Study on the mixed species of the rapido trawl fishery along the Italian Coasts. Study contract n. 99/051. Final Report

Rapido trawl is a bottom trawl gear planned and used in Italy mainly to exploit soles (Solea vulgaris) and scallops (Aequipecten opercularis, Chlamys glabra and Pecten jacobaeus). It resembles a toothed beam-trawl and is made of an iron frame provided with 4 skids and a toothed bar on its lower side. These gears are usually towed at a greater speed in comparison to the otter trawl nets. This fishing activity at present largely spreads along the Italian coast of the northern Adriatic Sea, while only a few number of vessels are fishing with this gear along the western Italian coasts. Due to the lacking information on the rapido trawl activity exploiting the common sole on the soft, sand-muddy bottoms along eastern and western Italian coasts, the present project has been conceived, in order to collect the basic information for a possible future management of this fishery. The aim of this study was to collect data on fishing areas, landings, fishing effort, fishing yields and composition of the retained and discarded catch, as well as the size composition of the target (S. vulgaris) and the most relevant commercial species. Investigation was carried out in three Mediterranean areas: the northern Adriatic Sea, the eastern Ligurian Sea and the central Tyrrhenian Sea. Data were collected from July '00 to March '02, both through a sampling activity on landings and fishing effort and by observations onboard of commercial vessels. The former was performed by monthly data collection at the landing sites of the rapido trawl vessels of Rimini and Ancona (northern Adriatic Sea) as well as of the vessels of Viareggio (eastern Ligurian Sea) and Fiumicino (central Tyrrhenian Sea). The latter was carried out at 20-day intervals adopting the same sampling protocol at all the investigated areas: observations were carried out aboard of commercial vessels belonging to Ancona, Viareggio and Fiumicino rapido trawl fleets. A total of 179 hauls were sampled in the first harbour, 177 and 87 in the other two, respectively. The catch obtained in each haul was studied according to the commercial categories adopted by the fishermen of each area: target species, "kept" by-catch and discarded by-catch; subdividing this last fraction in "commercial" and "not commercial" species. In addition, occurrence and composition (dead shells, wood, stones, waste, ecc.) of debris was registered. In each area, number and total weight were recorded for each species as well as the size of each individual of S. vulgaris and the most important commercial species. Abundance and biomass were standardised as number of individuals/km2 and kg/km2, in order to have comparable estimations among the three areas. Analysis of variance was used to perform statistical comparisons of the total catches in weight, for each area. This analysis was also applied to evaluate differences in the weight catches of the different categories as well as of S. vulgaris and the most relevant commercial species, with the season as the main factor. Only in the northern Adriatic this analysis was also performed in function of the depth stratum. When data deviated strongly from being normally distributed, a non-parametric method (Kruskal-Wallis test) was used. Ecological Use Efficiency, Stock Use Efficiency indices and the discard (kg) produced for one kg of retained biomass were computed as different approaches to evaluate the ecological impact produced by rapido trawling. The non parametric statistic test of Kolmogorov-Smirnov was utilised for the statistical comparisons among the seasonal frequency distributions of S. vulgaris and the other relevant commercial species. The study carried out inside this project, although temporarily limited, allowed to partially fill the gap of information on the rapido trawling, drawing a first global picture of this type of fishing activity and represented a starting point for any further investigation, especially concerning landings and fishing effort, as well as the composition of the catch. The results of the observations on the commercial fishing activity evidenced that "true" rapido fishing fleets are present only in the northern Adriatic Sea, where two consistent fleets exploiting the common sole with this gear are at present located in the Rimini (9 vessels) and Ancona (10-11) harbours. On the contrary, along the western Italian coast rapido trawls were scarcely employed (two vessels at Viareggio and only one at Fiumicino), due to the reduced presence of suitable seabeds and to legislative restrictions. Clear differences were also detected between the fleets of the northern Adriatic and of the other two areas as concerns the vessel size (considerably bigger in the two Adriatic harbours in terms of length, tonnage and engine power) and the fishing activity: rapido trawlers work day and night, all year round, and for 4-5 days a week in the northern Adriatic sea, while they fish only by day, unsteadily over the year and alternating rapido trawls with otter trawl net in the eastern Ligurian and central Tyrrhenian Sea. All the four fleets exploit sand-muddy, trawlable bottoms, placed at 10-60 m depth in the northern Adriatic, 20-50 m in the eastern Ligurian and 30-75 m in the central Tyrrhenian Sea, without interference with biotopes of particular ecological interest, as biogenic hard substrata and Posidonia meadows. The total landings recorded at Ancona and Rimini were generally more than ten times higher than those obtained at Viareggio and Fiumicino, either considering the overall study period and the seasons separately. This was especially due the greater fishing effort applied in the northern Adriatic area. In fact, this difference was reduced, although it remained still consistent, when the LPUEs were considered and it further decreased in the case of the catch values computed by square kilometre. Most of the retained catch was made of species belonging to the kept by-catch at all the sites. The contribution of the main target species (S. vulgaris) to the total landings varied from 36-39% (northern Adriatic) to 