Self-Governance in the North Sea Brown Shrimp (Crangon crangon) Fishery: Potentials, Perceptions, and Challenges

Abstract

Effective fisheries management increasingly relies on science-based harvest strategies that prioritize long-term ecosystem and stock sustainability over short-term yields. For short-lived and data-limited species like the North Sea Brown Shrimp (Crangon crangon), the use of traditional stock assessments is hampered by high natural mortality, rapid population turnover, and incomplete or inconsistent data on fishing effort. At the core of the harvest strategies for such species are thus harvest control rules (HCRs)—predefined protocols that regulate fishing opportunities based on real-time or near-real-time indicators of stock status. Despite being one of Europe’s most valuable fisheries, the North Sea Brown Shrimp has lacked regular stock assessment and population-based management measures, relying instead on technical measures. However, advances in growth rate modelling and integrated international datasets have revealed evidence of growth overfishing, highlighting the need for formal management. In response, a collaborative management framework was established, culminating in Marine Stewardship Council (MSC) certification of the main fleets and the adoption of a self-regulatory management plan based on Landings per Unit Effort (LPUE) thresholds, effort restrictions and gear modifications to reduce growth overfishing and delimit the risk of potential recruitment overfishing. This dissertation provides a comprehensive analysis of the recent dynamics in the Brown shrimp stock, the effectiveness of current management approaches, and the challenges faced in implementing effective and accepted management measures. In Chapter I, monthly VMS and logbook data of the North Sea Brown shrimp 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 larval release in March and April. The findings demonstrate that neither environmental drivers nor predation pressure alone can fully explain the recent decline in stock abundance. Instead, fishing pressure—especially during critical winter months when the stock appears most vulnerable—emerges as a key factor influencing population trends. 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. In Chapter II, the effectiveness of the current HCR is quantified and alternative, more stringent HCR designs are tested under two plausible scenarios: a general reduction in stock abundance and a recruitment failure event, both representing common risks for short-lived stocks. A comprehensive shrimp population simulation model, parameterized with recent fisheries and biological data from the German Exclusive Economic Zone, replicates the seasonal patterns of growth, mortality, and fishery removals. The performance of HCR designs was evaluated in terms of their effects on shrimp biomass, egg production, landings, and total fishing effort. Results demonstrate that all HCR variants yield modest improvements in LPUE, biomass, and egg production relative to unmanaged scenarios, while annual landings remain largely unchanged. Importantly, the magnitude and speed of stock recovery depend considerably on the stringency of the effort reduction. The currently implemented approach, which employs stepwise weekly effort limits when LPUE reference points are breached, achieves only moderate effort reductions (averaging 12–19%), with limited capacity to rebuild biomass after major declines. In contrast, simulated approaches involving greater reductions in weekly fishing time or short complete fishery closures (up to 99% effort cut for two weeks) trigger much faster LPUE recovery and higher increases in spawning biomass and egg production, especially following recruitment failure scenarios. However, such drastic measures could pose operational difficulties and may be less acceptable to stakeholders due to increased variability in landings and disruption to fleet activities. The current harvest control rule, based on empirical LPUE thresholds, provides a necessary foundation, but simulation results indicate that this mechanism may be insufficiently responsive under serious stock declines. More drastic, short-term reductions in effort, informed by timely and accurate abundance indices, are likely needed to prevent stock and fishery collapse, and could even result in increased future yields without compromising market supply. In Chapter III, the self-management regime of the North Sea Brown Shrimp fishery was analyzed, drawing on Ostrom’s design principles for robust self-governance of common-pool resources. The study employs a quantitative survey of shrimp fishers, complemented by qualitative insights, to examine user perceptions of the self-governance system. Fishermen’s attitudes toward management efficacy, enforcement, participation, and adaptation to local conditions were assessed across three clusters of fishers. Results indicate limited perceived need for management among fishers, with only a minority recognizing stock decline despite scientific evidence of increased fishing mortality and growth overfishing. While the fishery’s biological and institutional characteristics—restricted user group, monopolistic market structure, and homogeneity in economic interests—are conducive to self-governance, significant challenges remain. Notably, trust is lacking in the fairness and effectiveness of monitoring and enforcement, particularly across national producer organizations (POs). Many fishers express skepticism about the impact of measures such as increased mesh size, viewing technical adjustments as easily circumvented. Effort reduction is more widely accepted, especially during periods of low stock, but regionalization of management is resoundingly rejected in favor of equal treatment for all fleet segments. Fishermen report feeling insufficiently involved in the development and ongoing adaptation of management measures, leading to perceptions of remote, top-down decision-making—potentially undermining legitimacy and compliance. Nevertheless, there is strong interest, especially among less mobile, home-port-based fishers, for increased participatory roles in management design and rule-making. The North Sea Brown Shrimp fishery illustrates both the potential and limitations of self-management frameworks. While user-led regulation can stabilize and add value to the fishery, persistent gaps exist between scientific advice, institutional frameworks, and user perceptions. Addressing these gaps will require enhanced communication of scientific findings, fostering trust among user groups and POs, broader stakeholder engagement, and greater transparency in enforcement.