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Marine aquaculture of Atlantic salmon (Salmo salar) is a relatively new industry where breeding programs have led to rapid genetic change in the captive populations that were built up alongside conspecific wild individuals. Throughout its 50-years history, marine aquaculture of Atlantic salmon has been associated with escapes, and studies have shown that escapees may enter rivers, spawn successfully, and this may lead to farmed-to-wild genetic introgression and maladaptation in wild populations. Yet, an open question is what factors can best explain the variability in the proportion of farmed escapees in wild populations, and when present, which additional factors lead to introgression. Here, we combine two large-scale data sets from monitoring escaped farmed salmon and introgression in Norwegian rivers between 2006 and 2018 to model how anthropogenic, environmental, and population factors influence proportion of escapees and level of introgression. We found that increasing farming intensity and river discharge increase the expected proportions of escaped farmed salmon in rivers, whereas a larger wild salmon population size reduces the expected proportion of escapees despite increasing the expected absolute numbers of escaped farmed salmon. On a large scale, introgression is primarily a function of proportions of escaped farmed salmon, and only to a minor extent a function of local environmental factors or salmon population characteristics. This suggests that as long as salmon aquaculture is based on technologies where non-sterile fish can escape, all anadromous wild Atlantic salmon populations are at risk. Large marine protected areas without salmon aquaculture may slow down the rate of intrusion and introgression by increasing the distance between intensive aquaculture and wild populations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Heino, M., Porteiro, F. M., Sutton, T. T., Falkenhaug, T., Godø, O. R., and Piatkowski, U. 2011. Catchability of pelagic trawls for sampling deep-living nekton in the mid-North Atlantic. – ICES Journal of Marine Science, 68: 377–389. Material collected in summer 2004 from the Mid-Atlantic Ridge between Iceland and the Azores with three pelagic trawls was used to estimate relative catchabilities of common fish, cephalopod, decapod, and jellyfish species. Catchability is defined as the ratio of numbers caught between two trawls, standardized for towed distance. Taxon-specific catchability coefficients were estimated for two large pelagic trawls with graded meshes, using a smaller pelagic trawl with a uniform mesh size as the reference trawl. Two of the trawls were equipped with multiple opening–closing codends that allowed sampling of different depth layers. Generalized linear and mixed models suggest that most of the taxa have catchabilities much lower than expected from the area of opening alone, indicating that only a few species are herded by the large mesh at the mouth of larger trawls. Catchability coefficients across taxa show a very large spread, indicating that the sampled volume for the larger trawls with graded meshes was highly taxon-specific. Part of this variability can be explained by body size and taxonomic group, the latter probably reflecting differences in body form and behaviour. The catchability estimates presented here form the basis for combining data for quantitative analyses of community structure. [ABSTRACT FROM PUBLISHER]

To interpret long-term trends in age and size at maturation, new statistical methods have recently been devised for estimating probabilistic maturation reaction norms based on data collected for the management of fisheries. Here we apply these methods to three Newfoundland stocks of American plaice (NAFO Divisions 2J3K, 3LNO, 3Ps) and report a clear long-term shift in the maturation reaction norms of these stocks towards maturation at younger age and smaller size. Theory predicts that such trends could result from fishing acting as a selective force, inducing evolutionary changes in the life histories of exploited populations. Matching long-term trends in maturation reactions norms have already been documented for several stocks that have experienced high fishing pressures (Gulf of Maine and Georges Bank cod, Northeast Arctic cod, North Sea plaice). Our results add a new dimension to these earlier findings: since fishing pressures for two stocks of Newfoundland plaice (2J3K and 3Ps) have been relatively low, our results imply that fishing is likely to result in the evolution of life histories even when fishing mortality is low, or that natural mortality has played an important role in determining selective pressures in these populations. Both options suggest that conditions for rapid life-history evolution in exploited stocks are less restrictive than previously appreciated. [ABSTRACT FROM PUBLISHER]