Are spatial closures better than size limits for halting the decline of the North Sea thornback ray, Raja clavata?

Abstract

Abstract. A key challenge of the ecosystem approach to fisheries management is to sustain viable populations of largebodied less-productive vulnerable elasmobranchs that are the by-catch of fisheries that target more productive species. The North Sea population of the thornback ray (Raja clavata) is now mainly confined to the Thames Estuary and surrounding SW North Sea, which is subject to a flatfish trawl fishery. We explored the relative effectiveness of seasonal closures versus size-based landing restrictions using a four-season age-structured model. More than a third of adult thornback rays are currently removed by fishing each year, and without effective management, a further 90% decline within 30 years is likely. A three-season closure of the Thames Estuary was the shortest closure that ensured thornback ray recovery and minimal loss of fishery yield. Minimum and maximum landing size restrictions are nearly as effective at recovering thornback rays but less so at improving yield. While long seasonal closures and full marine protected areas are more effective at ensuring the recovery of thornback rays, length restrictions may be simpler to implement under the current institutional framework and may have less impact on the multispecies trawl fisheries operating in the area.

Modelling abundance hotspots for data-poor Irish Sea rays

“…Any such measures could be incorporated into a management strategy evaluation that considers the ramifications of area closures on the future of the stocks, and weighs these against the impact on the fishery, e.g. Wiegand et al (2011).…”

The importance of research and public opinion to conservation management of sharks and rays: a synthesis

“…Despite the growth in the use of spatial management in the ocean, the mobile nature of many sharks and rays means that marine protected areas (MPAs) may not always be the best approach to conservation management. A comparison of the potential effectiveness of seasonal closures and size limits for halting declines of thornback rays (Raja clavata) suggests that although MPAs might lead to more rapid recovery, the use of size limits would better suit the management systems and minimise conflict with trawl fleets targeting other species (Wiegand et al 2011). Such approaches are helping managers and policy makers design the best possible conservation management plans.…”

“…Life-history data inform decision-support tools such as ecological risk assessments (Braccini et al 2006), demographic models (Cailliet 1992), stock assessments (Walker 1992) and ecosystem models (Stevens et al 2000) that are widely used to set catch limits for many fisheries or species. Wiegand et al (2011) examined the sensitivity of their demographic model to the potential range of uncertainty in the input life-history parameters. Another source of uncertainty comes from the underlying assumptions of many demography methods; the pragmatic choice of one or other may profoundly affect the outcome (Braccini et al 2011).…”

Patterns of extirpation. I. Changes in habitat use by thornback rays Raja clavata in the German Bight for 1902-1908, 1930-1932, and 1991-2009

“…1), appears likely to enable recovery. In line with modeling studies (Wiegand et al 2011), a fishing ban had a significant positive effect for thornback rays in a 500 km 2 protected area in the English Channel (Blyth-Skyrme et al 2006), and even locally reduced fishing effort may provide effective refuges for elasmobranchs (Shephard et al 2012).…”