Distribution and abundance of Eledone cirrhosa ( Lamarck , 1798 ) and E . moschata ( Lamarck , 1798 ) ( Cephalopoda : Octopoda ) in the Mediterranean Sea *

The two octopods of the genus Eledone, the horned octopus Eledone cirrhosa (Lamarck, 1798) and the musky octopus Eledone moschata (Lamarck, 1798) are soft bottom species found throughout the Mediterranean. E. cirrhosa is quite common also in the northeastern Atlantic, where the northern limit is located around 66-67o N and the southern limit, still uncertain, is at the level of the Moroccan coasts (Guerra, 1992). In the Mediterranean Sea, the species displays a wide bathymetric distribution, generally down to the 700 m bathymetric line, with higher occurrence within the first 300 m (Belcari and Sartor, 1999). The musky octopus E. moschata is, on the other hand, a typical Mediterranean species, limited in the Atlantic Ocean to the southern coasts of Portugal and the Gulf of Cadiz (Guerra, 1992). In the Mediterranean, it is mainly distributed from 15 to SCI. MAR., 66 (Suppl. 2): 143-155 SCIENTIA MARINA 2002


INTRODUCTION
The two octopods of the genus Eledone, the horned octopus Eledone cirrhosa (Lamarck, 1798) and the musky octopus Eledone moschata (Lamarck, 1798) are soft bottom species found throughout the Mediterranean.E. cirrhosa is quite common also in the northeastern Atlantic, where the northern limit is located around 66-67º N and the southern limit, still uncertain, is at the level of the Moroccan coasts (Guerra, 1992).In the Mediterranean Sea, the species displays a wide bathymetric distribution, generally down to the 700 m bathymetric line, with higher occurrence within the first 300 m (Belcari and Sartor, 1999).

MEDITERRANEAN MARINE DEMERSAL RESOURCES: THE MEDITS INTERNATIONAL TRAWL
The two species represent a commercially important resource in the Mediterranean basin, and are fished mainly with bottom trawl nets.Catches made by means of other gear, such as traps and setnets are of less importance.E. cirrhosa is undoubtedly one of the most important commercial species among cephalopods.For this reason, it has currently been the object of a number of studies, related mainly to its biology (see Belcari and Sartor, 1999, for a review).The existing literature on the biology of E. moschata is, on the contrary, quite limited, although the species is economically important due to its abundance along the eastern and southern coasts of the Mediterranean Sea and in the Adriatic.The only studies on this species have been performed on a small geographic scale, especially in the Gulf of Lions (Mangold-Wirz, 1963;Moriyasu, 1981;Mangold, 1983).Some information regarding the populations of the Adriatic, the southern Aegean and the Thracian seas have also been reported (Manfrin-Piccinetti and Rizzoli, 1984;Lefkaditou et al., 1998;Belcari and Sbrana, 1999;Lefkaditou et al., 2001).
The two species are generally pooled together or with Octopus vulgaris in the commercial landings and in the Mediterranean fishery statistics (Sánchez and Martín, 1993;Belcari et al., 1998;Lefkaditou et al., 2000).This makes it impossible to assess catch levels per species as an index of stock biomass.To date there is still a lack of basic information for correct management of these resources.
The present paper aims at contributing to the knowledge of distribution, abundance and demographic structure of the two Eledone species, collected by means of experimental trawl surveys carried out with a common methodology in a wide area of the Mediterranean basin.

Sampling
Six annual bottom trawl surveys that were mainly aimed at obtaining estimates of abundance indices for a series of target species were carried out from late spring to mid summer in the Mediterranean Sea from 1994 to 1999 (Bertrand et al., 2000(Bertrand et al., , 2002)).The surveys covered 40 sub-areas belonging to 15 major areas (Table 1) (see Bertrand et al., 2002); the Mediterranean coasts of Morocco were included in the project since 1999.A total of approximately 1000 hauls was made during each survey in the depth range 10-800 m, by means of a standard trawl net GOC 73 having a cod-end mesh opening of 20 mm.Selection of sampling stations was based on a depth-stratified sampling scheme, taking into account the surface area of each stratum; five depth zones were considered: 10-50, 50-100, 100-200, 200-500 and 500-800 m.Specimens were counted, weighed, measured (mantle length, ML, to the next lower 0.5 cm), sexed and assigned to a maturity stage by macroscopic analysis of the gonads.The same sampling protocol was used in all cases (see Bertrand et al, 2000Bertrand et al, , 2002, for details), for details).Mantle length measurements of E. moschata were not available for the cruises 1994-1996, due to its late inclusion in the list of target species.

