Aspects of the ecology of the bigeye grenadier at South Georgia

Aspects of the ecology of the Bigeye grenadier, Macrourus holotrachys caught as bycatch in the Patagonian toothfish longline fishery conducted around South Georgia were investigated. Age estimates suggest that M. holotrachys is a long lived and moderately slow growing species, reaching ages of over 30 years and attaining lengths in excess of 80cm (Lt). Von Bertalanffy growth parameters were estimated for females and closely matched those of the congeneic species M. berglax from the Northern hemisphere. The size at which 50% of females had started to mature (Lint 50) for M. holotrachys was 21cm (Lpa) and occurred at around 9 years old. Estimates of natural mortality and Pauly’s growth performance index were found to be low (M= 0.09 And ? = 2.82 respectively). Gonad maturity stage was described from macroscopic and histological investigation. Mature ovaries had oocytes at all developmental stages with between 22 and 55% likely to be spawned each year. Absolute fecundity ranged from 22,000 to 260,000 eggs, an average number for a macrourid of this size. A highly skewed sex ratio of 33:1 females: males was found for specimens caught by longlines but not for shallower trawl-caught specimens (1:1). It is suggested that females are far more susceptible to longline capture than males. M. holotrachys is a bentho- pelagic predator/scavenger that feeds on a wide range of fish and invertebrates.