Working Group on the Ecosystem Effects of Fishing Activities (WGECO)
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 inverte-brates 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 (<5%) of total abundance for most taxa but as high as 25% for some prey. Benthic food resources do not appear to be limiting the feeding of benthivorous fishes. The second study examined the temporal trends in diet composition and consumption rates of haddock, which has experienced a northerly shift in abundance in Icelandic waters. Some prey types showed clear trends from 2006 to 2019, while others had more variable patterns. Consumption rates of benthic inverte-brates, fish and zooplankton/natantia prey types were calculated. The much greater consump-tion of fish prey relative to the other two prey groups was of interest.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 re-gions: 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 investi-gated 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 sen-sitivity analyses and applied to additional survey areas. WGECO recommends that these indi-cators be adopted by ICES for evaluating acceptable status for communities and supplemented where possible with risk-based targets from multispecies modelling approaches.
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