Reducing Sea Turtle Bycatch in the Mediterranean Mixed Demersal Fisheries

The sea turtle (Caretta caretta) is the most common sea turtle in the Mediterranean, where incidental catches due to fishing activities are considered the main threat to its conservation. Over 50,000 capture events and likely over 10,000 deaths are estimated to occur in the Italian waters alone. However, current knowledge on the interaction of sea turtles with fishing gears and the implementation of mitigation measures are still poor to hinder the decline of turtle populations in the Mediterranean. In this basin, where fisheries are multispecies, multi-gears and multinational, making demersal fishing activities profitable while preserving sea turtles is a challenge. This study aimed to develop bycatch reducer devices (BRDs) and alternative fishing gears to mitigate the impact of demersal fishing gears on sea turtles: (a) hard and flexible turtle excluder devices (TEDs) were tested in bottom trawling to immediately exclude turtles from the net; (b) visual deterrents (ultraviolet LEDs) were used to illuminate set nets and to alter turtle visual cues, avoiding entanglement during depredation activity. The results showed the different devices did not affect the commercial catch, while bycatch reduction was instead evident. Thus, the study highlights that introducing mitigation measures to reduce sea turtle bycatch in the Mediterranean, where the bycatch of vulnerable species seems as a global issue, can be possible at least in certain areas and periods. Considering fishermen reticence to change the gear traditionally used, determining the optimal gear configuration to minimize commercial loss while reducing bycatch, is the main issue while introducing new technologies. Therefore, a global effort should be done to introduce BRDs in different areas and fisheries of the Mediterranean

Reburial potential and survivability of the striped venus clam (Chamelea gallina) in hydraulic dredge fisheries

The striped venus clam (Chamelea gallina) is the main edible bivalve living in Italian waters. According to Regulation (EU) 2020/2237, undersized specimens (total length of the shell, < 22 mm) must be returned to the sea. C. gallina specimens of different size classes that had undergone hydraulic dredging and mechanized sorting were analysed for reburial ability in a laboratory tank and for survivability in the laboratory (135 clams, 21 days) and at sea (320 clams, 15 days). In the tank experiments, the reburial times ( T50 and T90) and the upper (+) and lower (−) confidence intervals (CIs) of the whole sample were about 4 h (CI+ 4.4, CI− 3.6) and 8 h (CI+ 8.2, CI− 7.7), respectively, and were significantly shorter for the medium-sized clams (22–24.9 mm) than for the smallest (< 21.9 mm) and the largest (> 25 mm) specimens. For the field survivability experiments, clams under and above the minimum conservation reference size were placed in separate metal cages. Survival rates were 94.8% and 96.2% respectively in the laboratory and at sea, without significant differences between the two experiments or among size classes. These findings conclusively demonstrate that C. gallina specimens returned to the sea have a very high survival probability and that they can contribute to mitigate the overexploitation of natural populations

Impact on Macro-Benthic Communities of Hydraulic Dredging for Razor Clam Ensis minor in the Tyrrhenian Sea

Bivalve molluscs fishery is of great importance along the Italian coasts, both in economic and landing terms, and different edible bivalve species are harvested both in Adriatic and Tyrrhenian Seas. A medium-term assessment of the impact of the hydraulic dredges targeting razor clam Ensis minor on macro-benthic community was made during two surveys carried out in winter 2017 and late spring – summer 2018 in central Tyrrhenian Sea, which represents the main fishing ground for this species. The study area was located between 1 and 4 m depth, within 0.3 nautical miles from the coast. A net sampler (40 cm width, 18 cm height, and 14 mm mesh size) was mounted on a commercial dredge (3 m width) and enabled to collect specimens of the smallest sizes for the entire community present in the areas. A control area was identified where fishing does not occur, in order to compare exploited and not exploited sea bottoms. The results show that benthic assemblages found in dredged areas are characterized by species living in high-energy habitat, due to the closeness to the shore, and thus showing a high resilience at medium-term disturbs. Differences in species richness were not clearly evident both for the entire community and for the mollusc assemblages evaluated over the two surveys, among the control and the impacted areas, with few exceptions mainly depending on local conditions and anthropic pressure. Thus, even if the benthic community is typical of a moderately disturbed environment, the effects of fishing on the community structure are still discernible over and above the natural variation

Dredge selectivity in a Mediterranean striped venus clam (Chamelea gallina) fishery

The striped venus clam Chamelea gallina is the target of a large fleet of hydraulic dredgers, which represent an important fishing sector in terms of income and landings in the Mediterranean Sea. Although there is information on the catch rates, impact and discards related to this fishery, the size selection process carried out by the dredge during trawl under commercial conditions is practically unknown. The present study aimed to fill this gap, assessing the selectivity of the gear at different haul durations. We demonstrated that 25% of the clams entering the dredge were not size selected by it. Clams with a length (i.e. maximum distance between anterior and posterior margins) of 18.9 mm had 50% retention probability and tow duration did not affect the size selection process in the dredge. The dredge catch efficiency was 79% in numbers of clams and 89% in weight. 58% of the clams caught were below the minimum conservation reference size of 25 mm. The study demonstrates that to land only the legal sizes of clams, the additional size selection process carried out on board the fishing vessels by the sorting sieves is necessary.publishedVersio

Approccio multidisciplinare alla gestione della pesca della vongola (Chamelea gallina)

