SCI. MAR., 66 (Suppl. 2): 233-242 SCIENTIA MARINA 2002 MEDITERRANEAN MARINE DEMERSAL RESOURCES: THE MEDITS INTERNATIONAL TRAWL SURVEY (1994-1999). P. ABELLÓ, J.A. BERTRAND, L. GIL DE SOLA, C. PAPACONSTANTINOU, G. RELINI and A. SOUPLET (eds.) Analysis of demersal assemblages off the Tuscany and Latium coasts (north-western Mediterranean)* FRANCO BIAGI1, PAOLO SARTOR2, GIAN DOMENICO ARDIZZONE3, PAOLA BELCARI1, ANDREA BELLUSCIO3 and FABRIZIO SERENA4 1 Dipartimento di Scienze dell’Uomo e dell’Ambiente, Università di Pisa, Via Volta 6, 56126 Pisa, Italy. E-mail: [email protected] 2 Centro Interuniversitario di Biologia Marina ed Ecologia Applicata, Livorno, Italy. 3 Dipartimento di Biologia Animale e dell’Uomo, Università di Roma “La Sapienza”, Roma, Italy. 4 Agenzia Regionale per l’Ambiente Toscana, Livorno, Italy. SUMMARY: A four-year time series (1994-1997) of groundfish trawl surveys performed within the European Union Pro- ject “MEDITS” (Mediterranean International Trawl Surveys), was analysed to identify and describe the fish assemblages along the continental shelf and slope of Tuscany and Latium (Italy), in the north-western Mediterranean. Cluster analysis was used to group samples with similar species composition in terms of abundance, biomass and frequency of occurrence. Results allowed the identification of four to five broad assemblages along the depth gradient: a strictly coastal group (< 50 m depth), two groups in the upper and lower part of the continental shelf (essentially 50-200 m), an epibathyal group (200- 450 m) and a group derived from hauls made at depths greater than 450 m. Each assemblage corresponded to a faunistic association with relatively homogeneous and persistent species composition, biomass and density indices. Key words: assemblages, cluster analysis, demersal resources, western Mediterranean INTRODUCTION Classical stock assessment tools indicate in most cases a general state of overexploitation and overca- The fauna of the Mediterranean basin is charac- pacity of the fishing fleets (Caddy and Sharp, 1986). terised by a high biodiversity, i.e. by a high number However, monospecific approaches are usually the of species, some endemic to the area, some with a rule. In the last 15 years there has been a growing wider geographical distribution and others recently perception that it is often ineffective to base fisheries immigrating into the Mediterranean waters (Mau- management policies exclusively on single species rin, 1962; Tortonese, 1987; Fredj et al., 1992). This approaches. High levels of variability in productivity, aspect is mainly due to the geological evolution of in abundance, in size and species composition, are the basin and to its ecological and climatic fea- not predictable from single species models. Howev- tures. The existence of such biological diversity er, problems can arise when choosing management certainly contributes to the health of the Mediter- options in a multispecies fishery context (Caddy and ranean fisheries. Sharp, 1986; Brugge and Holden, 1991). A more holistic approach to resource manage- *Received Novemver 7, 2000. Accepted February 14, 2002. ment is evidently needed. In practice, management DEMERSAL ASSEMBLAGES IN TUSCANY AND LATIUM 233 should be based on ecosystem principles: to main- MATERIAL AND METHODS tain the full ensemble of species while sustaining the harvest of desired species (Tyler et al., 1982; Tyler, This study was carried out within the framework 1988). Overexploitation alters not only the size of of the Mediterranean International Trawl Surveys stocks but also the ecosystems through the succes- Programme (“MEDITS”) (Bertrand et al., 2000, sive removal of large long-lived species and large- 2002). Four experimental bottom trawl surveys were sized specimens to the point where trophic linkages, carried out in the years 1994-1997, in the spring- biodiversity and resilience have all been lowered summer period (essentially from June to July). The (Caddy and Sharp, 1986). investigated area encompassed 36,536 km2, com- The aim of this work is to analyse the structure prising the Italian coasts along Tuscany and Latium of demersal assemblages (finfish, crustaceans and (Ligurian and Tyrrhenian seas). This area was sub- cephalopods), on a temporal and spatial scale and divided into three sub-areas: from Magra River to to specify the set of species characterising each Elba Island (sub-area 1), from Elba to Giannutri group and those species responsible for sample Island (sub-area 2) and from Giannutri to Garigliano groupings. From this perspective, data arising River (sub-area 3) (Fig. 1). from “MEDITS” trawl surveys carried out in the In the first survey, each sub-area was fully cov- years 1994-1997 along the Tuscany and Latium ered on a random stratified sampling basis according coasts, were analysed. Some previous information to the following depth strata: stratum A: 10-50 m; about fish assemblages in this area has been stratum B: 50-100 m; stratum C: 100-200 m; stratum analysed by Biagi et al. (1989) and Abella and D: 200-500 m; stratum E: 500-800 m. The allocation Serena (1995a, b). of hauls was proportional to the surface of each FIG. 1. – Map of the study area with the sampling stations. 