We present the first differentiated information on the biology and ecological niche of Macrourus whitsoni and M. caml in the Ross Sea region of the Southern Ocean. Fish were caught by New Zealand vessels in the fishery for Antarctic toothfish (Dissostichus mawsoni) between December 2011 and February 2012 from CCAMLR subareas 88.1B, 88.1C, 88.1G, 88.1H, 88.1K and 88.2H. In total, 227 M. whitsoni and 636 M. caml were examined.
Macrourus caml grows larger than M. whitsoni and is about 19% heavier for a given length. The largest M. caml examined was 84 cm total length (TL) and 5.4 kg, whereas the largest M. whitsoni examined was 65 cm TL and 1.3 kg. The two main morphological characters (number of rays in the left pelvic fin; number of rows of teeth in the lower jaw) were very effective at distinguishing between the two species. Scientific observers on New Zealand vessels had an success rate of identification of the two species of 94% overall. We found that total length (TL) was no less precise a measurement than pre-anal length (PAL), even after catching fish on longlines, freezing and rethawing. On the broad scale, Macrourus whitsoni and M. caml seem to be almost completely sympatric by depth and area, with both appearing to be abundant between depths of 900 and 1900 m. There was a small but significant increase in the proportion of M. whitsoni relative to M. caml caught on baited autolines with increasing depth. Catches of females of both species exceeded that of males (especially for M. caml) and this sex-selectivity was not explained by size or age of fish.
Otolith aging data show that the two species have very different growth rates; M. whitsoni approaches full size at about 10–15 years of age and can live to at least 27 years; in M. caml, full size is attained at about 15–20 years of age and they can live in excess of 60 years. However, sexual maturity in female M. whitsoni is reached at longer length and older age than in female M. caml (length at maturity 52/46 cm TL; age at maturity 16/13 y for M. whitsoni/M. caml respectively). Our data were insufficient to estimate onset of sexual maturity for males of either species. Gonad staging data imply that the spawning period of both species may be extended, starting within or before December–January, and with the main part of the spawning occuring later than February. Gonad stage data did not reveal any substantial differences in spawning characteristics between areas.
Most stomachs from both species everted on capture, so we augmented stomach contents data with examination of material from intestines. Our diet data are preliminary but are consistent with previous feeding studies of Macrourus spp., suggesting that M. whitsoni and M. caml are euryphagous predators and scavengers. Both species were found to be partially piscivorous, and amphipods were probably one of the main crustaceans consumed. Our data suggest that M. caml may feed more benthically than M. whitsoni (evidence of feeding on coral, ophiuroid, echinoderm, benthic polychaetes), though both species had been recently feeding on both benthic and pelagic prey, and the sample size was small. Isotopic analysis of muscle samples led to an estimated trophic level of 4.4 for M. caml and 4.1–4.2 for M. whitsoni.