CCAMLR

Target strength (TS) as a function of fish length (L) must be known to estimate fish density in acoustic survey. This TS-length function may be estimated by comparing in situ acoustic measurements with the fish size composition from trawl catches and should be accompanied with accuracy indices characterizing the uncertainty of its parameters. We show that the bootstrap–method is a highly efficient method of estimating the parameters of TS-length function and emphasize that calculation results depend on the adopted mapping rule of fishing data into acoustic data. The bootstrap methods has been applied to data from icefish measurements. The TS-length functions based on full probability density function (PDFs) of fish length and target strength at 38 kHz and truncated PDFs are presented. We reveal that it is probably necessary to use different regression equations for different length ranges.

This report presents results from a desktop study to consider approaches to monitoring and assessing toothfish in New and Exploratory fisheries. We review standardised CPUE analysis, quantitative research longline surveys, experimental manipulation of fishing effort, tagging programmes, bottom trawl surveys of juvenile grounds, and acoustic surveys. Because of potential biases we believe that it would be imprudent to rely solely on any one of these methods at the present time. We recommend that tag recapture experiments, or trawl surveys, be used in conjunction with experimental manipulation of effort to monitor toothfish – and perhaps as importantly – the wider ecosystem effects of the toothfish fisheries. We further recommend that simulation studies be carried out to determine the best way to use the effort manipulation approach in all New and Exploratory fisheries, and in particular the current active fisheries (e.g., Subareas 88.1 and 88.2, and Division 58.4.2). Such studies should examine the spatial and temporal scales required for this approach and possible methods (and sample sizes) required to implement the pulsed fishing experiments.

This report presents results from a desktop study to determine the feasibility of conducting trawl surveys for juvenile toothfish in Subarea 88.1 (Ross Sea). Potential problems including a largely unknown juvenile distribution, variable ice cover, large survey area, and rough bottom topography, were considered using data from published and unpublished literature, the exploratory longline fishery and previous research cruises.
The major limitation on a trawl survey in the Ross Sea is ice cover. On average over the last three years, 63% of the seabed area from 0–600 m depth was not fishable in January–February because of ice. Ice cover varied greatly between years, with the untrawlable area ranging from 31% in 2002 to 89% in 2002. Less than 10% of the potential trawl survey area was fishable in all three years.