234 F. BIAGI et al. bathymetric stratum. The same hauls were replicat- RESULTS ed in successive surveys. Globally, 129 hauls were made in day time hours in each survey, from 12 to During the four surveys, a total of 253 species 751 m of mean depth, distributed as follows accord- (32 Cephalopoda, 47 Crustacea, 21 Condrichthyes ing to the different depth strata: 18 hauls in stratum and 153 Osteichthyes) were collected (Table 1). Out A, 18 in stratum B, 36 in stratum C, 33 in stratum D, of these, 165 species (65.6%) occurred in all the and 24 in stratum E (Fig. 1). three sub-areas (71.0%, 68.1%, 52.4% and 63.8%, Haul duration was 30 minutes at depths shallow- respectively for cephalopods, crustaceans, cartilagi- er than 200 m and one hour at depths greater than nous and bony fishes). A total of 194 species (12 200 m. All the surveys were carried out with the exclusive) was collected in sub-area 1, 212 in sub- same otter bottom trawler and the same type of trawl area 2 (15 exclusive) and 209 in sub-area 3 (9 exclu- net, having a cod end with stretched mesh sizes of sive). 10 mm (knot to knot). Further information on the The number of species caught in each haul sampling design and on the characteristics of the ranged from a minimum of 10 to a maximum of 44. gear is available in Bertrand et al. (2000, 2002). However, no evident bathymetric trend in number of The species composition of the different taxo- species by haul was observed. Over the four years, coenosis (finfish, crustaceans and cephalopods) was the mean number of species per haul ranged studied by analysing abundance and biomass data between 20 and 40. coming from each survey. Raw catch data (biomass Four major clusters of stations recurrently and number of specimens collected) of each haul appeared in the dendrograms calculated for each were converted into abundance (number of speci- survey (Figs. 2 and 3). Each cluster was closely mens/km2) and biomass indices (kg/km2), utilising a associated with bottom depths. Actually, it was pos- specifically developed software (Souplet, 1996). In sible to identify a group composed of strictly coastal order to identify faunistic associations, cluster hauls down to 50 m depth, another group derived analysis was applied to the abundance and biomass from most of the hauls carried out on the continen- species-stations matrices of each survey; occasional tal shelf (essentially between 60 and 220 m depth), species (those caught only once time in each year) a third cluster based on the hauls made along the were not considered in this analysis. In addition, the upper part of the continental slope (from 200 to 450 strictly pelagic species (such as anchovy, Engraulis m depth) and the last group from the deepest hauls encrasicolus, and sardine, Sardina pilchardus) were (mostly deeper than 450-500 m). Regarding the con- eliminated from the analysis because, in most cases, tinental shelf group, a further subdivision into two they were assumed to be captured when the trawl sub-groups could be done: a shallower group, with gear was not sampling at the bottom; this could lead most hauls taken at depths of less than 130 m, and to some distortion in grouping the sampling into dif- another with the majority of hauls made at greater ferent assemblages. depths. Because the survey in 1994 was mostly aimed at Cluster structure (regarding both biomass and standardising and calibrating the sampling gear, abundance) remained essentially the same in the cluster analysis was performed using the data sets of three years: about 90% of hauls was assigned to the the years 1995-1997. To reduce variability and same clusters over the studied period when four skewness, data transformations were applied: dou- groups were retained, about 85% when five groups ble square root for the abundance data and log(x+1) were considered (Table 2). for the biomass data. Although many species occurred in more than Similarity levels between the stations were cal- one group, each faunistic association was charac- culated through the Bray-Curtis index (Clifford and terised by its own extent of species composition and Stephenson, 1975). A group average fusion strategy, catch rates (abundance and biomass) which distin- UPGMA, was used as clustering algorithm (Sneath guished them from each other. Moreover, these and Sokal, 1973). Species composition of each assemblages remained substantially unchanged over assemblage was described in terms of percentage the study period for both data sets (abundance and contribution of each species at the similarity level of biomass) (Table 3). each group. The above mentioned analyses were The coastal group was mostly characterised by computed with the PRIMER statistical package (rel.