Synoptic oceanography of San Jorge Gulf (Argentina): a template for Patagonian red shrimp (Pleoticus muelleri) spatial dynamics

Abstract

An extensive series of high-resolution satellite images from the advanced very high-resolution radiometer (AVHRR) and the sea-viewing wide field-of-view sensor (SeaWIFS) was used in a synoptic oceanographic characterization of San Jorge Gulf (SJG, Argentine Patagonia), an area of great significance for marine conservation and commercial fishing. Remotely sensed information was combined with on-board observer´s data and published information to investigate the role of distinctive oceanographic features in relation to the life cycle of the Patagonian red shrimp (Pleoticusmuelleri), main target of the industrial fleet in SJG. Three frontal systems?North, South and Outer SJG?are characterized. The North and South SJG fronts are associatedwith shrimp reproductive aggregations during late spring and summer. While both function as spawning/nursery grounds, they differ from each other in many respects. The thermohaline South SJG front has its maximum expression during the winter, reflecting the influence of the low-salinity Magellanic Plume, while the thermal North SJG front develops during spring and summer as the water column becomes stratified in the central basin of the gulf. Wind-related downwelling inshore of the front prevails in the North SJG, and upwelling in the South SJG frontal area. Chlorophyll a is concentrated near the thermocline on the stratified side of the North SJG, and for that reason, it is not detected by remote sensors during the spring bloomand the summer but becomes apparent offshore fromthe location of the frontwhen the thermocline deepens during the fall (May). In the South SJG front, Chl-a concentration is apparent inshore from the front all year-round, related in part to upwelling-mediated resuspension. The northern end of the outer front coincides in time and space with a recurrent non-reproductive aggregation of red shrimpbetween November and January and is presumably related to foraging. It is argued that keeping the North and South SJG frontal systems off-limits to fishing is a central precautionary requisite for the sustainability of the shrimp fishery.