CCAMLR

This note describes a successful method for tagging Dissostichus eleginoides and presents an analysis of a tag-recapture experiment carried during the Macquarie Island fishery for this species during the 1995-96 season. Simple population and tag accumulation models are used to account for the tags being released at various times throughout the season, and the effects of the removals by fishing. By assuming that recaptures have a Poisson distribution, maximum likelihood estimates are obtained for the pre-fishing abundance of the population in the season of 996 000 fish. However, this is adjusted to 901 000 fish if a small number of recovered but unreported tags are taken into account.

Estimates of absolute numbers of recruits to the stock of patagonian toothfish around the Heard Island region are calculated from density at length data derived from stratified trawl surveys. The estimates are referred to the four year-old age class. The parameters for a lognormal recruitment distribution suitable for use in a population model are calculated.

An experimental longline fishing cruise targetting D. eleginoides has been realised in the deep-sea zone (300 - 1700 m) off the Kerguelen islands (Division 58-5-1) from February to April 1996. The by-catch has been identified with 10 species of fish reaching a important part (36,1%) of the total catch in number. The grenadier Macrourus carinatus and two species of skates Bathyraja eatonii and Bathyraja irrasa are the dominant species in the by-catch. The morid Antimora rostrata is also a common by-catch. The other species are rare but the predatory effect of the two big sharks Somniosus microcephalus and Lamna nasus on longline caught D. eleginoides must be considered as not negligeable. Geographical and bathymetrical differences in the by-catch species distribution is noted. The mean abundances follow the differences. Size distribution in the main by-catch species show that some, like the grenadiers and the skates, have a commercial potentiality if a deep-sea fishery occurs in the future.

A number of weighted smoothing splines were fit to data on carapace length and size-specific, median chela height of Paralomis formosa. The second derivative of the spline which made the best tradeoff between goodness of fit and smoothness was used as an estimate morphometric size at maturity. Bootstrapping techniques provided a bias-corrected estimate of morphometric size at maturity (sBc) equal to 80- carapace width, and Pr(72mm ≤ sBC ≤ 90mm) ≈ 0.95. The spline modeling technique appears to overcome some of the philosophical and statistical problems associated with estimating size at maturity by fitting linear models to log-transformed morphometric data (see Somerton 1980a and 1980b), but improvements could be made to make the spline technique more robust to outliers in Y-space (crabs with regenerating claws). Given the uncertainty in sBC and the lack of information about body size growth rates, a wide range of alternative size limits are likely to be feasible options for managing the P. formosa fishery, but, despite the availability of new data and the arbitrary nature of the current size limit on this species, there does not seem to be a sufficiently strong biological reason to revise the size limit regulation set forth in CM 91/XIV, Paragraph 10.

Data collected from the FN American Champion during Phases 1 and 2 of the experimental crab fishery were analyzed with generalized additive models and depletion estimators. Results from the generalized additive models show that the density of fishable P. spinosissima is highest off the northern coast of South Georgia and at depths between about 100 and 300 fathoms. The Phase 1 results suggest that it would not be appropriate to extrapolate local estimates of abundance to the whole of Subarea 48.3 solely on the basis of depth-specific seabed area; extrapolations must consider location. Linear models fit to CPUE and cumulative catch data from the Phase 2 depletion experiments did not have significant, negative slopes. The insignificant regressions were probably a result of inter-haul variability in CPUE and crab movement and suggested that depletion estimators will not be appropriate tools for estimating local abundances of P. spinosissima. Approximately 6000 crabs were tagged and released during Phase 2. Four of the tagged crabs were recovered, and these crabs were at liberty for about 1 to 5 weeks. The recaptured crabs moved at rates between 0.08 and 0.25 n. mi./day. Mark-recapture estimates of standing stock and density were made for the area around Phase 2's third depletion square. The assumptions of the mark-recapture model were probably violated, but attempts were made to account for biased sampling of recaptured crabs and the movement of crabs away from their release sites. Density estimates from the mark-recapture data were on the order of 50,000-100,000 crabs/n. mi2, and there was a lot of uncertainty associated with these estimates. In general, the experimental harvest regime was successfU1. The regime provided important information about crab distribution and facilitated evaluation of local depletion estimators for use in stock assessment, but the WG-FSA may wish to re-evaluate the design of Phase 2.