Reproduction and recruitment of the brown shrimp Crangon crangon in the inner German Bight (North Sea): An interannual study and critical reappraisal

Kim Hünerlage,Reinhard Saborowski,Volker Siegel

doi:10.1111/fog.12453

Abstract

AbstractThe brown shrimp, Crangon crangon, is the most important target of the coastal crustacean fisheries in the German Bight. In order to evaluate the relation between the abundance of ovigerous females and larvae in spring and the recruitment success in autumn, we first analysed the seasonal appearance of ovigerous females and larvae from weekly samples throughout 2012. The spawning season in the German Bight extends over several months comprising multiple unsynchronized spawning events. The minimum shares of ovigerous females appeared in early autumn, and the highest shares in late winter bearing mostly early egg stages. We defined the putative start of the reproductive cycle for November when the frequency of ovigerous females started to increase. There was no distinct separation between winter and summer eggs, but a continuous transition between large eggs spawned in winter (the early spawning season) and batches of smaller eggs in spring and summer. Larval densities peaked in April/May. Consequently, regular annual larval surveys from 2013 to 2016 were scheduled for April/May and extended to six transects covering the inner German Bight. Ovigerous females were most abundant in shallow waters above the 20‐m isobaths, which also explained regional differences in abundance between the regions off North Frisia and East Frisia. No relation was obvious between the number of larvae in spring and recruited stock in autumn. Due to the short lifespan of C. crangon, the combination of various abiotic factors and predator presence seems to be the principal parameters controlling stock size.

Highlights

The brown shrimp (Crangon crangon, Linnaeus 1758) is a small but ecologically and economically important epibenthic decapod crustacean in the North Sea
In order to evaluate the relation between the abundance of ovigerous females and larvae in spring and the recruitment success in autumn, we first analysed the seasonal appearance of ovigerous females and larvae from weekly samples throughout 2012
We addressed the questions whether recruitment depends on the same year's spring abundance of ovig‐ erous females and larvae, and whether the C. crangon spawning stock size in autumn determines the number offspring in the fol‐ lowing season? Data on winter water temperature and predator occurrence were included to discuss factors affecting the repro‐ ductive performance of C. crangon and to scrutinize established views on the reproductive biology and life cycle traits of C. cran‐ gon in the German Bight

Summary

Introduction

The brown shrimp (Crangon crangon, Linnaeus 1758) is a small (up to 80 mm) but ecologically and economically important epibenthic decapod crustacean in the North Sea. First evidence for small‐ scale fisheries on C. crangon dates back to the 17th century (Lotze, 2007). The brown shrimp is the most important target of coastal crus‐ tacean fisheries in the southern North Sea, keeping 500 vessels in operation and providing income for about 1,000 fishermen. Stock estimates of brown shrimp yielded variable and occasionally extremely high numbers of up to 82 specimens per m2, including juveniles (Boddeke, 1986). The short lifespan in concert with high fertility enables this species to recover quickly from detrimental events (Berghahn, 1996; Siegel, Damm, & Neudecker, 2008)

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Conclusion