To responsively manage the striped venus clam (Chamelea gallina) fisheries, a multidisciplinary approach has been adopted through the investigation of new and updated biological aspects (e.g. age, growth, reproduction, size at first maturity, fecundity) and the interaction gear- target or non-target species (e.g. reburial ability, survival potential and exerted damage). The striped venus clam is an important socio-economic species in the Italian fishery context, highly regulated by national and international laws aiming at guaranteeing both social and ecological sustainability. Studies on growth and reproduction revealed that the size at first maturity is reached within the first year of life, whereas the present Minimum Conservation Reference Size of 22 mm is reached at two year of age. The annual reproductive cycle, which is driven by rises in seawater temperature and chlorophyll-a concentration, spans during the warmer months (late spring-summer) with multiple spawning events of different intensity occurring over the spawning period, and the number of potentially emitted gametes is positively related to shell size. Reburial tests conducted on undamaged specimens highlighted the ability of clams to rebury in the sediment once discarded, independently from the size. On the other hand, survival experiments in the laboratory and at sea, on both damaged and undamaged individuals, served to demonstrate that the species has a high survival rate, thus supporting the claim that discarded individuals can contribute to restock the natural populations. Moreover, the evaluation and quantification of damage induced by dredging on the discarded macro-benthic fauna living associated with C. gallina highlighted that soft-shelled or soft-bodied species are the most affected by the fishing process and subjected to a higher mortality. All these findings are of pivotal importance to rationally support the management measures to be adopted in the striped venus clam fishery

Chamelea gallina reproductive biology and Minimum Conservation Reference Size: implications for fishery management in the Adriatic Sea

Background: The striped venus clam Chamelea gallina is an economically important species in Adriatic Sea fisheries. The use of hydraulic dredging for its catch has a long history in Italy and its management faced several stages of development in the last 40 years. A great effort has been made in the past two decades to move from poorly or weakly managed fisheries to a well-structured co-management system to improve the sustainability of this fishery. However, a prerequisite for appropriate resource management is a sound knowledge of the biology and reproductive strategy of the species. Results: We investigated three major biological features– the gametogenic cycle, size at sexual maturity and partial fecundity – by microscopic, histological and video analysis techniques. We demonstrated that its breeding season is driven by rises in seawater temperature and chlorophyll-a concentration and that its spawning period lasted from March to September. Size at sexual maturity was reached very early in the life cycle. As regards partial fecundity – the number of mature oocytes potentially released by females with ripe gonads in a single release event – varied in relation to size. Nevertheless, the reduction on the Minimum Conservation Reference Size (MCRS) from 25 to 22 mm (Delegated Regulation (EU) 2020/2237) lead to a 40% reduction in the number of emitted eggs. Conclusions: We suggest that the ability of Adriatic clam stocks to withstand the strong fishing pressure of the past 40 years and the present one is due to their high reproductive potential and multiple spawning events combined with the effect of management measures (closed areas/seasons, quota, MCRS) and technical constraints on the gear and the sieve on board. Moreover, since the reduced MCRS for Venus shells is still larger than the size at maturity, it will probably not be detrimental to the reproductive capacity of the stock

Bottom trawl catch comparison in the Mediterranean Sea: Flexible Turtle Excluder Device (TED) vs traditional gear.

The Mediterranean Sea is a biodiversity hotspot where intense fishing pressure is associated with high bycatch rates of protected species (sea turtles and cetaceans) and top predators (sharks). Since the conservation of these species has become a priority, fishery scientists are faced with the challenge of reducing incidental catch, which entails high rates of mortality. Among the species threatened by fishing activities, the loggerhead turtle (Caretta caretta) is a charismatic species considered as "vulnerable" at the global scale. In the Mediterranean Sea trawl nets are the gears with the highest probability of catching protected species incidentally. A new flexible Turtle Excluder Device (TED) was tested for the first time on commercial bottom trawlers to assess its effectiveness in reducing bycatch in the Mediterranean Sea. Analysis of the total catches of the hauls made with and without the TED showed that the difference in terms of weight was not significant. The catch of the main commercial species showed similar rates without a significant loss of size (i.e. total length) with the exception of the largest anglerfish (Lophius spp.). The bycatch of control nets included mostly rays and sharks, but never turtles, although the authors learned from the crews of other vessels operating in the same areas at the time of the trials that they had caught some loggerhead turtles. Our study demonstrates that TED scan be adopted without significantly affecting commercial catch. This informs fishers and managers for a practical and effective means that may reduce the bycatch of threatened species in coastal Mediterranean demersal multispecies fisheries. The measures involving gear modifications require significant investment but they are technically feasible and are capable of improving the conservation prospects of these endangered species. Besides ensuring normal earnings, the TED induced a significant reduction of debris and litter in the codend, thus reducing catch sorting time and improving catch quality

Estimating selectivity of experimental diamond (T0) and turned mesh (T90) codends in multi-species Mediterranean bottom trawl

This paper evaluates the effect of changing from a diamond mesh codend (T0) to a 90° turned mesh codend (T90) on the size selectivity of seven commercially important species in the Mediterranean bottom trawl fishery. In sea trials conducted in the north-western Adriatic, two experimental codends made of 54 mm nominal mesh size netting and differing only in mesh configuration were alternately mounted on the same trawl. Overall, the T90 mesh significantly improved codend size selection for all the species analysed. The difference in the predicted average L50 values between the T90 and T0 codend was particularly marked in European hake (Merluccius merluccius, 21.26 vs 11.26 cm total length), common squid (Loligo vulgaris, 12.06 vs 7.88 cm mantle length) and mantis shrimp (Squilla mantis, 20.78 vs 13.35 mm carapace length). Both codends showed an excessive size selectivity, which involves a commercial loss, especially for red mullet (Mullus barbatus), Mediterranean horse mackerel (Trachurus mediterraneus) and whiting (Merlangius merlangus). These findings demonstrate the efficiency of the T90 configuration in excluding undersized specimens, especially of hake, whose average L50 was above the minimum conservation reference size of 20 cm. The adoption of this practical and inexpensive solution can contribute to improve the management of the demersal resources targeted by the Mediterranean bottom trawl fishery