Structure of the populations and assessment of the biomass of the coastal demersal resources in the Golfo di Castellammare

D'Anna, G. and Badalamenti, F. and Pipitone, C. and Patti, B. and Cuttitta, A. and Lipari, R. and Coppola, M. and Di Stefano, G. and Modica, D. and Perrone, V. (1994) Structure of the populations and assessment of the biomass of the coastal demersal resources in the Golfo di Castellammare. Project Report. CNR-IRMA, Castellammare del Golfo. (Unpublished)

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Our research, carried out in the continental shelf of the Gulf of Castellammare (N/W Sicily), aimed at supplying new data regarding the assessment of the abundance and biomass of the main demersal commercial species, and at depicting the nekton assemblages structure. Other important aims (sub-tasks) were to detect the effects of the trawling ban four years after its deployment, to identify areas important from a biological point of view and to supply general management advice. The study, involving both trawlable and non-trawlable areas, used trawl surveys for the first ones, and trammel-gill net (on sandy and rocky bottoms inside the 16-25 m bathymetric range), beach seine (on sandy bottoms from 0 to about 10 m) and visual census methods (on rocky, sandy and Posidonia oceanica areas from 0 to 25 m) for the non-trawlable areas. Four seasonal trawl surveys, based on a stratified random sampling design, were carried out from autumn 1993 to summer 1994. Three bathymetric strata were adopted, each of which was subdivided in elementary sampling units (ESUs). 245 diurnal and nocturnal hauls of 30 minutes were made. In order to estimate the biomass of the demersal resources the swept-area method was used. Biomass was expressed as tons in the whole sampled area (about 57 square nautical miles). The catch per unit effort (CPUE as gr/haul) of each species caught was calculated. Twelve target species were selected taking into account both their commercial importance and abundance. For these species, some reproductive aspects, the length frequencies distribution, the length-weigth relationship, the growth parameters, the istantaneous total mortality rate were calculated. Moreover, maps illustrating the distribution of the biomass indices of the target species were drawn. Monthly diurnal and nocturnal trammel-gill net samplings were carried out for one year on hard and soft bottom areas. Seasonal maps of the CPUE (gr/500m of net/12h), based on the diurnal and nocturnal samples obtained in each area were drawn. Qualitative samples were monthly collected for one year with a beach seine in the shallow sandy bottoms of the central area of the Gulf. Two visual census techniques were adopted to obtain furher seasonal data on the fish assemblages of the non-trawlable areas on different subtrates (i.e., rocky bottoms, sandy bottoms and artificial reefs). The average values of density and biomass were estimated. Community structure indices were elaborated for each sampling method. Multivariate analyses were also computed. The principal results from the trawlable areas can be summarized as follows. Considering the overall catch, neither significant differences were found among the four seasons nor between diurnal and nocturnal catches. Anyway, the highest CPUE calculated as average value per stratum, was obtained in the diurnal winter samples of stratum B (50983 gr/haul) and the lowest in the nocturnal autumn samples of stratum C (14616 gr/haul). In general diurnal catches were larger than nocturnal. The yields per stratum were in many cases significantly different between each other. The red mullet (Mullus barbatus) was nearly always the dominant species in every stratum in the diurnal hauls, as well as in stratum A (except in winter) in the nocturnal ones. The highest yield for this species was recorded in the winter samples from the stratum A in the day (18559 gr/haul). The dominant species in stratum B and C at night was the picarel (Spicara flexuosa), with the highest values recorded in summer (16440 gr/haul) in stratum B and in autumn (2919 gr/haul) in stratum C. Considering the demersal trawlable fauna as a whole, the outer belt of stratum A and the whole stratum B were the richest zones inside the study area. The biomass estimates of the twelve target species showed that the red mullet was by far the most abundant species (ranging from 34.08 t in summer at night to 170.34 t in winter in the day), followed by hake (from 5.81 t is summer at night to 62.58 t in winter in the day), pandora (from 6.93 t in summer at night to 24.94 t in wintwr in the day) and striped seabream (from 0.47 t in in summer in the day to 10.90 t in autumn at night). The comparison between the CPUE of experimental trawl surveys carried out before the trawling ban and our data showed that the commercial demersal resources of the Gulf underwent a mean seven-fold increase. In particular the red mullet yields increased 33 times (mean value on the whole study area). A total of 171 taxa were collected in the four surveys. In general the highest values of specific richness and diversity were recorded in stratum C at night in winter, while the maximum number of specimens was attained in winter in the diurnal samples of stratum B. The multivariate analyses separated six areas characterized by different nekton assemblages, which could be seen as management units. Nocturnal samples from the trammel-gill net surveys were always significantly higher than the diurnal ones. The highest yield was recorded on the western rocky bottom, where an average CPUE of 2895 gr was attained, calculated on an annual basis. The lowest values were recorded on the soft substrata in the central area (1149 gr), while the artificial iii reef area showed intermediate values (2811 gr). On the whole sampled area, cuttlefish attained 25% of the total catch on an annual basis, followed by the annular seabream (19.4%). In the western rocky bottoms the most important species were the scorpionfishes, in the artificial reef the annular seabream and the red mullet and in the soft bottom areas the wide-eyed flounder and the striped seabream. Trammel-gill net surveys displayed a significantly higher species richness and abundance in number of specimens in the nocturnal than in the diurnal samples. From the visual census surveys damselfish, rainbow wrasse and swallowtail seaperch were the most abundant species. Sparids (common two-banded seabream, annular seabream) and serranids (comber and painted comber) were the most frequent and abundant commercial species observed. The highest biomass (gr/m³) was reached in summer on the artificial reef (52.7), while the lowest was recorded on the sandy bottoms in autumn (0) and in spring (0.19). The highest density (number of specimens/m³) was found in summer on the artificial reef area (2.85) and the lowest in autumn (0) and spring (0.01) on the sandy bottoms. The community structure indices of the rocky and artificial reef areas were significantly higher than those of the soft bottom areas. The FCA put into evidence a spatial gradient from the rocky to the sandy bottom fish assemblages. The artificial reef area was plotted just in the middle of the geometrical model. The beach seine surveys highlighted the role of the shallow sandy bottoms as a nursery area for several species. The nekton assemblages of the four sampled sites were very similar among each other, whereas a strong seasonality was evident by the cluster analysis results. These data along with those of the trawl surveys showed that the sandy and muddy bottom areas located between 1 and 40 m are a nursery ground for several important species: red and striped mullet, pandora, striped seabream and several sparids and mugilids

Item Type: Monograph (Project Report)
Uncontrolled Keywords: Beach seine, Biomass, Continental shelf, Demersal fauna, Fish assemblages, Mediterranean Sea, Multivariate analysis, Sicily, Trammel net, Trawl survey, Visual census
Subjects: 500 Scienze naturali e Matematica > 570 Scienze della vita; Biologia > 577.7 Ecologia marina
600 Tecnologia - Scienze applicate > 630 Agricoltura e tecnologie connesse > 639 Caccia, pesca, tutela delle risorse biologiche, tecnologie connesse
Depositing User: Carlo Pipitone
Date Deposited: 18 May 2023 16:15
Last Modified: 18 May 2023 16:31

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