Aspects of the sustainability of creel fishing for Norway lobster, Nephrops norvegicus (L.), on the west coast of Scotland
This thesis describes a study of a number of aspects of the Nephrops creel fishery. Various aspects have been studied. In particular, work has focused on how the biology of Nephrops affects its catchability in creels, as well as assessing the environmental impacts of the Nephrops creel fishery. A smaller study relating to the physiological effects of the capture and transport of live creel-caught Nephrops, has also been undertaken. Fishery-directed sampling of Nephrops displayed strong seasonal trends in catch composition and catch rate, reflecting patterns in the Nephrops moult and reproductive cycles. Most females were caught in the early spring, following moulting and mating, with catch rates becoming low after spawning in autumn. Catches of males were greatest around February following moulting. Of those animals caught, males were larger than females. These seasonal patterns reflect the exploitation of different components of the stock throughout the year. Fishery sampling also showed there to be considerable stock heterogeneity both among and within creel fisheries. Global positioning system (GPS)-linked data loggers fitted to selected fishing boats provided a more detailed assessment of spatial variability in catch rate. Results showed both effort and landings per unit effort (LPUE) to be patchily distributed in the area studied, further illustrating considerable stock heterogeneity throughout the area. Both approaches illustrated that catch rate varied with sediment type, with high catch rates of small animals on sandy to firm mud and high catch rates of large animals on softer mud. Additionally, population modelling indicated that, in the areas studied, Nephrops are not currently being growth overfished. Behavioural observations in both the field and laboratory indicated a low probability of entry to creels by approaching Nephrops, particularly if other crustacean species were inside the creel. Larger Nephrops appeared to have a greater threshold for disturbances such as and the presence of conspecifics or other species as well as reduced gear avoidance, suggesting that Nephrops creels select larger animals in the population. Using sea pens as an indicator of physical disturbance of the sea bed showed that impacts from creeling were low. The sea pens Virgularia mirabilis and Pennatula phosphorea were caught in moderate quantities in the creel fishery, but were still observed in high densities on creel grounds. Comparison with trawled areas showed some differences in density, the most notable being that the tall sea pen Funiculina quadrangularis was less abundant on trawled grounds, indicating an adverse effect of bottom-towed gear. Bycatch was assessed in two distinct fisheries and varied both between and within areas; however, levels of bycatch (both target and non-target species) in all areas were much lower than previous estimates from the trawl fishery. Additionally, in contrast to the trawl fishery, commercially exploited fish made up a small proportion of the bycatch in the Nephrops creel fisheries studied. The survival of Nephrops discards is considered to be high, although preliminary results suggest that predation by seabirds may cause significant mortality. The effects of ‘ghost fishing’ were investigated experimentally at two sites and it seemed that lost creels do not constitute a serious issue in this fishery. Creels were found to be very selective for their target species, with both target and non-target species being able to escape. Only a few crabs and a small proportion of the captured Nephrops died in the creels, indicating no evidence of a ‘re baiting’ effect from dead animals. Stress associated with the capture, storage and live transport of creel-caught Nephrops was found to be greatest during the transport process, during which Nephrops are kept out of water for long periods of time. Temperature was particularly important, suggesting cooling prior to and during transport was essential in order to ensure their survival. Reduced salinity was found to be a major source of stress. An assessment of the management measures implemented in the Torridon creel fishery show that escape gaps placed in creels are effective in enabling smaller Nephrops and other bycatch species to escape. Limiting effort of individual fishermen has been successful, but this has been followed by an increase in the number of boats fishing within the area, leading to an overall increase in effort. Management measures (both statutory and voluntary) currently applied in the Torridon area are discussed and considered in relation to other areas. It is concluded that creel fishing areas need to be considered on a case-by-case basis, accounting for biological characteristics of individual stocks, as well as management issues at a local scale. The findings of the present study are discussed in the context of providing a detailed description of the characteristics of the Nephrops creel fishery on the west coast of Scotland. Findings are also discussed in relation to other crustacean creel fisheries, enabling issues directly relevant to the specific fishery to be considered in a much broader context. The present study has been particularly valuable in a wider context, as Nephrops are found in a habitat which is very different from the majority of other commercially exploited crustaceans, and their life history characteristics are also different. This study provides an ideal platform to assess what general principles from other creel fisheries apply to the Nephrops creel fishery, as well as what characteristics are unique to this fishery.