The efficacy of releasing caught Nephrops as a management measure

The objective of this work was to examine if a policy of releasing caught lobsters, Nephrops, could be used to protect the Nephrops population off the south coast of Portugal. Such a measure, as an alternative to closing the fishery completely, would allow the continued operation of the fleet in targeting other species, resulting in a reduced economic and social impact compared with a complete closure. For such a proposal to be considered, information on survival rates of released lobsters is required. Nephrops were sampled randomly from the catch of commercial trawlers and put in cages, in an area with similar conditions to the area where they were caught, in order to evaluate survival rates. The cages were immersed for periods of 5–9 days. Several factors were considered to potentially affect survival, including duration of the tow and season, and biological characteristics of the individuals (e.g. size, sex and ovigerous condition). Survival was only affected by season (increased mortality inwarm months).Aglobal estimate of survival of released lobsters, taking into consideration survival and proportion of the catches for each season, was 35%. Simulation models were used to investigate the potential effect of a release measure on the population. Assuming the number of recaptured individuals is negligible, Z (instantaneous total mortality rate)would be considerably reduced if a release policy was implemented. This is particularly important if the levels of exploitation are high, which seems to be the case in this stock. It is our belief that the implementation of amanagement measure of returning all caught lobsters to the sea is worth considering as a means of aiding recovery of the resource. This measure would have the side effect of discouraging the choice of Nephrops as a target species, potentially reducing the fishing mortality even more significantly.

Fate of discards from deep water crustacean trawl fishery off the south coast of Portugal

Trawling for crustaceans takes place on the south coast of Portugal at depths between 200 and 800 m. Large amounts of discards are released back to sea, reaching the bottom in the general area where trawling occurs. The objective of this work was to study the time taken for decomposition of the discards, to identify the most important scavenging species involved, and to evaluate the impact on the species targeted by the fishery. We conducted a series of 22 trials, using traps baited with samples of the discards. The time of immersion varied between 1 and 40 h. The amount of tissue removed from the discards was evaluated on a qualitative scale of 1–3, and quantitatively. The species caught in the traps were identified. Considerable consumption of the bait had occurred after 5 h of immersion. After 24 h of immersion only fish bones were left; past 40 h, consumption was complete. The conger eel (Conger conger) was the most common fish species present in the traps. Two species, the amphipod Scopelocheirus hopei and the isopod Natatolana (Cirolana) borealis, were identified as most important for the recycling of organic matter in the system. The stomach contents of a sample of species caught in trawls were analysed for the presence of small scavengers. Preliminary results show that S. hopei and N. (C.) borealis are part of the diet of some of the target species of this fishery, such as the Norway lobster, Nephrops norvegicus and the rose shrimp Parapenaeus longirostris.