CCAMLR

Length-at-age data for the Patagonian toothfish (Dissostichus eleginoides) caught by trawl in Division 58.5.2 were fitted using von Bertalanffy (VB) and segmented-linear models using a likelihoods that either did or did not incorporate fishing selectivity and the length-bin sampling frequencies for ageing relative to those for random length frequency (LF) samples to give the variable probability (VP) of sampling lengths. A segmented linear trawl fishing selectivity function was used where the upper-arm was estimated, as described elsewhere, from comparisons of longline LF samples to that for trawls for seasons where both fishing methods were used. Due to the small number of fish in the aged sample that were older than 20 years the general trend in the data of length with age did not exhibit obvious asymptotic behaviour so we compare the fit of the VB model to a segmented linear model. We also adjust the VB model by incorporating a multiplicative term that reduces the VB length-age trajectory below an estimated young-age to give a more realistic predicted length at age zero than that for the unadjusted model. We also tried fixing the t0 and L? parameters of the VB model to give age zero length of 30 mm and asymptotic length of 2000 mm but the fit to the data was very poor. The Akaike Information Criterion indicated that the 2-segment linear model gave the best fit to the length-age data. We compare predictions from these models with growth increments obtained from mark-recapture data for a large sample of recaptured fish. Mean annual growth increment from mark-recapture data for days at liberty greater than 175 days was 36.6 mm yr-1 while that predicted from the 2-segment linear model for lengths above 557 mm was 37.6 mm yr-1. Mean annual relative growth rates for mark-recapture data corresponded most closely to predictions from the VB model fitted by maximising the VP likelihood. For the 5 to 25 year age range predicted lengths from this VB curve and the 2-segment linear model were almost identical.

Gonad samples from female Antarctic toothfish (Dissostichus mawsoni), collected during the 2003/04 and 2004/05 commercial fishing seasons in the Ross Sea to improve estimates of size of maturity, were analysed macroscopically and histologically. Progress was, however, limited because few samples of toothfish less than 80 cm and over 120 cm total length were obtained. The first part of the study used classic histological techniques to classify ovary stages by the most advanced state of oocyte development visible in histological sections of the 2003/04 samples. Using this methodology, it is theoretically possible to determine the proportion of fish maturing to spawn, and from that, determine the mean size at maturity. The calculated Lm50 of 113.0 cm was very close to the value of 115.2 cm estimated in 2000/01. GSI data collected from across the fleet, however, still raise doubt about the true Lm50.
The second part of the study examined histological sections from the 2004/05 season for indicators of spawning activity that may also be useful in ascertaining the maturity ogive of D. mawsoni in the previous season. A range of staining techniques was used to determine variations in the presence of PAS bodies (macrophage aggregates), encapsulated residual eggs, ovary wall thickness, atretic oocytes, the configuration of lamellae tips and oocyte packing. The latter three attributes showed the greatest potential in identifying post-spawners, but require calibration over a much wider range of size samples than obtained for this study. Recommendations for further sampling and character measurements are given.

The New Zealand longliners Janas and San Aspiring were authorised to operate in the Exploratory Fishery in Divisions 58.4.1 and 58.4.2 during the 2004-05 season. The results from their first season in this fishery are presented. Janas undertook 44 sets in Division 58.4.2 for a catch of 37.9 tonnes of D. mawsoni and 0.395 tonnes of D. eleginoides. Two sets were undertaken in Division 58.4.1 SSRU C for a catch of 0.372 tonnes of D. mawsoni. San Aspiring undertook 20 research sets in Division 58.4.1 SSRU G for a catch of 20.9 tonnes of D. mawsoni and 0.4 tonnes of D. eleginoides.
The first sign of the onset of spawning activity for D. mawsoni was noted this season for this part of the Indian Ocean with 1 ripe female and 10 males sampled in Division 58.4.2 during March.

Patagonian toothfish, (Dissostichus eleginoides) have traditionally been aged assuming one opaque and one translucent increment is formed each year on the sectioned face of sagittal otoliths. It has been recorded that differences in otolith interpretation exist between institutes routinely ageing D. eleginoides, resulting in differences of ageing data presented. One area that was assumed to cause interpretation differences was the position of the first increment that corresponded with the end of the first year of growth. While there is a general consensus that the observed increments represent annual events, no direct validation has been provided for the early growth of this species. In an attempt to determine the position of first increment formation daily increments in 7 sagittal otoliths were counted. An average daily increment count between the hatch mark and the outer edge of the opaque nucleus of 229 days was recorded. Due to the resolution of light microscopy, increments could not be confidently counted in the first translucent zone immediately succeeding the opaque nucleus. It was assumed that the outer edge of the first translucent zone would correspond with the end of the first year’s growth. The average measurement from the primordium to the outer edge of the first translucent zone on the ventral side of the otolith section was 0.630mm. This is consistent with the measurements in previously aged samples from the Heard and Macquarie Islands fisheries. These data suggest that the protocol currently used for the determination of the first annual increment is valid.

