INDICATIVE ESTIMATES OF BIOMASS AND YIELD OF WHITSON’S GRENADIER (M. WHITSONI) ON THE CONTINENTAL SLOPE OF THE ROSS SEA IN SUBAREAS 88.1 AND 88.2

Catch limits currently in place for macrourids in Subareas 88.1 and 88.2 are defined as being equal to 16% of the catch limit of Dissostichusspp. in these subareas. The 16% was based on the ratio of the by-catch limit for macrourids to the catch limit for Dissostichus spp. in Division 58.5.2 in 2002/03 (CCAMLR-XXI, para 11.53). The bycatch limit for macrourids in Division 58.5.2 had in turn been based on the extrapolation of catch rates from a trawl survey of M. carinatus on Banzare Bank. Two bottom trawl surveys (IPY_CAML and BioRoss) have recently been carried out by New Zealand on parts of the continental slope of the Ross Sea in SSRU 88.1H. Whitson’s grenadier (M. whitsoni) was the most abundant species caught during both surveys, with mean catch rates in some strata being considerably higher than those recorded for M. carinatus on Banzare Bank. Because the strata covered by the New Zealand surveys represented only 25% of the continental slope of the Ross Sea, the data could not be used to calculate a biomass estimate for the entire slope directly. Instead, indicative biomass estimates for the rest of the area were made by extrapolating the more recent IPY-CAML survey catch rates across the entire slope. The extrapolations were carried out in two ways: (i) assuming M. whitsonidensities were constant across the entire slope and (ii) assuming M. whitsonidensities were proportional to the commercial macrourid CPUE from the fishery. The uncertainty of the extrapolated biomass was estimated through a bootstrap procedure. Indicative estimates of M. whitsoni biomass for the Ross Sea slope ranged from 26 892 t (cv = 29%) to 41 823 t (cv = 28%) depending on assumptions. Applying estimates of gamma (γ) =0.01439 and 0.01814 gave indicative estimates of yield in range 386–602 t and 487–759 t respectively. These indicative yield estimates provide tentative support for the bycatch limit of 374 t currently in place for M. whitsoni on the Ross Sea slope (SSRUs 88.1H–L and 88.2A–B).