No abstract

Species that have a high likelihood of surviving the discarding process have become great concern since the European Union reformed the Common Fisheries Policy and enacted a landing obligation prohibiting the discarding any individuals of species under quota. Among species presenting an elevated survival potential, plaice (Pleuronectes platessa) is one of the most discarded in the coastal otter trawl fishery in the English Channel. The objective of this study is to provide the most reliable estimates of plaice survival after release in commercial conditions, and to identify the factors that influence survival rates. A captivity experiment was conducted in January-February in the English fishery to assess the survival of discarded plaice as a function of a semi-quantitative index of fish vitality, which has been demonstrated to be a good proxy of fish survival in comparable fishing and environmental conditions. This study examined the potential of this index to estimate discard survival in three trials from the English and French fisheries and at three different seasons. The vitality index was then used to analyse the influence of several factors (fishing practices, environmental conditions and fish biological characteristics) on the discard survival. The survival rates for plaice were accurately estimated at 62.8% in January-February, 66.6% in November and 45.2% in July. While these rates remained substantial whatever the fishing, environmental or fish biological conditions, the time fish spent on the deck, the bottom and air temperatures, the tow depth and the fish length had a significant influence on plaice survival. In practice, plaice survival could be enhanced by releasing the fish early during catch sorting and avoiding exposure to extreme air temperatures.

The new Common Fisheries Policy may exempt some species from the landing obligation if a high survival rate can be demonstrated among discards. This study focuses on the survival capacity of Nephrops norvegicus discarded from trawlers of the Bay of Biscay (France). Three sea trials were conducted on board 3 commercial trawlers and 15 fishing operations were sampled. The vitality of Nephrops was classed into three categories (healthy, moribund or dead) before release at sea and samples of healthy and moribund individuals were kept in captivity on Nephrops grounds for three days. A Generalised Linear Model was used to examine the variability in the proportion of healthy Nephrops discarded, which was significantly influenced by tow duration, duration of air exposure and temperature, as well as individual length. By combining the results on vitality before release at sea with those on survival in captivity following re-immersion, resampled by bootstrapping, it was calculated that 51% [42-60%] of discarded Nephrops would survive after three days of re-immersion. We discuss the sources of variability in survival rate, as well as the implications of a potential exemption from the landing obligation for this species.

In the context of the landing obligation set by the new Common Fisheries Policy (CFP), Norway lobster Nephrops norvegicus was identified as a species likely to have high survival rate when discarded in the bottom trawl fishery of the Bay of Biscay. Previous studies in this area reported a survival rate between 30% and 51%, but the experiments were done on a limited monitoring period and the seasonal variations were not investigated. This study was designed to obtain a reliable value for survival rate after a 14-day monitoring period in onshore tanks allowing considering delayed mortality. The study also tested the effect on the survival rate of using a discarding chute system, a sorting device that was made mandatory on the 1st of January 2017 for Nephrops trawlers in the Bay of Biscay. This device, which enables fishermen to discard undersized Nephrops back to the sea while sorting, led to an increased average survival rate (51.2%) compared with the standard sorting practice (36.9%). The impact of biological, environmental and fishing operation related variables on survival from the first day of captivity to the end of the monitoring period was examined using a generalized linear model. The results of the GLM indicate that injuries, season and duration of the air exposure, significantly influence the survival from the 1st day of captivity to the end of the monitoring period. The survival rate was higher for noninjured Nephrops as well as for Nephrops that have undergone short air exposure, in summer and autumn. Highlights ► Survival of discarded Nephrops was monitored for 14 days in tanks. ► Injuries and air exposure reduced survival significantly. ► Survival was highest in summer and autumn. ► Use of a discarding chute system improved survival.

By 2019, the Common Fisheries Policy will prohibit discarding in all European fisheries of any pelagic, demersal or shellfish species for which removals are managed by TACs and quotas or minimum sizes. However, the regulation allows for exemptions from the prohibition for species for which scientific evidence demonstrates high survival rates associated with discarding. Producing reliable evidence of high survival typically requires long and costly studies involving tagging or captivity. This paper proposes to use the capacity to resist air exposure, a key stressor for discarded animals, as a proxy for survival that can be used to prioritize candidate species for more in-depth discard survival studies. The time required to induce mortality (TM) in air-exposed fish was estimated for ten discarded species under commercial fishing conditions for two artisanal French otter trawlers in the Bay of Biscay and in the English Channel. European seabass, plaice, sole and skates had extended TM values on average, suggesting that these species are good candidates. The three species observed in both regions (plaice, sole and skates) had larger TM values in the English Channel experiment compared to the Bay of Biscay experiment. Among the four measured external conditions that could influence TM (air temperature, fish length, tow depth and tow duration), the air temperature was the most important and the factor that most distinguished the two experiments. Highlights► Discard survival was investigated in small-scale bottom trawl fisheries. ► Mortality due to air exposure was used as a proxy of discard survival potential. ► Air exposure mortality was higher in the Bay of Biscay than in the English Channel for three common species. ► High survival potential was demonstrated for sole, plaice, seabass and skates.