Distribution of paralarvae and small juvenile cephalopods in relation to primary production in an upwelling area off southern Brazil

Abstract

The distribution of paralarvae and small juvenile cephalopods sampled by a rectangular midwater trawl (opening area 8 m²) over the continental shelf off Cape Santa Marta Grande, southern Brazil (28º09’S–29º56’S) during spring 1989 is discussed. An intrusion of Brazil Current Tropical Water (22ºC; 36.5) separates warm, less-saline water (22ºC; 35.2) from cooler, more-saline water (15ºC; 36.4). Prevailing northeasterly winds led to upwelling of South Atlantic Central Water over the shelf, promoting high Chl a concentrations. Three species constituted 99% of the 628 cephalopods collected: Illex argentinus (n = 540; 4–40 mm mantle length, ML), Argonauta nodosa (n = 46; 2–19 mm ML), and Loligo sanpaulensis (n = 42, 2–21 mm ML). Segregation of I. argentinus juveniles of similar size suggests school formation as small as 10 mm ML. The presence of mature males along with fertilized female A. nodosa indicates mating early in life. There was a consistent and direct link between high plankton production and high densities of juvenile cephalopods through a short and ecologically efficient food chain. The relationship between production, pycnocline intensity, and the density of paralarvae and juveniles revealed suitable conditions for survival and growth during the upwelling season.