SEAwise Report on improved predictive models of recruitment under different habitat scenarios and incorporating experimental results

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. This SEAwise report 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 higher mean individual weight of age-1 fish was achieved at high food densities accompanied with low-moderate SSB levels resulting in high explanatory power of the models. Climate conditions were less important than trophic linkages with processes during both the hatching year and the second year of life being significantly important. Notably, decoupling of individual weight of the fish from prey resources was evident since the early 2000s. Recruitment of Western Baltic cod and herring showed decreasing reproductive potential as temperature increased.

In the North Sea, environmentally-mediated stock-recruitment relationships have been developed and used to project the dynamics of commercially important fish stocks under climate change. For cod and saithe, temperature is a major driver of recruitment and water warming under projected climate change is expected to adversely affect recruitment. Whiting, in contrast, may be exposed to more favourable conditions. Currents and salinity at the beginning of the spawning season are important drivers of autumn spawning herring recruitment. Projections show no relevant changes in herring recruitment under RCP4.5 and a decreasing trend under RCP8.5. Plaice recruitment would decline slightly under RCP8.5 and increase under RCP4.5, but from a lower level than RCP8.5. Results are, however, critically affected by the uncertainties associated with the different climate models. For small pelagic forage fish, stock-recruitment models were fitted for three sandeel stocks and the North Sea sprat stock, revealing various relationships between recruitment and environmental variables.

In 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 the environment, but impacts for sole may occur in sole nursery areas. Preliminary models were developed to assess predictors of sole recruitment in the northeast Atlantic Ocean and will be further refined. A dynamic factor analysis was applied to cod, haddock, sole and megrim stocks in the Celtic Seas and identified zoo-mesoplankton and temperature as the most likely drivers of stock dynamics. The inclusion of environmental variables or ocean circulation indices in stock-recruitment models for Bay of Biscay anchovy and sardine, black-bellied anglerfish, northern hake, sole, mackerel and blue whiting improved predictive ability compared to naïve models, while no significant improvement was obtained for seabass, southern hake, white anglerfish, horse mackerel. The Iberian sardine stock has undergone changes in productivity over time, with a major regime shift around 2005 leading to lower productivity. An individual-based dynamic energy budget model was developed to reconstruct past population dynamics of European seabass. Results of lab experiments on the same species show that parental acclimation does not fully compensate for the negative impact on gamete quality induced by water acidification expected under climate change.

In the Mediterranean Sea, environmentally-mediated stock-recruitment relationships were developed for European hake, red mullet, deep-water rose shrimp, giant red shrimp and blue and red shrimp in GSAs17-18-19 and showed better predictive performances than classical models. Recruitment projections under climate change scenarios indicate a possible negative effect for hake and red mullet, while deep-water rose shrimp, giant red shrimp and blue and red shrimp could benefit from warming. In the Adriatic Sea, connectivity for hake larvae could increase under climate change scenarios, possibly due to improved ocean circulation that would compensate for the reduced duration of the pelagic larval phase and the earlier reproductive period. However, expected changes in the structure of ocean circulation under more extreme climate change scenarios may lead to greater separation of the northern from the southern basin, possibly affecting the metapopulation dynamics of the stock. In the Eastern Ionian Sea (GSA 20) improved predictive models of recruitment for European hake and red mullet incorporated environmental factors. Hake recruitment showed overcompensating density dependence and was affected by microphytoplankton-related chlorophyll. Red mullet recruitment was also density dependent and influenced by chlorophyll-a and salinity.

Read more about the SEAwise project at https://seawiseproject.org/