CCAMLR

The upper-arm of the length-dependent fishing selectivity function for trawlers fishing for Patagonian toothfish around Heard Island (Division 58.5.2) was modelled as the exponential of a quadratic function in length. This function was calibrated using random length frequency (LF) data collected from commercial operations of each of trawlers and longliners fishing concurrently for 3 seasons assuming that, for the upper range of lengths, fish are fully selected by longliners but not by trawlers. To estimate the trawl gear selectivity while simultaneously adjusting for relative availability of length classes as a function of fishing depth, a log-linear hazard function for each gear type was fitted by a technique used to model grouped survival data. The hazard function relates to the event of a fish falling in a particular length bin, analagous to a death in a given time interval in survival analysis, and the ratio of the hazard function for trawlers to that of longliners for the length range where this ratio is less than 1 defines the upper-arm of the trawl gear selectivity function. This model of selectivity was estimated using the LF data and a fitted binomial generalized linear mixed model (GLMM). The GLMM was fitted to the number of fish in a bin using a complementary log-log link function and binomial sample size defined as the number of fish in that bin or a larger length bin. The resultant conditional binomial probability of the event is linked to an approximate integral of the hazard function to give the GLMM formulation. Random effects of haul identifier and ‘season by length bin’ were included in the GLMM which was fitted using an approximation to the marginal quasi-likelihood (MQL). The fitted trawl gear selectivity function showed a decline from 100% to 1% selection for a corresponding length range of 1030 to 1610 mm and relative availability of fish was also predicted to decrease across a similar length range for the median trawl depth of 580 m relative to the median depth of longline sets of 1200 m.

A descriptive analysis of the toothfish tagging programme carried out in Subareas 88.1 and 88.2 since 2001 is presented. The paper updates and revises tag-release and tag-recapture data which were presented in July 2006 at WG-FSA-SAM. In addition, the paper presents release and recapture data for non-New Zealand vessels for the 2005-06 season for the first time. However, tag data were still only available for about half of the non-New Zealand vessels for 2003-04.
A reported total of 10 775 Antarctic toothfish have been released and 225 recaptured, and 818 Patagonian toothfish released and 25 recaptured. Tagging rates by area over the past three years have been in the same proportion as the catch by area. However, recapture rates have tended to be higher in SSRUs 88.2E, 88.1C, and 88.1H. About 17% of the recaptures could not be matched to a release observation, mainly because of missing release data.
Two fish tagged and recaptured in the fishery have moved over 200 km, but the majority (>80%) of Antarctic toothfish have moved less than 50 km. Sub adult fish (80–100 cm) have tended to move further than adults over all time periods at liberty. In 2006, New Zealand vessels greatly increased the size of toothfish being tagged so that for the first time the size distribution of the tagged fish in the Ross Sea was very close to the size composition of the catch.

The CCAMLR system for assessing risk of fisheries mortalities to seabirds has been applied for 10 years and helps to define mitigation and monitoring requirements for fisheries within the waters of the CCAMLR convention area. Annual review of the information by used CCAMLR Working Group on Incidental Mortality Associated with Fishing for the assessments of risk, and performance of fisheries within these zones is undertaken, resulting in a dynamic system of assessment and response by the CCAMLR commission. Data used in the assessments relate primarily to seabird biology and their threat status – the areas most used by breeding and foraging seabirds of high threat status are given highest risk ratings (six areas, levels 4-5). Those with few foraging birds and no breeding populations are given lowest ratings (six areas levels 1-2). A suite of mitigation measures have been specified for each risk rating, and these are refined annually with input from a technical working group. The thresholds and data-types used in the CCAMLR risk assessment system are documented and some recommendations made for minor modifications to this system to increase the transparency of the assessments.

Over 9,000 skates have been tagged and released in the Ross Sea over a period of seven years, and 47 (0.5%) have been recaptured. The recapture of tagged Amblyraja georgiana after up to four years at liberty shows that some skates survive and recover from being hauled out of depths of around 1,000 m and tagged. In-water tagging greatly reduces the incidence of broken jaws in skates, and probably increases the survival of released animals. Unfortunately, this means that length at release can not be determined, so no useful data were gathered for estimating skate growth rates. Amblyraja georgiana moved very little, even after four years at liberty. There was no movement between SSRUs, and the maximum distance travelled was less than 70 km. The distance travelled did not increase with period at liberty for skates at liberty more than nine months. It appears that this species makes only small-scale movements along depth contours.

Biological parameters were reviewed and updated for the two main species of skates taken as bycatch in the Ross Sea toothfish fishery: Amblyraja georgiana and Bathyraja cf. eatonii. Differences in length-weight relationships among regions raise doubt about the conspecificity of A. georgiana from the Ross Sea and South Georgia, and of B. cf. eatonii from the Ross Sea and Heard Island and the MacDonald Islands. The taxonomy needs to be resolved before biological parameters derived from populations outside the Ross Sea can be applied to Ross Sea skates. Length-weight regression relationships for male and female A. georgiana and B. cf. eatonii from the Ross Sea are provided. Male and female relationships differ significantly for both species. Male A. georgiana attain 50% maturity at 92 cm total length (TL). The length at maturity of females is not well determined but is probably in the range 95–100 cm TL. Length at maturity of B. cf. eatonii is poorly determined, but is about 90–100 cm TL for males and 100–110 cm TL for females. The best available, but unvalidated, growth curve for A. georgiana for both sexes combined is: TLt = 101.3(1 ? e?0.308[t+1.30]). The best estimate of longevity in A. georgiana is 14 years. However this may be a considerable underestimate, and true longevity may exceed 20 years. Age at sexual maturity for A. georgiana is estimated to be about 6–7 years for males and 8–13 years for females. The most plausible range for the natural mortality rate, M, for A. georgiana is probably 0.15–0.25. No growth curve, longevity estimate or age at maturity estimate is available for B. cf. eatonii.