Data analysis
Catch data (number and weight of octopuses collected) were analysed by means of specifically developed software (Souplet, 1996), taking into account the surface of each sub-area and depth stratum, in order to obtain estimates of abundance indices expressed in terms of both number of specimens and kg per km 2 .The abundance estimations of the two species were calculated as a mean value per year of sampling, geographic sector and bathymetric stratum.In addition, the summary statistics were estimated by sub-area and major depth stratum (shelf: 10-200; slope: 200-800 m).The above estimates indicated that the abundance of E. moschata -List of the major areas covered by the surveys.*Area 114 was not included in the analysis since only one-year data were available.A map can be found in Bertrand et al. (2000Bertrand et al. ( , 2002)).In order to examine the effects of year and major areas on the abundance and identify overall trends, the detailed relative abundance indices, i.e. catch per unit effort (CPUE) by sub-area and depth stratum, were standardised using General Linear Model (GLM) techniques (Gulland, 1956;Kimura, 1981;Hilborn and Walters, 1992).CPUE indices were expressed in g/km 2 and the general model used was of the form: where: µ: overall mean A i : effect of major area i Y j : effect of year j D k : effect of depth stratum k interaction: any possible combination of interaction between two effects ε: error term assumed to be distributed normally The constant 1 was added to all CPUE rates to account for the zero observations.The coefficient of determination (r 2 ) was considered to express the goodness of fit of the model with the data.All statistical inferences were based on the 95% confidence level.
The demographic structure of the two species was studied by computing the size frequency distributions for every major area; all the years were pooled, since the cruises were always performed at the same period of the year.Length frequency distributions in major areas were compared by means of the Kolmogorov-Smirnov test.Variations in the size distribution patterns of E. cirrhosa between shelf and slope were also examined.

Abundance indices
From the examination of the all sectors investigated, it appears that E. cirrhosa was present in the whole western basin; in the central basin the species was fairly sparse, increasing again in the eastern Mediterranean, in the Argosaronikos and northern Aegean Sea (Tables 2 and 3).The species showed a wide depth distribution, the largest catches occurred generally from 50 to 500 m.E. moschata was sparse or absent in some geographic sectors, such as the northern Ligurian Sea, north-eastern Corsica, Ionian Sea and Morocco, although in the latter case only data pertaining to the 1999 survey were available (Tables 4 and 5).In contrast, in other sub-areas, catches were comparable to the maximum amounts reached by the closely related species E. cirrhosa, and vastly exceeded these amounts in the northern Adriatic Sea off the Slovenian coasts in 1995, reaching the values of 2,396 individuals per km 2 and 336.1 kg per km 2 .The species was mostly restricted to within 100 m; below 200 m catches were scarce, and only occasional at further depths.
From the analysis carried out for the major areas, the computed r 2 indicated that the adopted GLM models explained a large proportion of the variance for both species (Table 6).The Analysis of Variance (ANOVA) for the applied GLM model, indicated that differences among major areas, depth strata and their interaction were always statistically significant.In the case of E. cirrhosa, differences among years and the year-major area interaction were also significant.Overall, yearly fluctuations do not seem to be important and this is particularly true for E. moschata (Fig. 1).The spatial distribution of both species followed a rather complex pattern and there were noticeable fluctuations among areas, particularly for E. moschata (Fig. 2).In some areas, such as the northern Tyrrhenian seas, the northern Adriatic and the southern Aegean, there appeared to be an inverse relationship regarding the abundance of the two Eledone species.In other areas, both species showed high abundance, as in the Gulf of Lions.E. cirrhosa was more abundant in the western Mediterranean and its highest index was computed for the Gulf of Lions.In this area a high abundance of E. moschata was also observed.Both species were found in relatively lower abundance in the southern Adriatic and Ionian seas.The highest abundance of E. moschata was observed in the southern Aegean Sea.
The standardised abundance indices by depth stratum shown in Figure 3, confirm the narrower depth distribution of E. moschata.The highest indices were computed for depths up to 100 m.In contrast, E. cirrhosa was found at all depths but it was more abundant in the 50-500 m depth strata.

Size structure
The distribution of the annual length frequencies by sub-area was far from normal in most of the cases.
Two cohorts were generally present in E. cirrhosa catches (Figs. 4 and 5).Small individuals (ML: 1-5 cm) were present in the entire study area except from the southern Aegean Sea, where recruitment was sparsely observed during the sampling season.Recruits with modal lengths from 2 to 4 cm constituted the dominant mode in the Gulf of Lions,  Catches obtained in the Gulf of Lions, northern Adriatic Sea and southern Aegean Sea, showed a relatively restricted size range.In other macro-areas, size range proved to be wider (ML:1-19 cm) and it was more difficult to single out only one predominant size.The modal mantle length varied considerably between the areas, presenting the lowest values (3-5 cm) in Aegean, Ligurian and northern Tyrrhenian seas.
The length structures of the populations of both eledonids during the MEDITS sampling season were found significantly different between the major areas (P< 0.05).

