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    Reports on biological information of toothfish with special reference to bycatch, depredation and spawning dynamics in Division 58.4.4 a & b by Shinsei maru No. 3 during 2008–14 seasons

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    Document Number:
    K. Taki and T. Ichii (Japan)
    Submitted By:
    Dr Kenji Taki (Japan)

    This report arranged the biological information with special reference to bycatch, depredation and spawning dynamics in Division 58.4.4 a & b during 2008-14 seasons in response to the recommendations in the last WG-SAM meeting and their related issues.

    D. eleginoides (TOP) was highest both in weight and number, accounting for 93% and 76% of the total catch, respectively, among total of 16 species/genus occurred in Division 58.4.4a & b. Grenadiers (GRV) showed the second highest catch and was widely spread in the division as TOP, but the proportion in number to the whole species/genus (17%) was considerably smaller than that in Kerguelen (Division 58.5.1) and Crozet (Subarea 58.6). The fish assemblage for the three dominant species/genus was mainly divided into three groups using clustering. The three groups were generally divided by depths.

    Depredation by whales was analyzed using three methods; the CPUE method and grenadier bycatch method by Gasco et al. (2014) using two size levels of unit cell, and generalized linear model with stepwise process for separating two areas, SSRUs 58.4.4aA & 58.4.4bB and SSRU 58.4.4bC. The depredation rate in weight of whales toward toothfish was estimated to be only 3.8-4.8 % in SSRU 58.4.4bC during 2008-12 using the CPUE method, while the presence/absence of whales was not considered to be a significant parameter for the catch rate of toothfish in the generalized linear model in the same SSRU. The depredation rate in weight was estimated to be 20-25% in SSRUs 58.4.4aA & 58.4.4bB during 2008-14 using the CPUE method. The standardized CPUE in absence of whales in the SSRUs was estimated to be 2 times higher than that in presence of whales in the generalized linear model, and the depredation rate of 25% estimated from this ratio and the occurrence rate of whales of longlines (49%) was identical with the rate estimated using the CPUE method with the 0.5˚x0.25˚unit cells. The grenadier bycatch method by Gasco et al. (2014) is currently rather difficult to be applied in the division because of considerably low proportion in abundance of GRV to sum of GRV and TOP.

    Some spawning activity in this division seems to occur in the slope areas during April-August. The GSI values along with macroscopic stages tended to increase with their body sizes for both sexes. Because of some spawning activity along with the evidence of recruitment of small sized fish, the stock is likely to be self-sustainable in each SSRU. Further discussion was made in the separated document of research plan in Division 58.4.3a.