Population biology of the myctophid fish Gymnoscopelus nicholsi (Gillbert, 1911) from the western South Atlantic

Thirty-three collections of Gymnoscopelus nicholsi were made between 1976 and 1981 in the South Atlantic using both krill and bottom trawls. In the Antarctic waters the main part of the stock consisted of adult fish 3–7 years old, characterized by a benthopelagic mode of life. Fish of year-class I were observed only in the vicinity of South Georgia, South Sandwich Islands and the Scotia Sea; year-class II individuals were absent from Antarctic samples. In the notal zone of the western South Atlantic, fish aged 1–5 years were found. The existence of only one population in the region is a possible explanation of these findings: this population is divided into a pelagic stock of juveniles and sub-adults which occur in the offshore waters of the western South Atlantic and benthopelagic stocks of adults distributed over the slope regions off Argentina, South Georgia and Antarctic archipelagos. The computed values of K (von Bertalanffy growth) were 0.48, 0.41 and estimated total annual instantaneous mortality rates (Z) were 0.45, 0.67, and 0.70 for South Georgia, South Shetlands and notal zone of western South Atlantic stocks, respectively.