Skip to main content

    Revised biological parameters for the Antarctic skates Amblyraja georgiana and Bathyraja cf. eatonii from theRoss Sea

    Request Meeting Document
    Document Number:
    M.P. Francis (New Zealand)

    Revised weight-length and length-length regressions are provided for Amblyraja georgiana and Bathyraja cf. eatonii. Significant differences between males and females occurred for all comparisons except for B. cf. eatonii length-length regressions. Some observer maturity staging data were unreliable, as indicated by comparison with lab staging, and the presence of very large skates scored as immature. For A. georgiana, this resulted in flatter maturity ogives for observer data than lab data, and different estimates of median length at maturity. The lab estimates were based on small datasets that did not cover the maturation range adequately, so using them instead of the observer estimates is not necessarily a good option. Logistic regressions differed significantly between the sexes for both datasets, but observer data suggested that females matured at a larger size than males, whereas lab data suggested the reverse. The differences between sexes may therefore be spurious, and an artifact of insufficient and inaccurate data. Until better data are available, length at maturity should be regarded as the same for both sexes. The median lengths at maturity estimated for both sexes combined were similar for both data sources: 65.9 cm for lab data and 66.5 cm for observer data, suggesting a reasonable overall value of 66 cm PL. There are still insufficient reliable data to estimate the median length at maturity of B. cf. eatonii, but it is probably around 65–75 cm for males and 70–80 cm for females. Reasonable point estimates are therefore 70 cm for males and 75 cm for females.