Exploring the causes of differences in growth rate of the Patagonian scallop Zygochlamys patagonica along its commercial bed distribution in the SW Atlantic

BETINA J. LOMOVASKY; BALDONI, A.; RIBEIRO, P.; ALVAREZ, G.I.; LASTA M.,; CAMPODÓNICO S.,; IRIBARNE O.

Congreso; 19th International Pectinid Workshop; 2013

The maximum age of Z. patagonica varies between 19 and 25 years old across different beds. Our study provides evidence of growth patterns variation in its SW Atlantic distribution range, where an increase in H from the fishing Management Unit (MU) 3 (39°37´S) to MU 6 (42°15´S) was observed with similar results between the rest of the beds with exception of MU 10 (44°45´S) and Tres Puntas bed (46°47´S) (Table 1) which could not be compared given the lack of individuals younger than 6 years old. The k parameter decrease to the South with difference between MU 3 and MU 6, showing similar tendency when compared beds at the south and north of 39º30´S. Individuals from Tres Puntas bed that are influenced by coastal waters showed the similar trend than the North beds with lower Hoo and higher k. The beds analyzed in this study (from MU 3 to MU 10) are located at the Shelf-Break Frontal System which represent the transition between the Malvinas Current, advecting northward nutrient rich Subantartic waters, and the diluted Subantartic waters of the continental shelf, strongly governed by a seasonal cycle of thermocline formation and breakdown which showed a gradual decrease of seasonal stratification with increasing of latitude (MU 3 to MU 10). The sea bottom temperature showed that the maximum values were similar between beds (MU 3 to MU 10), but distinct timing was observed across beds between one-two months for MU 3 and MU 10, respectively, and between four months for MU 6 to MU 9. The Shelf-Break Frontal System presents a band of high Chl a concentration and is associated with several biological aggregations including scallops beds. When we observed the difference of Chl a (SeaWiFS) throughout a meridional gradient, the maximum values occurred in spring-summer being the highest values in MU 3 and decreasing from MU 5 to MU 10 in summer. The minimum values were observed in autumn-winter with a decreasing clearly trend to high latitudes from MU 3 to MU 10.   The latitudinal trend found between beds in the von Bertalanffy parameters increasing Hoo, and decreasing k from North to South could be related to the difference of Chl a concentration along the meridional gradient and the timing of the thermocline (both decreasing from North to South) showed in this study. In either case the food availability was determinant of the good fitness of this species and thus, the possible difference along a latitudinal gradient in the oceanography variables could be the cause of the difference observed in our study. Thus, the observed differences between beds may be related to hydrographic processes that influence primary productivity with concomitant effects on growth.