ICES working group uncovers Crangon crangon connections

WGCRAN looks at the connectivity of local brown shrimp sub-stocks in the North Sea and the risk of local recruitment overfishing.
Published: 7 April 2022

​The brown shrimp, Crangon crangon supports the fourth most valuable European fishery in the North Sea (EUR 169 million in 2018). The fishing fleets of Germany, the Netherlands, and Denmark are responsible for over 90% of the yearly landings.

A new publication stemming from a joint data analysis (VMS/logbook) by ICES Working Group on Crangon Fisheries and Life History (WGCRAN) indicates that not only are abundances of brown shrimp significantly decreasing in the most important hatching months in all major fishing areas, but also hints towards a possible impact on the following recruitment by excess fishing effort in winter.

Despite the fishery's high value, neither regular advice, EU-wide management of the target species, nor regular stock assessments are carried out. Although large advances in the understanding of the life cycle have been made, the assessment of the stock status of the brown shrimp remains a challenge and has been for decades. This is largely due to the lack of coherent effort data from the international fishery. In addition, biological traits such as the impossibility of age determination, the short life cycle, and high predation mortality have impaired or complicated analytical assessments.

Brown shrimp females carry their fertilized eggs attached to the body until the larvae hatch. This coupling of the fate of the eggs and the adults presents a risk of recruitment overfishing, especially in winter. The added relevance of the effect of the winter fishery comes from new results on the life cycle, which highlight the importance of the winter egg production for the first peak of adult shrimp in late summer. Since 1990, the winter fishery has intensified due to the shift of large vessels from the Dutch flatfish fishery into the shrimp fishery, provoking discussions about the potential negative effects of the fishery in the Sylt area, at least for the northern regions. However, as overall landings increased from an average of 20,000 t before 1990 to 30,000 t in the subsequent decade, this discussion faded. Recent years, however, were characterized by very large variations in annual landings, especially in the northern regions. Both 2016 and 2017 were very poor years for northern German and Danish areas and gave new relevance to the issue of winter fishing.

A significant decreasing trend of LPUE in the first quarter of the year​

In 2019, WGCRAN had - for the first time - access to a full set of spatially resolved landings and effort data that included the fleets of the main fishing nations Denmark, Germany, and the Netherlands. VMS and logbook data for 2009–2018 were aggregated and further analyzed. 

The results show a significant negative trend in landings per unit effort (LPUE) in the first quarter of the year for all regions, with the northern regions showing the steepest decrease. In two southern areas, up to 86% of the variability of the following summer landings in various regions could be explained by the effort in the preceding winter months. This is clearly more than previous attempts that explained the variability in survey abundance of shrimp in German waters with the North Atlantic Oscillation (​NAO) index of the previous year, winter temperature, river run-off and a predator index leading to 57% explained variance. 

The strongest correlations and regressions involve the effort in January and February which are the months before the eggs of the winter period are released. The large female shrimp are concentrated in characteristic areas during those months, showing a certain depth preference of 10–20 m. Along the Dutch and East Frisian coasts, the relevant depth range is compressed into a narrow area, making potential aggregations quite vulnerable. 

The mechanism behind WGCRAN's correlation is most likely a reduced spawning stock impacting negatively the subsequent recruitment, as landings in July–August stem from eggs of the previous winter which were released as larvae in March–April. Most surprisingly, the correlations and linear regression of effort in NL-E in winter and LPUE in the following season in northern regions are highly significant even though three years with extreme LPUE values are included (2011, 2016, and 2018). 

To prevent economic and ecological consequences for the shrimp stock and fishery, transboundary management measures need to be considered and implemented. WGCRAN have added further investigations of migration and drift patterns of brown shrimp to their agenda. The current results highlight the importance of access to spatially resolved, coherent fishing effort and landings data for ICES working groups, a topic which should be put forward in cooperation with ICES Data Centre and the Working Group on Spatial Fisheries Data (WGSFD) for the coming years. ​​​

Read the full study, Connectivity of local sub-stocks of Crangon crangon in the North Sea and the risk of local recruitment overfishing in the Journal of Sea Research.

A priority for ICES Working Group on Crangon Fisheries and Life History​ (WGCRAN) is to understand the interactions between the brown shrimp population and fishing effort as well as between the shrimps and the environment and ecosystem. In the future this work will lead to better knowledge of the stock status, allowing for sound advice for sustainable management of the population.​

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Empty nets. The recent paper by ICES Working Group on Crangon Fisheries and Life History ( WGCRAN) suggests high fishing intensity in winter leads to empty nets in summer. Image: ICES WGCRAN.

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ICES working group uncovers Crangon crangon connections

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