High survivability of discarded invertebrates from bottom trawling fisheries

The impacts of bottom trawling in coastal ecosystems are significant, not only for targeted species but also for non-commercial ones. However, the critical gaps in scientific knowledge associated with these impacts on untargeted but functionally important species remain to be filled. This is notably the case for a great proportion of discarded invertebrates, for which only a handful of studies have investigated their survival. In this study, 600 individuals from six different benthic invertebrate species were collected in commercial conditions in the Bay of Biscay for short-term survival experiments. Overall, the observed survival after 100–130 h and the predicted survival via mixture models were very high (>93%) for Asterias rubens, Aphrodita aculeata, Buccinum undatum and Pagurus sp. Survival of Maja brachydactyla was lower though still high (>80% overall) and Atelecyclus undecimdentatus was more vulnerable to trawling and handling, with 50% of survival. Both showed 100% mortality when presenting carapace cracks and survival of M. brachydactyla was lower when missing appendages. No biotic nor abiotic parameters were correlated to survival, except injury class for M. brachydactyla and A. undecimdentatus. This study shows an overall high survival but highlights the fact that otter trawl fisheries may differentially affect discarded benthic invertebrates, therefore bringing light for future studies on longer-term impacts on benthic communities and coastal ecosystems. Furthermore, we recommend incorporating survival rates of discarded invertebrates in ecosystem-level modelling studies and encourage the use of detailed information and biomass reports of benthic invertebrates for a better management of natural populations and fisheries resources

Comparative assessment of two proxies of fish discard survival

Two vitality index approaches have been demonstrated to be good proxies of discard survival in similar fishing and environmental conditions: Reflex Action Mortality Predictors combined with injuries (RI) scores that measure the proportion of impaired reflexes and injuries, and a simpler, quicker, but less precise approach based on ordinal categories related to fish body movements and injuries (semi-quantitative assessment, SQA). This study assessed and compared these two approaches for five species (two flatfishes, skates, a seabass and a catshark) discarded in a coastal otter trawl fishery in the Bay of Biscay. All species displayed good vitality status according to both indices and were not very sensitive to the fishing nor their biological conditions. Still, flatfishes were more sensitive to discarding while the catshark was very resistant. Furthermore, depending on species, the impairment of some reflexes and injuries were associated to stressing factors, highlighting the potential of the approach but also the complexity involved in building a relevant score. The SQA index was highly correlated with the RI score and was sensitive to more sources of stressing factors for similar predictive performance. While the relevance of these two approaches should be ultimately determined by the strength and consistency of their association with discard survival, these results suggest that the RI approach was not more effective than SQA despite the extra labour and time involved in collecting the data

Reducing discards of demersal species using a 100 mm square mesh cylinder: Size selectivity and catch comparison analysis

We assessed the impact of an additional 100 mm square mesh cylinder (SMC) on the selective property of the demersal whitefish trawl in the Celtic Sea. Sea trials were conducted on board a French trawler using twin trawl rigging. We tested the effects of the position of the SMC (in front of and behind the mandatory 100 mm square mesh panel) and of the insertion of a dispersive float. Selectivity analysis revealed the 50% retention length () to be greater than the Minimum Conservation Reference Sizes regardless of species (haddock, whiting or megrim) or rigging configuration. Results did not reveal a clear effect of the SMC position. However, the insertion of the float led to a decrease in selection ranges suggesting enhanced contact probabilities with the square meshes. With the largest values, the SMC placed in the front position (and without the float) proved to be the most selective rigging configuration and was then tested under commercial conditions. Catch comparisons revealed that the test gear retained less fish across all size classes than the commercial gear. For haddock, the test gear retained less fish below 50 cm in length. Catch comparisons also indicated significant discards of fish above the MCRS, i.e. “high-grading” practices. Under the requirements of the landing obligation (LO), unwanted catches must be stored on-board and landed regardless of economic value. The SMC is thus a valuable tool to mitigate the impact of the LO on the demersal whitefish fishing fleets operating in the Celtic Sea

Comparison between "underway" and "on station" acoustic measurements made during bottom trawl surveys

