Modelling the variability of the drift of blue whiting larvae and its possible importance for recruitment

by Einar Svendsen, Morten Skogen, Terje Monstad Institute if Marine Research, Bergen, Norway Stephen Coombs Plymouth Marine Laboratory, U.K. The first 8 months of the drift of blue whiting larvae has been simulated and studied for the 20 year period 1976-1994, using the 3-dimensional numerical model system NORWECOM (the NORWegian ECOlogical Model system). The model particles assumed to represent larvae are released identically in space and time each year according to best "average" knowledge. Large variations in the drift pattems are seen from year to year, but generally much larger amounts of larvae than expected seems to drift southward from the Porcupine Bank area west of Ireland to the Bay of Biscay. Many of these larvae drift into deep water where chances of survival are assumed to be small. Typica.J.ly larvae hatched on the northem side of the Porcupine Bank drift northwards through the Faeroe-Shetland Channel, and most of these larvae are located on the shelf. Several distribution parameters are selected and compared with ICES time-series of recruitment. Introduction Quantitative knowledge of the actual processes leading to the observed large year to year variability of recruitment are for most fish stocks quite unknown. Much work has been focused on studying empirical relations with temperature and size of spawning stocks, but few clear conclusions has been drawn. W e know that temperature has an effect on the whole ecosystem and that the size of the spawning stocks are related to recruitment when the spawning stock is small, being directly related to the amount of eggs and larvae being produced. However no clear relations are found to variations in large spawning stocks, and the correlations with temperature are quite weak (Svendsen et al., 1996). This indicates that other environmental factors are important, and it is generally accepted that it is the processes acting during spawning and earl y life stages of the fish which are most important for the recruitment variability. Rothschild and Osborn (1988) showed theoretically the importance ofturbulence for plankton contact rates, and Sundby and Fossum (1990) has further confrrmed this theory to measured feeding conditions for cod larvae. The turbulence is in most areas directly related to wind conditions. Svendsen et al. (1995) used a multivariat statistical approach to study the recruitment variability of several North Sea stocks, and he indeed found that the wind generated turbulence could explain much of the variability, together with indirect measures of varying inflow of Atlantic Water and heat con tent in the North Sea. During the 1990's 3-dimensional numerical circulation models have improved significantly, and access to supercomputers have made it possible to make simulations over many years. Berntsen et al. (1994) used such a model for studying North Sea sandeellarvae, and they concluded that unfortunate drift of the larvae in the northem North Sea may lead to bad recruitment. However the know ledge of the behavior of the larvae ( which only drift a few months in the water be fore settling on the bottom) was too weak to come to frrm conclusions on the importance of varying drift on recruitment. Several modelled larval drift studies of other species, particularly of herring in the North Sea has been published (Moksness et al., 1996, Bartsch et al., 1989, Bartsch et al. 1993), and similar work is carried out in Alaska (Stabeno et al., 1995) and on Georges Bank (Werner et al., 1995). During the EU-project SEFOS (ShelfEdge Fisheries and Oceanography Studies), 19 years of modelled larval drift simulations are performed to study the year to year variability of: retention in • spawning areas, drift onto the shelf versus into deep water, drift northward versus southward, and drift into the North Sea. Finally these time-series are related to the 0-group recruitment to see if any of the variability are linked to the larval drift. The Model The NORWegian ECOlogical Model system (NORWECOM) is a coupled physical, chemical, biological model system applied to study ocean circulation, primary production, transport of nutrients and dispersion of particles (fish larvae, pollution). The model system is full y described in Skogen (1993), Aksnes et al. (1995) and Skogen et al. (1995). In this study a coupled system with a circulation module and a transport module is used. A fine-scale version of the circulation module is evaluated towards data in the eastern North Sea and Skagerrak by Skogen et al. (1996) and Svendsen et al., (1996)