheries during subsequent months, partly because they had grown large enough to be caught in the gillnets used, but more importantly because migratory behaviour diminished towards the end of the year. The mean recapture rate of adult salmon was high after releases in fjords (7.33%), lower after coastal releases (4.7%), and zero on the outer coast. Most of these recaptures were immature fish recaptured in sea relatively close to the release site during their first months post-release. Recaptures of adult escapees after 1.2 years in the wild were very rare (0.09%), probably because of their low survival. A Monte-Carlo method was developed to estimate the annual numbers of escapees from Norwegian fish farms based on reported catches of escaped farmed salmon in te sea and in rivers and the recapture probabilities reported here. The model provides a tool to estimate numbers of escapees independently from the reported numbers. Importantly, our analysis suggests that the total numbers of post-smolt and adult escapees have been two- to fourfold as high as the numbers reported to the authorities by fish farmers, depending on whether the incomplete sea fishery statistics are compensated for

Reduced daylength stimulates size-dependent precocious maturity in 0-plus male Atlantic salmon parr

To study relationships between fish length, photoperiod and the onset of precocious maturity in 0+ parr during intensive rearing of Atlantic salmon (Salmo salar L.), 0+ juveniles of 55-89 mm length were individually tagged with Passive Induced Transponders (PITs) in May and exposed to reduced daylength (LD12:12) in May, June or July, or kept as control fish under continuous light (LD24:0). Relationships between the length of the fish and maturity were studied by the use of probabilistic maturation reaction norms. The incidence of mature males and the proportion of fish of lower modal group size in autumn were highest in the groups exposed to short days in May, and lowest under continuous light. In contrast with the expectation that high growth rates promote maturation, the future mature male parr were smaller than the immature males at the start of the experiment, and they also grew more slowly during the subsequent maturation process. Variability in condition factor was low until autumn, when the condition factor of maturing males rose well above that of immature males during autumn as the fish grew from ~80 to ~95 mm length. Reduced day-length increased the probability of the onset of precocious maturity in the fish smaller than, but not above a threshold length of ~ 90-100 mm (9-11 g). Intensively reared parr of 60-90 mm, and possibly also smaller fish, may be particularly sensitive to photoperiod manipulations that may influence the probabilities of fish adopting one of the three life-history alternatives, to enter lower or upper mode or to mature precociously

Domesticated escapees on the run: the second-generation monitoring programme reports the numbers and proportions of farmed Atlantic salmon in >200 Norwegian rivers annually

Norway is the world’s largest producer of farmed Atlantic salmon and is home to ∼400 rivers containing wild salmon populations. Farmed escapees, a reoccurring challenge of all cage-based marine aquaculture, pose a threat to the genetic integrity, productivity, and evolutionary trajectories of wild populations. Escapees have been monitored in Norwegian rivers since 1989, and, a second-generation programme was established in 2014. The new programme includes data from summer angling, autumn angling, broodstock sampling, and snorkelling surveys in >200 rivers, and >25 000 scale samples are analysed annually. In 2014–2017, escapees were observed in two-thirds of rivers surveyed each year, and between 15 and 30 of the rivers had >10% recorded escapees annually. In the period 1989–2017, a reduction in the proportion of escapees in rivers was observed, despite a >6-fold increase in aquaculture production. This reflected improved escape prevention, and possibly changes in production methods that influence post-escape behaviour. On average, populations estimated to experience the greatest genetic introgression from farmed salmon up to 2014 also had the largest proportions of escapees in 2014–2017. Thus, populations already most affected are those at greatest risk of further impacts. These data feed into the annual risk-assessment of Norwegian aquaculture and form the basis for directing mitigation efforts

Growth and osmoregulation in Salmo salar L. juveniles 1+, 1½ + and 2+ reared under restrained salinity

The freshwater phase of Atlantic salmon Salmo salar L vary between one and eight years. The reduction of the freshwater phase is desirable to reduce freshwater usage, human resources and to increase year round availability of pan-sized salmon. Three trials were conducted to investigate the possibility of supply the market in a year-round basis, with pan-sized Atlantic salmon Salmo salar L. (250-300 g) in Portugal (southern limit of the natural distribution of this species). This study primarily aimed to compare the osmoregulatory ability and growth of different fish sizes, smolts 1+ (trial 1), 1.5+ (trial 2) and 2+ (trial 3), in freshwater and seawater conditions. Additionally, effects of photoperiod were determined in smolts 1.5+ (trial 2) for both freshwater and seawater groups. The increments in the plasma osmolality and chlorine concentrations after seawater transfer suggest an identical development in the hypo-osmoregulation capacity among the different age classes. In all trials, weight gain was smaller after 30 d of saltwater transfer when compared to fish reared in freshwater. However, the growth depression was temporary. Seawater group showed a compensatory growth in the immediate months, which permitted an improvement in growth rates. At the end of trials there were minor differences on growth performance between freshwater and seawater groups. Specific growth rates varied between 0.7 and 1.0 % day-1, according to the age and /or size and transfer season