Gonçalves, J.M.S. Fisheries biology and population dynamics of Diplodus vulgaris (Geoffr.) and Spondyliosoma cantharus (L.) (Pisces, Sparidae) from the Southwest coast of Portugal.

Species of the family Sparidae occupy an important place in the fishing activity of the southwest coast of Portugal, and are the main group supporting the artisanal fisheries of the region. The objective of this work was to study aspects of the biology of two sea breams, Diplodus vulgaris (the two-banded sea bream) and Spondyliosoma cantharus (the black sea bream), namely feeding ecology, reproduction, age and growth, as well as to estimate the selectivity parameters of the most important gears used to catch these species and to carry out a preliminary evaluation of the state of the stocks of the two sea breams. To fulfil these objectives the landings in the Sagres and Sines fish auction halls were sampled from July 1992 to March 1994; laboratory studies were carried out using samples obtained from Sagres between December 1992 and March 1994, and a comparative selectivity study of gill nets and longlines was carried out in 1997/1998 with 40 experimental fishing trials with each gear.

The feeding habits were studied based on the analysis of stomach contents, using the three basic methods of evaluating diet composition, five ‘combination’ indices and three proposed indices. Based on the comparison of the different indices it was concluded that the Index of Relative Importance (IRI) along with the Weighted Feeding Index (WFI) were the most suitable for describing the diets of these species. The percentages of empty stomachs were high, largely due to the characteristics of the fishing gear. The diet of the two-banded sea bream consisted essentially of ophiuroids, polychaetes, amphipods and echinoids, while the black sea bream was found to consume primarily polychaetes, amphipods and hydrozoans. The two diets were significantly different, largely because of the greater consumption of echinoderms in the case of the two-banded sea bream. The structure and the dimensions of the mouths of these sea breams are distinctly different, influencing their trophic behaviour. As with the majority of the sea breams, D. vulgaris and S. cantharus are benthic species that are characterised by a generalist feeding strategy, strongly influenced by prey availability in their habitat.

Analysis of the reproductive cycle showed that both species had extensive spawning period: from December to March with a peak in intensity in January/February for Diplodus vulgaris, and from February to April with a peak in March for S. cantharus. No significant differences in the sex ratios were found along the year and for different size classes in the case of D. vulgaris, while female S. cantharus were more abundant throughout the year and in the smaller size classes. Size at first maturity (L50%) for both sexes as well as indeterminate individuals of D. vulgaris was 18.33 cm, with no significant difference between males and females. For S. cantharus, the size at maturity based on individuals of both sexes as well as those whose sex could not be determined was 20.10 cm, with significant differences between males (22.41 cm TL) and females (19.98 cm TL). The estimated sizes at first maturity were considerably greater than the minimum legal landing size of 15.0 cm for D. vulgaris in Portugal, and slightly smaller in the case of S. cantharus (23 cm). The mean absolute fecundity (Fa) and the relative fecundity (Fr) for D. vulgaris and S. cantharus were: Fa = 131127 eggs, Fr = 526 eggs/g, Fa = 61396 eggs and Fr = 346 eggs/g respectively. The relationships between absolute fecundity and length (TL) and between absolute fecundity and somatic weight (SW) were the following: D. vulgaris: Fa = 0.1853TL4.1903; Fa = 57.715SW1.4067; S. cantharus: Fa = 436.27TL1.5747; Fa = 2979.7SW0.585. The reproductive strategies of these two species can be characterised as hermaphroditic, rudimentary, and potentially protandric in D. vulgaris and protogynic in S. cantharus.

The determination of age was based on the analysis of hard structures; otoliths in the case of D. vulgaris and otoliths and scales for S. cantharus. The length-at-age data was well described by the Von Bertalanffy model. Estimation of the growth parameters was carried out by non-linear fitting procedures using pairs of length-age, with age corrected by date of capture. In addition to the greater statistical validity of this method of estimation, the parameters obtained were judged to be the most realistic: D. vulgaris (otoliths): L¥ = 28.1 cm K=0.30 yr-1 e t0= -1.618 yr (validity: 12.5-30.5 cm; 1-10 years); S. cantharus(scales): L¥ =35 cm, K=0.32 yr-1 e t0=-0.481 yr (validity: 14.3-33.5 cm; 1-9 years). Length frequency analysis resulted in the most realistic estimates of L¥ : L¥ =39.6 cm, K=0.32 yr-1 e t0=-0.481 yr (D. vulgaris); L¥ =40.0 cm, K=0.24 yr-1 e t0=-0.646 yr (S. cantharus). The weight-length relationships (W=a Lb) for D. vulgaris and S. cantharus from the south-west coast were defined by the following parameters: a=0.0223 and b= 2.895, r2=0.89 (D. vulgaris); a=0.0106 and b= 3.085, r2= 0.89 (S. cantharus). Natural mortality (M) values, estimated empirically, were 0.39 and 0.30 yr-1, respectively for D. vulgaris and S. cantharus. A preliminary estimate of M for Sparidae can be obtained by the following multiple linear regression: M = -0.162 + 1.714K + 0.00273L¥ (r2=0.77). Catch curve analysis was used to estimate total mortality (Z): D. vulgaris: Z=0.642 (otoliths) e 0.727 yr-1 (length frequency analysis); S. cantharus: Z=0.676 (scales) and 0.576 yr-1 (length frequency analysis).

The most important gears in this region for the two species are longlines and gill nets, and to a lesser extent the purse seine. The "diverse sparidae" group, that includes the black sea bream, and which should be discriminated by species, constitute the only category of fisheries and sea bream resources that shows a negative trend in landings from 1987 to 1998. The demographic structure of the landings of the two-banded sea bream consists of 86.9% of individuals between 20.5 and 27.5 cm (2 to 6 years of age), while for the black sea bream, 85.0% of the individuals are between 21.5 and 27.5 cm (2 to 6 years of age).

In the comparative selectivity study 4 hook sizes (nº11, 12, 13 and 15) and mesh sizes (80, 70, 60 and 50mm) were used. While D. vulgaris and S. cantharus were two of the most important species in the longline catches, their contribution was less in the gill net catches. In general, there was a decrease in catch rate with increasing hook size, and the intermediate mesh sizes were relatively more efficient. Although longline catch size frequency distributions were highly overlapped, those for gill nets were clearly separated for both species. The logistic and the normal models were used to respectively describe the size selectivity of hooks and gill nets for both species. With regards to the minimal landing size and taking into account the sizes at maturity and the demographic structure of the catches, mesh sizes 70 and 80 mm are the most appropriate for D. vulgaris and S. cantharus respectively. Hook number 13 is the most suitable for D. vulgaris, given that the use of the smaller number 15 hook is not very practical. In the case of S. cantharus, catches with the number 11 hook had the smallest proportion of illegal size individuals.

Results of yield per recruit analysis suggest that for the longline, the two-banded sea bream is moderately exploited, at levels below the maximum exploitation rate (Emax). In contrast, the black sea bream appears to be intensely exploited, at levels close to the maximum sustainable. Alterations in exploitation rates and/or size at first capture should take into consideration the multi-species nature of this fishery. With regards to the management of these fishery resources, the emphasis should be on the control of fishing effort, in order to avoid over-exploitation, and the revision of minimum landing sizes. The establishment of marine reserves and the deployment of artificial reefs may contribute to sustainable exploitation and conservation of biodiversity.