It becomes in many cases, routine, to collect acoustic data during bottom trawl surveys, both during and between trawls providing “on station” and “underway” acoustic data. Still, the combination of trawl and acoustic data is not operational and could represent a cost-effective way of improvement: bottom trawl survey use samples taken from very small areas (say 2 n.mi.) as representative of much larger areas (the statistical rectangles) while an acoustic transect from one trawling station to the next one, covers a 20-30 n.mi. strip. This study uses 3 different surveys (Norwegian acoustic and bottom trawl survey for cod and haddock in 2000; IBTS (France) in 2002; Northern Irish bottom trawl survey in 2000) where “on station” data are integrated over the trawling sections while “underway” data are regularly integrated over 1, 0.5 or 0.1 n.mi. The objective of this exploratory data analysis is to evaluate the consistency between “underway” and “on station” acoustic data. Experimental means and variances of NASC values are first presented. Acoustic observations being autocorrelated, we analysed the consistency between acoustic data using geostatistical tools. The spatial structures (i.e. variograms) of the ‘underway’ acoustic data indicate that fish assemblages are autocorrelated in space. If the variograms of “underway” and of “on station” data indicate the same large distance structures, the “underway” acoustic data provide the opportunity to observe short scale structures that are smaller than the interstations distances. These structures appears as nugget effect on “on station”variograms and may explain why very few structure are observed sometimes “on station”

Using acoustic telemetry to estimate post-release survival of undulate ray Raja undulata (Rajidae) in northeast Altantic

The landing obligation introduced in the reform of the European Union Common Fisheries Policy requires landing all catches of species under quota. However, exemptions may be granted for species for which “scientific evidence demonstrates high survival rates”. Captivity experiments are often used to determine fish survival rates, but they are difficult to perform for large species such as rays due to the limited storage capacity of tanks. Thus, little information is available on ray survival after release despite an identified potential for high survival. We used acoustic telemetry to study rays discarded from a coastal bottom trawl fishery in their natural environment and present a new ad hoc approach to derive a minimum survival rate. After capture under commercial conditions in a semi-enclosed bay, 144 rays were tagged with a miniature acoustic transmitter. Survival was assessed based on detections from 15 acoustic receivers deployed in the area and a mobile reception antenna. Then, combining detection data with information on currents provides useful results from a management perspective, as at least 49% of the rays were found to have survived the first 14 days after released, and because the only factor identified that likely decrease survival (i.e. smaller individuals) cannot be improved easily by changing fishing practices

The low impact of fish traps on the seabed makes it an eco-friendly fishing technique

Besides understanding the effects of fishing on harvested fish stocks, effects on non-target species, habitats and seafloor integrity also need to be considered. Static fishing gears have often been mentioned as a lower impact fishing alternative to towed gears, although studies examining their actual impact on the seafloor are scarce. In this study, we aimed to describe fish trap movements on the seafloor related to soaking time and trap retrieval. Impacts on the seafloor of lightweight rectangular traps and heavier circular traps were compared. We used 3D video cameras to estimate sweeping motion on the seabed and penetration into the sediment during soaking time. The area and distance swept by each type of trap during retrieval was determined by a camera set up facing the sea bottom. The potential rotation of the traps around the mainline was assessed using an Acoustic Doppler Current Profiler. Results showed that no penetration and almost no movements could be detected during soaking time for either lightweight or heavy commercial traps, even for high tidal coefficient (maximum 6 cm). No rotation could be observed when the tide turned. The swept area covered by a trap during retrieval was low (maximum 2.04 m2) compared to towed fishing gear and other static gear

Using a trait-based approach to understand the efficiency of a selective device in a multispecific fishery

Improving the selectivity of a fishing gear is one technical management measure to significantly reduce by-catch of non-commercial species or undersized individuals. The efficiency of selective device is mainly estimated by comparing species composition, the biomass and length spectrum of caught individuals and escapees while the functional traits of species are rarely accounted for. Using an innovative technical device to reduce catches of undersized individuals in a multispecific bottom trawl fishery in the Bay of Biscay, namely a T90 mesh cylinder, we measured functional traits on both caught and escaped individuals of 18 species. Using a Principal Component Analysis and K-means partitioning, we clustered species into 6 groups illustrating 6 different locomotion strategies. We identified functional traits related to body size, visual ability and locomotion, differing between caught individuals and escapees using Linear Mixed-effects Models. As expected, escapees were smaller on average but also tended to be more streamlined, with a high position of the eyes and fin features characteristic of manoeuvrability and propulsion. Here, we present how a trait-based approach can shed light on the biological characteristics influencing the efficiency of selective devices

