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    Review of biological parameters for Ross Sea skates

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    M.P. Francis (New Zealand)
    Agenda Item(s)

    Biological parameters were reviewed and updated for the two main species of skates taken as bycatch in the Ross Sea toothfish fishery: Amblyraja georgiana and Bathyraja cf. eatonii. Differences in length-weight relationships among regions raise doubt about the conspecificity of A. georgiana from the Ross Sea and South Georgia, and of B. cf. eatonii from the Ross Sea and Heard Island and the MacDonald Islands. The taxonomy needs to be resolved before biological parameters derived from populations outside the Ross Sea can be applied to Ross Sea skates. Length-weight regression relationships for male and female A. georgiana and B. cf. eatonii from the Ross Sea are provided. Male and female relationships differ significantly for both species. Male A. georgiana attain 50% maturity at 92 cm total length (TL). The length at maturity of females is not well determined but is probably in the range 95–100 cm TL. Length at maturity of B. cf. eatonii is poorly determined, but is about 90–100 cm TL for males and 100–110 cm TL for females. The best available, but unvalidated, growth curve for A. georgiana for both sexes combined is: TLt = 101.3(1 ? e?0.308[t+1.30]). The best estimate of longevity in A. georgiana is 14 years. However this may be a considerable underestimate, and true longevity may exceed 20 years. Age at sexual maturity for A. georgiana is estimated to be about 6–7 years for males and 8–13 years for females. The most plausible range for the natural mortality rate, M, for A. georgiana is probably 0.15–0.25. No growth curve, longevity estimate or age at maturity estimate is available for B. cf. eatonii.