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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 δ13C and δ15N 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. δ13C values were related statistically to observed salmon body condition (Fulton’s K), and fish with higher δ13C values tended to show significantly better condition. In contrast, δ15N 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 δ13C values partly reflected ambient ocean temperature and recent marine feeding environment before return migration, such that the observed relationship between higher δ13C 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.

The 2019 meeting of WGECO was held at the International Council for the Exploration of the Sea, Copenhagen, Denmark from 8–16 April 2019. The meeting was attended by 16 delegates from eight countries and was chaired by Stefán Ragnarsson (Iceland) and Jeremy Collie (USA). The work conducted centred on three Terms of Reference that had been made by WGECO. In addition, a list of sensitive species prepared by WGBYC was reviewed. WGECO continued the work initiated in 2018 to examine the ecological consequences of stock rebuilding, with emphasis on benthivorous fish (ToRa). Two case studies were carried out to estimate the predation pressure of fish on benthos. In one, the consumption of benthic invertebrates by demersal fish on the US continental shelf regions was compared with their biomass in sediments. The estimated annual consumption of benthic organisms was a small proportion (WGECO scrutinized methods to estimate density-dependent effects on fish growth and made suggestions on its estimation from stock assessment or survey data. A simulation study showed that the age-specific growth increment may not be a useful variable to test for density-dependent growth, because it depends on body size. In the WGECO 2018 report, the case had been made that North Sea plaice show density-dependent growth reduction. In 2019, we conducted a more thorough analysis of the spatial and temporal trends in distribution, growth and status of plaice in the North Sea. As the plaice population increased in abundance, the spatial distribution did not expand, apart from an increased occupancy of marginal areas. The observed reduction in plaice weight-at-length may thus be explained by the metabolic stress associated with the effects of rising sea temperature on plaice growth rates. However, the two mechanisms, temperature and density, that can explain reduced weight-at-age are not mutually exclusive. As part of ToR b, WGECO compiled fisheries-dependent size composition data from four regions: West of Scotland, Irish Sea, West of Ireland, and the Northern Celtic Sea. This novel dataset was used to examine changes in the length composition of the catch in relation to changes in total catch (used as a proxy for fishing pressure). In the Irish Sea, for example, the commercial fishery now catches larger individuals of larger demersal species as the total catch has declined. Size composition of the landings was compared with size composition in the surveys. In the northern Celtic Sea and West of Scotland, the commercial fleets generally take fish of a much larger size than those caught by the survey. By contrast, in the Irish Sea the commercial fleets tend to capture fish across the whole size range available in the environment. Demersal fish community indicators of species composition and size–structure were investigated for several sea areas, using survey data for state, and fisheries-dependent data for pressure. Together, these indicators can be used to track the wider impacts of fisheries on the ecosystem and monitor the evolving nature of the relationship between pressure and state. Simple empirical approaches for setting targets and baselines for demersal fish communities were tested with sensitivity analyses and applied to additional survey areas. WGECO recommends that these indicators be adopted by ICES for evaluating acceptable status for communities and supplemented where possible with risk-based targets from multispecies modelling approaches. A multispecies model framework was advanced to determine more ambitious risk-based targets for demersal fish community indicators that can be used to minimise the risk that stocks suffer from impaired reproductive capacity consistent with commercial fish stock indicators. Ensemble modelling demonstrated that the fish community in the North Sea is recovering in response to fisheries management. WGECO reviewed potential indicators of spatial distribution with special attention to the simplicity of use and ease of communicating the results to non-experts (ToRc). Metrics including aspects of latitude, longitude or depth (e.g. range and centre of gravity) and the area containing a fixed percentage of the population (termed core area) were considered simple to use and easy to communicate. Other measures were considered useful for scientific purposes but less easily applied and communicated to managers. The accuracy of selected indicators for spatial distribution were explored in a simulation study. The metrics produced from the simulated raw survey data and modelled data accurately reflected the underlying mean abundance of the simulated data, but more accurately for the more-common species. Changes in the centre of gravity were detected with high accuracy. The percent of occupied area is generally accurate in its trend when there is a strongly aggregated population, whereas classifying occupied area is more complicated when the population is more dispersed. This preliminary analysis provides the context for analysing distributional metrics at the scale of large marine ecosystems. The spatial distributions of three age groups of plaice were analysed as a case study. The observed northerly and easterly movements of plaice distribution and into deeper waters are interpreted as a response to warming sea temperatures. Consistency in area usage over time was examined to identify those areas critical to plaice. This analysis revealed that the proportion of the population using the ‘critical areas’ (defined as the area holding 75% of the population in 60% of years) was stable over the study period, though some decreases have been noted in recent years. WGECO compared the list of sensitive species identified by WGBYC with the underlying list of species to be monitored under the protection programmes in the EU, using methods reviewed previously in WGECO as well as the sensitivity ratings carried out by IUCN. A resulting list of sensitive species and vulnerable/endangered/critically endangered species is provided. Halibut (Hippoglossus hippoglossus) was the only species suggested by WGBYC which is considered sensitive to fishing. The remaining species, with the exception of the two Hippocampus species, are diadromous species likely to be most sensitive to deterioration of their freshwater habitat. Eleven sensitive species are absent from the list of species to be monitored under protection programmes; WGECO recommends that these 11 should be added to the Commission list to provide information on sensitive and vulnerable species. Further, WGECO recommends that the requirement to return specimens of endangered and critically endangered species to the sea should not be restricted to parts of EU waters and that the status of sensitive species should be monitored on a regular basis, with the aim of providing advice on the adequacy of existing landing restrictions.