Estudos sobre a população de Portunus spinicarpus (Stimpson, 1871) (Crustacea: Decapoda: Brachiura) da região do Cabo Frio, Brasil

Abstract

During 33 months (January/86 to December/88) 99 trawlings have been made at 30, 45 and 60 m depth in front of Massambaba beach (Arraial do Cabo, R.J.), with lhe purpose to know the softbottom benthos of the in·fra littoral from an area characterized by the upwelling phenomenon. A Van-Veen grab, a rectangular dredge and an "otter-trawl" "' type net have been used. Portunus spinicarpus was the dominant species, summing up 85, 31. of all animals captured with the net. They are of small size (average size: CL= 5, 5 cm and W= 12, 03 g) and thus not comercialized in the region. Although captured in large amounts (120. 523 animals in 33 months) during daytime, it's peak of activity is nocturnal so the biomass of 630 g/20 min. trawls is underestimated. Males are larger (CL= 5, 65 cm) than females (CL= 5,35 cm) and the sex-ratio is 1, 1 m. :1 f.; however this relation changes according to depth, being males practically the double of females at 60 m. The egg-bearing females occur during the whole year, being their greatest numbers in Summer at 45 m and in Spring at 60 m. The smallest ones carrying eggs measured 3,9 cm (CL) and 3,65 g. Portunus spinicarpus population shows migratory movements apparently restricted to 45 and 60 m depth, which seems to be linked to food and temperature. The greatest abundance and benthic fauna diversity was observed at 45 m depth, whereas during Summer (and Spring, with intermitlent downwellings) this area remains watered by lhe cold upwelled waters from the ACAS (Central Waters of South Atlantic). It is discussed wheter this "migration" to 45 m would be dependent only on lower temperatures, or on the additional available food provided by the "trashfish" discarded from shrimp trawlers at this depth in Summer. The biological cycle of Portunus spinicarpus is performed in 12 months, with the development from egg to juveniles (CL= 3 cm) taking approximately 4 months. It is inferred from the obtained results that spawning occurs in Summer at 45 m depth, the larvae appear in Autumn, juvenile in Winter and pre-adults in Spring.