Cross-frontal variations in recruitment and biomass of the Patagonian scallop (Zygochlamys patagonica) in the SW Atlantic Shelf Break Front

MAUNA,C; LASTA, M; BALDONI, A; FRANCO, B; IRIBARNE, O

Halifax, Nova Scotia, Canada

Workshop; 16th International Pectinid Workshop.; 2007

Nationa Shellfisheries Association

Cross-frontal variations in recruitment and adult abundance of the Patagonian scallop (Zygochlamys patagonica) at beds located between 38°S and 39°30´S, under influence of the Shelf Break Front (SBF) in the Argentine Sea, were studied based on integrated data from commercial fleets (1996-2005) and research cruises (1998-2005). Annual and seasonal mean climatologies of sea surface temperature (SST) gradients and chlorophyll-a concentration (chl-a) were used to locate the SBF mean position and its variability, and estimate the concentration of primary production respectively. The SST gradients were computed using a centered difference scheme based on 13 years (1985?1997) of satellite data of the Pathfinder+Erosion monthly climatology of about 9 km x 9 km resolution. Climatological analyses of chl-a concentration were employed based on 7 years (1998-2004) of sea surface color images from Standard Mapped Images (SMI), supply by SeaWiFS. Data from research cruises were used to calculate variations in abundance of recruits (i.e, scallops smaller than 16 mm of total height; lesser than 1 years old) and abundance of adults (i.e, scallops larger than 55 mm of total height; age range between 4-14 years old). In this sense, fine scale variations in biological and fishery data were analyzed according with their positions across the SBF. To evaluate the effect of the front on scallop abundance the scallop ground was divided in 6 boxes to minimize the latitudinal influence (Fig. 1). The data were related within each box to a cross-frontal location according the initial position of tows obtained by GPS. Differences across-front on recruitment and adult abundance of scallop were tested by one-way ANOVA. The results show that the scallop beds were located mainly at central-western positions of the SBF and their extensions were associated with the seasonal variability of the zonal displacement of the front (~35 km). The annual mean position of the front has a strong effect on the spatial variability of the recruitment (in 3 boxes, p< 0.004), within areas where this was successful. The spatial pattern in Catch Per Unit Effort (CPUE) of recruits  showed the highest values located to the center-western of the front matching with the highest concentrations of chl-a. A similar pattern was found in CPUE of scallop adults considering the data from research cruises (in 5 boxes, p< 0.0002) and commercial fleet (in 6 boxes, p< 0.001). The observed spatial variability in scallops abundance could be a result of i) front variability effect on the sinking and settlement at the bottom of recruits and ii) the spatial variability in the food supply from the surface to the beds. Our results highlight the importance of phenomenon like larval retention and bentho-pelagic coupling at the SBF. Fig. 1. Study area in the South Western Atlantic. Box 1, 2 and 3 belong to ?MDQ? bed, box 4and 5 are including in ?Reclutas? bed and box 6belongs to ?San Blas? bed. In the lower right corner there are the schematic box cells. Box 1, 2 and 3 belong to ?MDQ? bed, box 4and 5 are including in ?Reclutas? bed and box 6belongs to ?San Blas? bed. In the lower right corner there are the schematic box cells. Box 1, 2 and 3 belong to ?MDQ? bed, box 4and 5 are including in ?Reclutas? bed and box 6belongs to ?San Blas? bed. In the lower right corner there are the schematic box cells. Box 1, 2 and 3 belong to ?MDQ? bed, box 4and 5 are including in ?Reclutas? bed and box 6belongs to ?San Blas? bed. In the lower right corner there are the schematic box cells. Thin contours correspond to the 100, 200 and 1000 m isobath.