Oceanographic flow regime and fish recruitment: reversed circulation in the North Sea coincides with unusual strong sandeel recruitment

The chase for environmental descriptors of fish stock production is ongoing. Although, numerous correlations between environmental variables such as food abundance or sea surface temperature have been proposed in the past, few are operational in a fisheries management context today. Reasons for this may be many, but spurious correlations, the use of higher level climate-change indicators, and wrong perception of causal relationships has been pointed out.

In the present study, we demonstrate how modelled oceanographic data, describing local conditions, combined with a simple probabilistic risk assessment can be used to forecast fish recruitment. We used the lesser sandeel (Ammodytes marinus) in the North Sea as an example and focused on the circulation patterns experienced by the first feeding larvae on the Dogger Bank.

A strong link between the net-direction of the water transport in the surface and unusual strong year-classes of sandeel were found. For example, the most extreme recruitments only took place in years with a particular type of flow-regime in February, which may be associated with the occasional reversals of the North Sea circulation.

Using risk-ratios, we put forward the potential for using flow-regime in probabilistic short-term forecasts of unusual strong year-classes. Lastly, we propose a hypothesis for recruitment in sandeel, which could be extended to other species, and thereby contribute in future pursues for predictors in recruitment forecasting.