12-19% (Fiumicino and Viareggio, respectively). Other relevant commercial species were Squilla mantis, Sepia officinalis everywhere, Penaeus kerathurus, Trigla lucerna, Eledone moschata, Eledone cirrhosa, Bolinus brandaris in the northern Adriatic, while Citharus linguatula and Raja asterias were important only at Viareggio and Fiumicino. The seasonal LPUE values (all the catch) showed an opposite pattern in respect to the applied fishing effort at both the Adriatic harbours, while it was more difficult to evidence a clear pattern in the eastern Ligurian and the central Tyrrhenian Seas. Conversely, both in the northern Adriatic and in the eastern Ligurian Seas the landings of the common sole appeared to be directly related with the applied fishing effort. Seasonal variations were observed, especially in the northern Adriatic area, in the catches of most of the commercial species, including the common sole; they were mainly due to their biology and the distribution pattern of their population at sea. The differences are explicable with the different wideness of the continental shelf among areas, considerably higher in the northern Adriatic Sea and so allowing a noticeable spatial and temporal partitioning of the marine organisms on the basis of the size classes and the reproductive behaviour. The rapido trawls revealed to be highly efficient both in the exploitation of the common sole and most of the other important accessory species, being the discard of these species negligible in many cases. This is due to the fact that all the specimens caught of these species are marketable, independently from the size, and also the rare individuals damaged by the gear can be easily sold, even though they have lower prices in respect to the intact ones. A noticeable discard was detected only for T. lucerna in the northern Adriatic, C. linguatula at Viareggio and Fiumicino and S. mantis in this last site. It was mainly a discard by size, especially when considerable amounts of small and often damaged specimens were caught and discarded because of their negligible commercial value. About 16% of the specimens of S. vulgaris recorded during the observations aboard at Ancona were under the minimum landing size (MLS), even though this fleet adopted a larger mesh size than the minimum legal one, while the occurrence of specimens smaller than the MLS was negligible both at Viareggio and at Fiumicino, where the used mesh sizes were smaller than those employed in the Adriatic Sea. This difference is due to the fact that in the eastern Ligurian and central Tyrrhenian Seas the common sole population is more spatially segregated, with the small individuals mostly concentrated inshore and consequently caught almost exclusively by passive gears. A high species diversity characterised the catches. During the observation onboard in the overall sampled period a total of 172 taxa was recorded at Ancona, 170 at Viareggio and 127 at Fiumicino; fishes and molluscs were the taxa with the greatest number of species at all the areas, followed by crustaceans and echinoderms. Considerable amounts of discards were recorded at all the investigated areas. The percentage of biomass discarded at Ancona was always higher than 65% of the seasonal total catch, while it was generally lower than 60% in the other two harbours. In the northern Adriatic the main species in the discards were the echinoderm Astropecten irregularis, the bivalves Corbula gibba, Anadara inaequivalvis and Ostrea edulis, followed by the crab Liocarcinus depurator and by the gastropod Aporrhais pespelecani. In the eastern Ligurian Sea the dominant species was the gastropod Turritella communis, followed by A. pespelecani, A. irregularis and L. depurator. A slight different species composition was found in the central Tyrrhenian Sea, due to the deepest depth range exploited; in fact, together with T. communis and A. irregularis, a high number of holothurians, mainly belonging to the genus Trachythyone, were found in the catches. Commercial species represented the main fraction of discard in the Adriatic, while the non commercial species dominated in the other two areas. This was especially attributable to differences in the commercial value given locally to some species, as in the case of A. pespelecani, which is marketable in the northern Adriatic markets, but not at Viareggio and Fiumicino. Debris was also very abundant at Ancona, while the amounts recorded at Viareggio and Fiumicino were notably lower. It was mainly constituted by organic matter (mainly gastropod and bivalve shells) at all the areas. Results of the present study demonstrated that, apart from fishes, most of the discarded species were still alive when rejected at sea even though they sustained different levels of damages depending on their fragility and sensitivity. Damages sustained by the crabs discarded by the Viareggio rapido trawl fishery resulted similar to those sustained by the same species discarded by beam and otter trawling, while they were lower if compared to those reported for rapido trawls used in the scallops grounds of the northern Adriatic Sea, area characterised by biogenic hard formations and by the presence of "structural species" as the bivalve Athrina fragilis. Therefore, on the basis of the results of this study, it is reasonable to suppose that the rapido trawl fishery targeting the common sole on sand-muddy bottoms is not more significantly destructive than the otter trawl, both concerning the physic and the biologic point of view. Even though the catch of some species comprised a considerable fraction of small sized specimens, it is difficult at present to draw evaluations of the real impact of this fishing activity, being still lacking evaluation studies on these resources. The only possible signal of a non-equilibrium state of exploitation emerged from this study could be seen in the opposite patterns followed by the seasonal values of the total LPUE and the applied fishing effort at the two Adriatic harbours, even though a more consistent monitoring would be necessary, in order to verify if it represents a provisory status or a proper situation of these two fleets.