Biological parameters for icefish (Chionobathyscus dewitti) in the Ross Sea, Antarctica

Icefish (Channichthyidae) specimens were randomly collected by observers during the 2005–06 fishing season. These observers were placed aboard three longline vessels targeting Antarctic toothfish in the Ross Sea (Areas 88.1 and 88.2). Biological data from 303 returned specimens was collected. These data included species identification, fish length, weight, sex, meristics, reproductive biology, diet, and age estimation. All of the icefish sampled were identified as Chionobathyscus dewitti, and showed no significant difference in sex ratio. Meristic, diet and age data were consistent with previous research. Regression equations for converting standard length to total length and for defining length-weight relationships were calculated and presented for both male and female fish. Gonad maturity stage data showed that most fish were either immature or resting (mature). Gonado-Somatic Indices (GSI) were calculated and plotted against sample month. There was a weak positive trend in GSI between December and February, but this was limited, probably due to the short temporal distribution of the data. Length-at-maturity and age-at-maturity ogives indicated that 50% of the fish sampled were mature at about 340–360 mm (TL) and about 3–4 years of age and that 95% were mature at about 370–400 mm (TL) and 6–8 years of age. Counts of growth zones in sectioned otoliths were used to determine ages and von Bertalanffy growth parameters. Fish growth was rapid for both sexes, and females approached a significantly larger mean asymptotic maximum size than males. Maximum ages of 8 and 11 years were obtained for male and female fish, respectively. Diet analysis showed most icefish stomachs were empty and the few prey items recovered were generally in advanced stages of digestion. This may be due to regurgitation of prey during capture.