Connectivity of local sub-stocks of Crangon crangon in the North Sea and the risk of local recruitment overfishing

Monthly VMS and logbook data of the North Sea Brown shrimp (Crangon crangon) fleets of Denmark, Netherlands and Germany from 2009 to 2018 were analysed for trends and regional patterns in five subareas. Effort increased by 12% while landings decreased by 9% from the first five to the second five years of the time series. All areas showed a significant decreasing trend of LPUE in the fishery of the first quarter of the year from 2009 to 2018. Fishing effort in Dutch and East Frisian waters during winter was negatively correlated with LPUE in the same and adjacent areas in March – April. Furthermore, highly significant negative correlations were found between fishing effort in January and February in Dutch waters and LPUE in July and August in areas further north, explaining up to 86% of the variance. Together our results support the hypothesis that early recruitment in the Northern areas partially depends on a new cohort coming from the south and that reduced recruitment in Northern areas may be a consequence of previous local depletion of egg-bearing females further south. Egg bearing shrimp appear to concentrate in southern areas in January and February and migrate to adjacent northern areas for egg release in March and April. To prevent economic and ecological consequences for the shrimp stock and the fishery, transboundary management measures need to be considered and implemented. Further investigations of migration and drift patterns of brown shrimp are recommended.