Using fish behavior to design a fish pot: Black seabream (Spondyliosoma cantharus) case study

The French fishing industry is becoming increasingly environmentally aware and likely to adopt more sustainable fishing gears. As a result, fishers are showing a growing interest in sustainable fish pots. This experiment aimed to develop a fish pot concept specifically based on target species behavior in French coastal waters. First, the consultation led with fishers indicated the black seabream as the main species of interest. Then, the pot’s characteristics were defined to comply with fishing regulations. The conception process of the pot was led step-by-step by gradually testing a pot design appropriated to black seabream’s natural behavior. The approach and feeding behaviors of black seabream were described using underwater video combined to an ethogram. The experiment led to a prototype of fish pot, which provide fishers with an innovative fishing gear

Gut content metabarcoding of six crustaceans provides detailed diet description and insights into their roles as predators and scavengers

While many studies have analyzed the diet of marine consumers using traditional morphology-based methods, the integral diet of many species of functionally important groups such as benthic invertebrates remains poorly resolved. This hinders our ability to precisely establish trophic links among species, which is necessary to understand food webs and ecosystem dynamics. Here, we applied DNA metabarcoding to obtain a high resolution picture of the animal fraction of the diet of six species of crustaceans in the Bay of Biscay; the edible crab Cancer pagurus, the velvet crab Necora puber, the circular crab Atelecyclus undecimdentatus, the spider crab Maja brachydactyla, the shrimp Crangon crangon, and the hermit crab Pagurus bernhardus. We used two pairs of primers targeting the 18S rDNA (generalist) and 12S rDNA (fish-specific) regions to identify prey that makes up the animal fraction of the diet. Based on occurrence data, 54 prey taxa belonging to 5 different phylum mostly represented by teleost fishes and malacostracans were identified. Through a high-level of prey detection, results indicate a scavenging strategy across all consumers mostly based on the consumption of preys likely originating from fishing discards. This study also reveals patterns of niche overlaps among consumers differentiating C. crangon from others and suggesting high overlap in animal resource use between N. puber and M. brachydactyla as well as C. pagurus. High resolution diet description revealed the complexity of trophic interactions on a local scale

Survivorship of discarded sole (Solea solea) characterised via telemetry, vitality, and physiology

International audienceWhile the discarding practices of commercial fisheries might greatly contribute to fish mortality, a better un-derstanding of discard survival patterns and their drivers is crucial to assess fishing mortality and develop effective mitigation measures. Bottom trawling in particular is likely to induce multiple forms of stress in captured fish. In this study, we used three experimental methods to explore discard survivorship of the common sole (Solea solea). We focused on fish captured via a coastal bottom trawler of the Bay of Biscay and conducted sampling during commercial fishing operations. First, we used acoustic telemetry to determine the survival rate of 160 discarded soles. While this technique makes it possible to study discarded fish in their natural habitat, inferring fish fate from these data can be challenging, especially when acoustic receiver network coverage is fragmented. Second, we scored the vitality of the sole before their release. Third, we examined three plasma markers of fish physiological status over a 90-min period following capture. The method based on acoustic telemetry estimated a minimum discard survival rate of 41%. This metric was positively correlated to the vitality. Furthermore, both survival and vitality were associated to tow duration and the presence of algae in the catch consistently. The three physiological markers reflected the presence of various levels of stress and fatigue among discarded individuals, but also a fish's ability to recover after being discarded. Individuals with excellent vitality (33% of discards in this study) suffered less stress, were less fatigued and, therefore, were greater equipped to react to other sources of stress than individuals in lower vitality. The consistency of the results demonstrates the reliability of our approach to estimate post-release survival from acoustic tagging even from fragmented network coverage, and that vitality can serve as a proxy for discard survivorship