This paper reviews the distribution of seabird species with the WCPFC Area. From a review of the distributions of 99 species of albatross and petrel, 16 species of albatross and 60 species of petrel occur within the area of the WCPFC, and are potentially vulnerable to fisheries bycatch. These include species with IUCN classification of Critically Endangered (n = 1), Endangered (n = 7), Vulnerable (n =26) and Near Threatened (n = 7). The remaining 30 species are classified by the IUCN as Least Concern. This paper, having identified key risks, is intended to assist with the development of advice on mitigation of seabird mortality in fisheries in the WCPFC Convention Area. Recommendations on future approaches to mitigating risk are made.

The exploratory fishery for Antarctic toothfish (D. mawsoni) has been operating for nine years in Subarea 88.1 and for five years in Subarea 88.2 with a large amount of data collected on toothfish and the associated bycatch. All SSRUs in the two subareas except for 881D and 882C have now been fished. The 2006 D. mawsoni catch was the second highest on record with a total of 3388 t against a combined catch limit of 3451 t. The subarea catch limit was almost reached in both Subarea 88.1 and 88.2. The management of the SSRUs within the two subareas was changed for the 2006 season as part of a 3-year experiment (SC-CAMLR-XXIV). One of the aims of the experiment was to simplify the administration of the fishery by having fewer catch limits. This appeared to be moderately successful, with only one catch limit being slightly exceeded in the 2006 season. Concentration of effort within a smaller spatial area also increased the recovery of tags by over 50% (Dunn & Hanchet 2006).
Interesting patterns are beginning to emerge concerning the size distribution of fish from the different areas in Subareas 88.1 and 88.2. The occurrence of large adult fish from relatively shallow waters on the Ross Sea Shelf as well as the occurrence of small juveniles in much deeper water on the continental slope in Subarea 88.2 suggests that depth alone does not explain all the variation in toothfish length in the areas. Further sampling from along the continental slope of the continent is needed to determine the extent of these small fish.

This project evaluated the prevalence and intensity of an ectoparasite Eubrachiella antarctica as a marker for stock discrimination of Antarctic toothfish in the Ross Sea. New Zealand Ministry of Fisheries Observers on four toothfish longline vessels recorded the number of E. antarctica on the fins and in the buccal cavity of 621 large D. mawsoni (mostly 120-150 cm). Up to five D. mawsoni per set were examined for parasites in Small Scale Research Units (SSRUs) 88.1C, 88.2E and 88.2F; and two fish per set in SSRUs 88.1H - 88.1J. Up to 15 E. antarctica, one from each of the first fifteen sets in each SSRU, were dissected out and stored in ethanol for laboratory identification. All of the preserved copepod parasites were identified as female or male E. antarctica. D. mawsoni from 88.1H had a higher prevalence of E. antarctica compared to D. mawsoni from 88.1C, 88.1I, and 88.1J (and to 88.2E) and higher intensity compared to D. mawsoni from 88.1C, 88.1I, and 88.1J. However, neither intensity nor prevalence were significantly different between 88.1H and 88.2F. These area differences do not appear to be sampling artefacts produced by differences in host length structure, sex ratio, seasonality, or observer experience, but represent a regional difference in prevalence and intensity of E. antarctica on D. mawsoni. The small-scale regional differences suggested by the parasite marker are inconsistent with the genetic and tagging studies which suggest homogeneity at small spatial scales. Genetic, tagging, and parasite studies measure different biological parameters and results from these independent studies provide a picture of toothfish movement over different time scales. The stability of this regional difference in prevalence and intensity of E. antarctica needs to be tested with large scale sampling in future years.

The stomach contents of 190 sub-adult (51-100 cm TL) Antarctic toothfish (Dissostichus mawsoni) captured by bottom longline in the western Ross Sea were analysed. Fish were sampled during the 2005 fishing (Dec 2004 to Jun 2005) season and caught in 398-1678 m depths. Sub-adult D. mawsoni are primarily piscivorous, feeding on a wide variety of small to medium sized fish. Icefish (Channichthyidae) were the most important prey by frequency, weight and IRI, while small notothens (Nototheniidae) were more numerous. Whitson’s grenadier (Macrourus whitsoni), dragonfish (Bathydraconidae), and eel cods (Muraenolepididae) were also important. Glacial squid (Psychroteuthis glacialis) were found in about 20% of stomachs, but only 6% by IRI.
Samples were collected mainly from the continental slope along the northern edge of the Ross Sea. Although the study provides an important comparison of diet of adult and sub-adult fish collected from the same area, the results may not reflect the diet of the main part of the subadult population which is thought to reside on the main shelf to the south. It is therefore recommended that further stomach samples be collected from further south on the Ross Sea shelf.