Macroscopic scale of gonadal stages and size at first maturity of Zygochlamys patagonica (King & Broderip, 1932) in Management Unit 2, SW Atlantic

SILVANA CAMPODÓNICO; MARIO LASTA,; BETINA JUDITH LOMOVASKY; MACCHI, G.

Halifax, Canada

Congreso; 16th International Pectinid Workshop; 2007

One of the basic goals of fisheries management is to conserve sufficient reproductive potential of a stock to allow a sustainable exploitation supported by the knowledge of reproductive variables such as size at first maturity and extension of spawning period. The Patagonian scallop Zygochlamys patagonica inhabit the SE Pacific extending from Chiloé Island (42ºS) to Cape Horn (55º56’S), and the south-western Atlantic northwards to off the Río de la Plata estuary (35º50’S) overlapping with the Shelf-break Frontal System at the 100 m isobath. The Patagonian scallop fishery is conducted since 1996 in the SW Atlantic. Due to the commercial importance of this scallop, and in view of importance of increase the knowledge about reproductive characteristics of this specie, the main objectives of this study were to: (a) describe a macroscopic scale of gonadal stages and (b) determinate the size at first maturity. Scallops were collected in a closed fishing area within Reclutas Bed (39°20’S - 56°W and 39°30’S - 55°52’W, 1085 km2, SW Atlantic Ocean). Samples were obtained monthly between November 2000 and February 2001. Specimens (n=864) were grouped in 1 mm size classes and classified histologically as mature (with oocytes or sperm present in the follicle) or immature (with scantly and small alveoli containing few sexual cells). To estimate shell height (SH) in which 50 % of the specimens had already reached the ripe phase (L50), a logistic model was fitted to the proportion of mature individuals by SH class using the maximum likelihood method. Coefficients of the regression obtained for males and females were compared using a Chi2 test. A macroscopic scale of gonadal stages was determined mainly based on color, external appearance and size of testes and ovaries. Samples were collected in early March 2001.  Specimens (n = 247) were shucked and gonads were examined macroscopically for determination of sexual maturity. Macroscopic appearance observed was validated using a histological scale of maturity. A macroscopic gonadal scale of  five stages for both sexes was defined: I. Juvenile – Early maturation: small gonads, transparent or colorless, with an angular and flattened appearance. II. Advanced maturation: gonads near to maximum size, testis is white or cream and the ovary is orange or bright orange. III. Ripe: gonads larger than other body organs (is the most conspicuous organ in the scallop), it is rounded, and the surface smooth and glossy. Intense coloration: testis are white or cream; the ovary is bright orange. IV. Spawning: gonads reduced in size and collapsed, with empty areas. Testis is white or practically colorless  and the ovary is amber or dull orange. V. Spent: gonads flaccid. Testis containing sex cells cloudy white, rest transparent. The ovary is amber or pale orange, or transparent and colorless. It is possible to discern the intestinal loop within them. In some cases, sexes are not differentiates. The size at first maturity was 36.63 mm SH for males and 36.31 mm SH for females corresponding to 2-3 years old. No significant differences were observed between both sexes (p > 0.05). The smallest male and female with active gonads corresponded to 25 and 32 mm SH, respectively. Several authors have defined macroscopic gonadal scales of scalops with five or more stages, depending on the possibilities of reaching an accurate identification of each stage of development. Regarding macroscopical differences, sometimes it is difficult to differentiate clearly between the termination of one stage and the beginning of another. Therefore, to gain accuracy in the classification, we defined a macroscopical scale of five gonadal stages clearly differentiable. We observed that the stage II, where the gonads are clearly mature is reached around 36 mm SH (L50). Previous studies reported the size of first maturity of Z. patagonica from a wide latitudinal range (40º-54ºS) showing a value of 45 mm SH. The differences with the present study could be attributed, however, to the different geographic scales of analysis. It is know that the reproductive cycle in marine bivalves shows high intraspecific variations, moreover, in scallops with a broad geographic distribution, many authors have mentioned the important role that latitudinal variations play in their reproductive cycles. Our work is restricted to micro or small scale (Reclutas Bed) where population processes (trophic or reproductive) correspond to the neighborhoods of individuals suggesting that the spatial scale is important to define the reproductive characteristic of the species.