DISCUSSION
The MEDITS project allowed data to be collected on the two octopuses E. cirrhosa and E. moschata in a wide area of the Mediterranean, providing useful information on distribution, demographic structure and abundance from areas not yet widely investigated.Comparisons between the different areas were allowed by the common methodology used in the surveys.The two species showed a wide geographical distribution, as they were collected in all the major areas investigated although notable differences were evidenced among the 40 geographic sectors.The pattern of spatial distribution confirmed data already reported in the literature, E. cirrhosa being present in a wide bathymetric range, up to the 800 m isobath, with recruits concentrating mainly on the shelf, and E. moschata mostly restricted to within 200 m, but sometimes found at greater depths, as in the Aegean Sea (Boyle, 1983;Mangold, 1983;Würtz et al., 1992;Belcari and Sartor, 1993;D' Onghia et al., 1995;Sánchez et al., 1998).
The highest densities of E. cirrhosa were found in the Gulf of Lions, in the Ligurian and northern Tyrrhenian seas and in the northern Aegean Sea.The latter finding is of interest since the presence of E. cirrhosa has only recently been reported in this area (Tursi et al., 1995): until just a few years ago it was considered uncertain because of the small number of studies conducted there (Mangold and Boletzky, 1987).Biomass estimates available for the western coasts of Italy (Würtz et al., 1992) corroborate the present finding that the main biomass of the species in Italian waters is concentrated in the eastern Ligurian and northern Tyrrhenian seas.
The present study highlights a decreasing trend of the overall annual abundance of E. cirrhosa, while E. moschata abundance seems much more stable.Considering that the official landed catch statistics generally pool the two species together or with Octopus vulgaris (Worms, 1979;Sánchez and Martín, 1993;Belcari et al., 1998;Quetglas et al., 1998), comparisons with previous data are possible only in the few areas investigated by means of experimental surveys or observations on commercial landings.In the northern Tyrrhenian Sea, a decreasing trend in experimental catches of E. cirrhosa was noticed (Biagi et al., 1998), while from the evaluation of landings in the Ligurian Sea, Relini et al. (1998) reported an increase in catches per unit effort.
Despite the economic importance of E. moschata, especially on the southern and eastern coasts of the Mediterranean and in the Adriatic Sea (Mangold, 1983), few fishery surveys and analysis of landings are available.Investigations reporting a slight decrease of annual catches date back to several years ago (Gamulin-Brida, 1963;Moriyasu, 1981).
The present results confirm the discontinuous pattern in the catches presented by both octopods.Apart from environmental conditions that may markedly affect the stocks, annual fluctuations in abundance should be related to the peculiarities of species dynamics and to fishing strategies, that take advantage of these characteristics.
Because of the short life span, rapid population turnover and reproductive period, which displays seasonal sexual maturity and post-reproductive mortality (Mangold-Wirz, 1963;Boyle, 1983), marked differences in the demographic structure of the population sampled in different seasons or different areas can be detected.Size frequency distributions of E. cirrhosa from MEDITS surveys, clearly show two components in almost all areas, even if with different amplitude, corresponding to the two cohorts of trawl net recruits and adults, respectively.The exception of the southern Aegean Sea is not easily interpretable: the lack of recruits could be due to the scarcity of catches or to the postponement of the reproductive period.In the Mediterranean, sexual maturity occurs generally earlier in the western basin (spring-summer) than in the eastern basin (summer-autumn) (Belcari et al., 1990;Sánchez and Martín, 1993;Lefkaditou and Papacostantinou, 1995;Tursi et al., 1995;Agnesi et al., 1998).
The reproductive period of E. moschata investigated by Mangold-Wirz (1963) in the Banyuls population (Gulf of Lions) seems to extend from winter until spring, with the disappearance of large size individuals from the catch in April-May (Mangold-Wirz, 1963).Experimental research cruises conducted in the Adriatic in May (Manfrin-Piccinetti and Rizzoli, 1984) still show the contemporary presence of two age classes: young individuals and adults.MEDITS samplings, carried out from May onwards, highlight the presence of one age class in most of the investigated areas.However, the prolonged spawning season and long brooding period could explain the wide size range detected in some areas, and support the existence of cohorts with different growth rates depending on the hatching period, as hypothesised by Mangold (1983), who proposed a model of alternating short-long cycles which would be very flexible according to environmental conditions.
FIG. 1. -Standardised CPUE estimates (marginal means) for Eledone cirrhosa and Eledone moschata by year.Vertical lines indicate the 95% confidence intervals of the corresponding estimates.

FIG. 2
FIG. 2. -Standardised CPUE estimates (marginal means) for Eledone cirrhosa and Eledone moschata by area.Vertical lines indicate the 95% confidence intervals of the corresponding estimates.Areacodes correspond to the area names mentioned in Table1.

TABLE 2 .
-Eledone cirrhosa: mean abundance (in number of individuals/km 2 ) estimated from the MEDITS trawl surveys by depth stratum, geographical sector and year(1994 -1999).Not sampled strata are indicated by *.Values higher than 300 individuals/km 2 are presented in bold.

TABLE 5 .
-Eledone moschata: mean biomass (in kg/km 2 ) estimated from the MEDITS trawl surveys by depth stratum, geographical sector and year(1994 -1999).Not sampled strata are indicated by *.Values higher than 20 kg/km 2 are presented in bold.

TABLE 6 .
-ANOVA for the General Linear Models fitted to the Catch per Unit Effort (g/km 2 ) indices.