Two species of grenadier are predominatly taken as bycatch in the Ross Sea region, Macrourus whitsoni and M. caml. A total of 220 otoliths from M. whitsoni and 307 otoliths of M. caml from fish taken as bycatch in the Ross Sea on New Zealand fishing vessels in the 2011–12 fishing year were used to test whether the otoliths of the two species can be discriminated. Samples of both species of macrourid were obtained from SSRUs 88.1B, 88.1C, 88.1H, 88.1J, 88.1K and from 88.2H. Lengths of M. whitsoni in the sample were 34.5–65.1 cm total length (TL), and 12.0–24.0 cm pre-anal length (PAL); lengths of M. caml in the sample were 34.5–81.5 cm TL and 11.0–30.0 cm PAL. Both males and females were included in the sample, but females predominated. A linear function of fish total length (cm), depth of the whole otolith (Depth, mm), and maximum cross-sectional area of the otolith (Area, mm2) gave excellent discrimination between the two species:
α = 1.254 + 0.03512*TL - 0.02463*Depth - 0.7668*Area
Where α<0.5 indicates M. caml and α>0.5 indicates M. whitsoni. Based on withholding data for testing in 10 folds, the multiple linear regression coefficient, R=0.776 [F(4, 522) = 272.9, p < 0.001], the proportion correctly identified was 92% (483 / 527), and the area under the receiver-operator characteristic (ROC) was 0.968. A similar discriminant function based on PAL (cm) rather than TL performed almost as well (90% correctly identified, ROC 0.97):
α = 1.330 + 0.09022*PAL - 0.02453*Depth - 0.6915*Area
There was no significant variation in the discriminant values (α) according to sex of the fish or for Subarea of capture for M. caml (CCAMLR Subareas 88.1 versus 88.2). However, there was a small but significant difference in α for M. whitsoni between subareas 88.1 and 88.2 [t(218) = 4.20, p < 0.001 ***], which provides some evidence that they could be separate stocks in the two CCAMLR Subareas.