Estimation of brown shrimp stock and fishery dynamics in the German Bight using a novel biomass index

The brown shrimp (Crangon crangon) stock of the Wadden Sea supports one of the most important fisheries in the region due to its high productivity and market price. Given the species' short life history and seasonal dynamics, fisheries-based indicators (i.e., landings per unit effort, LPUE) are used to monitor stock biomass; however, these indicators may be biased by fishing behavior and thus require further evaluation. Using survey data from the German Bight area of the Wadden Sea, this study presents a novel species distribution model to estimate annual stock biomass changes for different size fractions (‘small’, <50 mm; ‘large’, >50 mm; and ‘combined’, all sizes), which are compared to LPUE and catch per unit effort (CPUE, with modelled discards) to evaluate their use as indicators of biomass. The results indicate that changes in LPUE are well correlated with biomass changes, although CPUE is shown to have an even stronger relationship to the combined biomass index, representative of exploitable biomass. This relationship is used to reconstruct monthly historical stock dynamics and estimate fishing exploitation rates (monthly harvest rate and annual fishing mortality). Estimated fishing rates reflect changes in fishing effort, which varies seasonally, in response to recruitment and subsequent exploitable biomass dynamics, and interannually, relating to reductions in fleet size and shorter-term pauses in effort. The resulting time series serve as a basis for further stock assessment models that can provide more biologically-based advice for the stock. The methodology’s combination of survey and fisheries-dependent data should be of interest to other data-limited applications; particularly, short-lived species where seasonal dynamics of the stock and fishery are of importance.