SEAwise report on improved predictive models of recruitment under different environmental scenarios
The SEAwise project works to deliver a fully operational tool that will allow fishers, managers, and policy makers to easily apply Ecosystem Based Fisheries Management (EBFM) in their fisheries. This SEAwise investigates how key environmental variables influence the recruitment process of target fish stocks. Understanding how the environment affects recruitment may allow more accurate predictions of fish stock dynamics under scenarios of environmental change and in particular their response to global warming, supporting the development and implementation of robust management policies. Case studies from the four Seawise case study regions have been analysed, and the main results obtained so far are summarized below.
In the Baltic Sea, the Gulf of Riga spring spawning herring showed effects of spawning stock biomass on individual weight of age-1 fish, with prey abundance in May and previous year feeding period temperature also playing significant roles. The explanatory power of the final model was moderate. Higher weight of herring is achieved at higher prey densities, lower SSB levels and lower temperatures during the main feeding season of age-0 fish. Recruitment of Western Baltic cod and herring showed decreasing reproductive potential at increasing temperature.
In the North Sea, the effects of temperature, salinity, currents, chlorophyll and zooplankton on recruitment of cod, haddock, saithe, whiting, plaice, sole, sprat and herring were investigated using a semi-automated, machine learning framework. The incorporation of environmental signals in recruitment predictions showed improved predictions over a stock-recruitment model without environmental effects in six out of the eight stocks. For small pelagic foirage fish, four stock-recruitment models were fitted for three sandeel stocks and the North Sea sprat stock Linear regressions revealed various relationships between recruitment and environmental variables. Short-term time scales based on monthly averages produced a noisier and less consistent pattern for most stock.
In the Western Waters, Bayesian online change point detection models were applied to three Irish Sea stocks: Atlantic cod, whiting and common sole. Irish Sea cod recruitment was reduced at high sea surface temperatures and low copepod prey. Whiting and sole recruitment was less affected by environment but impacts for sole my occur in sole nursery areas. The Bay of Biscay anchovy recruitment was impacted by upwelling and turbulence whereas the Iberian sardine, productivity was low in the regime from 2006 onwards. Additional regime shifts for sardine were identified around 1993 and 2015. A numerical model was developed for European seabass simulating the impact of environmental factors on reproduction. The western channel sole recruitment is affected by spawning stock biomass and mean winter temperatures. The slightly closer link to Channel average temperatures rather than inshore site temperatures is consistent with temperature impacts directly on the survival rates of fish that were about to recruit.
In the Mediterranean Sea, the influence of sea surface temperature, bottom temperature, salinity, bottom salinity and net primary production on recrutiment were investigated in the Adriatic and western Ionian Seas (GSAs17-18-19), and in most cases, these variables improved the predictive power. A study of the exchange of hake larvae and early juveniles between spawning grounds (the areas where adult hakes lay their eggs) and nurseries (where juveniles settle at the end of the larval phase) is still ongoing. In the Eastern Ionian Sea (GSA 20) improved predictive models of recruitment for European hake and red mullet incorporated environmental factors. Hake spawning stock biomass has a positive effect on recruitment up to a certain stock size but recruitment decreases at larger stock sizes. Recruitment is also influenced by microphytoplankton-related chlorophyll (chl4). Red mullet recruitment was also density dependent and influenced by chlorophyll-a and salinity.
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