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    Mitigation of Macrourus by-catch in research block 58.4.1_6 and estimation of Macrourus biomass and sustainable catch in Divisions 58.4.1 and 58.4.2

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    Numéro du document:
    WG-FSA-17/23
    Auteur(s):
    D. Maschette, P. Burch, P. Yates and P. Ziegler
    Soumis par:
    Philippe Ziegler (Australia)
    Approuvé par:
    Dirk Welsford (Australia)
    Résumé

    Macrourus spp. are the major by-catch species in the exploratory toothfish fishery in Divisions 58.4.1 and 58.4.2. CCAMLR has previously identified three main steps to minimise bycatch; i) avoidance, ii) mitigation and iii) the setting of sustainable by-catch limits if mortality is not preventable (SC-CAMLR-XXII, paragraph 5.230). Here, we undertake a risk assessment of the effects of exploratory fishing for D. mawsoni on Macrourus spp. in Division 58.4.1 and 58.4.2 in general and in research block 58.4.1_6 in particular.

    We investigate the distribution of D. mawsoni and Macrourus spp. catch rates in research block 58.4.1_6, and conclude that the current research grid includes fishing locations with high Macrourus spp. by-catch rates in depths between 750 m and 1150 m. To mitigate Macrourus spp. by-catch in future fishing seasons, we propose to either (1) modify the research grid such that the grid includes only areas of low Macrourus spp. by-catch rates, or (2) replace the requirement for vessels to fish within the research grid with structured fishing. The latter option is similar to research fishing in other research blocks within Division 58.4.1 that do not have research grids, where fishing effort is distributed across a range of depth strata (<1000, 1001–1500, 1501–2000 m) with at least 5 longlines in each depth strata per fishing member.

    We also provide estimates of biomass and sustainable catch for the M. whitsoni species morph in all research blocks of Division 58.4.1 and 58.4.2 by applying the CPUE-by-seabed-area method using biomass estimates of M. whitsoni from the 2008 Ross Sea assessment as the reference biomass. Macrourus spp. biomass was estimated separately by longline gear type in each research block due to differences in reported catch rates. We recommend that the biomass and sustainable catch estimates from autoline, and if not available, from trotline be considered as the basis for management advice.

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