Species identification and age estimation for the ridge-scaled macrourid (Macrourus whitsoni) from the Ross Sea
Samples of macrourid rattail by-catch from the toothfish fishery in the Ross Sea dependency in 2002 were identified by observers and returned to New Zealand for subsequent confirmation of their identity by researchers from the National Institute of Water and Atmospheric Research (NIWA). Two species were found, Macrourus whitsoni and M. holotrachys. A previously aged sample from 1999 was confirmed as being solely M. whitsoni, based on otolith comparisons and the geographic source of the aged sample.
Counts of translucent zones were made from baked and cross-sectioned otoliths of the ridge-scaled macrourid (M. whitsoni) sampled from the commercial longline fishery in the Ross Sea (CCAMLR Subarea 88.1). The zones are assumed to form annually as this ageing methodology has yet to be validated for this species. Von Bertalanffy growth parameters were calculated from the readings, separately by sex. M. whitsoni appears to be a slowgrowing and long-lived species, living to at least 50 years. Females grow at a faster rate, and reach a larger size, than males, but both sexes exhibit comparable maximum ages. Regression relationships were calculated, by sex, between total and snout–vent lengths, snout–vent length and weight, and total length and weight. The species exhibited a degree of sexual dimorphism. The age at which 50% of fish are sexually mature was estimated to be from 12 to 14 years.