Patagonian toothfish, (Dissostichus eleginoides) have traditionally been aged by counting presumed growth increments on the sectioned face of sagittal otoliths. While there is a general consensus that the observed increments represent annual events, no direct validation has been provided for this species. This study attempts to validate the annual increments using strontium (Sr) marked otoliths from tagged-recaptured samples. Since 1996, a large scale tag-and-release program on D. eleginoides in the Heard Island and Macquarie Islands fisheries has included injection of most fish with Strontium Chloride (SrCl2) on release. One hundred and forty two of these strontium marked fish were selected for analysis. Strontium (Sr) marks were detected in 139 of the 142 otoliths examined. Using the number of observed increments after the mark in relation to the time-at-liberty between tagging and recapture, we determined that for age classes 5 to 18, one increment is laid down per year. The expected number of increments was the same as the observed increments for 88% of cases (when the time-at-liberty was known at the time of reading and 52% when the time-at-liberty was not known These results indicate that while interpretation difficulties of increment structure in sectioned otoliths of D. eleginoides may result in variability from the absolute age, reliable estimates can still be produced.

Trials were conducted during the 2004/5 mackerel icefish (Champsocephalus gunnari) season in sub-area 48.3, to quantify seabird interaction with fishing operations, to trial different mitigation methods in an attempt to reduce seabird mortality caused by entanglement with the net and to collect baseline data on general seabird abundance while fishing.
Priority was placed on developing a new idea of binding the net with sisal string on shooting, in order to prevent it opening until it had sunk beneath the sea surface, and thereby reduce the chance of seabirds becoming entangled, while attempting to feed from the net. Observations suggest this technique to be highly efficient and adoption of this technique across the fleet is recommended.
The use of streamer lines during shooting and hauling of the net were also tested however due to low vessel speed this proved to be impractical and their use was discontinued.
In addition other ideas were considered and reviewed. These included reducing the size of the ‘dangerous’ 200mm net meshes to a safer 140mm, the use of additional weighting on the net body during hauling to reduce the amount of time that the net is on the surface, and the placing of a small-mesh ‘jacket’ on top of the dangerous meshes. Established techniques, such as cleaning the net and reducing offal discharge prior to shooting and hauling were also monitored.
Unfortunately only 39 trawls were completed and due to the small number of replications it was not possible to show significant difference between the treatments.

The ASPM assessment of the Prince Edward Islands toothfish resource by Brandão and Butterworth (2004) that permitted annual fluctuations about a deterministic stock-recruitment relationship is updated to take account of further catch, GLM standardised CPUE and catch-at- length information that has become available for the years 2004 and 2005. The assessment is also generalised to allow for a second fleet to accommodate data from a pot fishery that has been in operation since November 2004. Results obtained show a much greater selectivity for larger toothfish for the pot than for the longline fishery – a feature which has important implications for the status of the population. The possible extent of cetacean predation and its consequences are also investigated. Twenty year biomass projections under the assumption of various constant annual catches for the two-fleet model are computed. Although higher sustainable yields may be possible, it is suggested that a prudent management approach at this stage would be for the annual legal catch not to exceed 500 tonnes for the time being, together with encouragement that this be taken more by pots than by longline to reduce the impact of cetacean predation. Industry observations of the extent of cetacean predation for the longline fishery have proved helpful, and should be extended to a more formal data recording basis in the future.

One UK vessel fished around the South Sandwich Islands (Subarea 48.4) in in July, 2005 in accordance with CM 41-03. During this time fish were tagged in order to start a mark-recapture programme to assess the toothfish population. Catch rates were similar to those experienced in Subarea 48.3. The UK proposes to undertake a more extensive mark-recapture experiment in Subarea 48.4 over the 2006 – 2007 fishing seasons. The objective of the experiment will be to assess toothfish population structure and size. The research will be undertaken during April in each year by up to two vessels, details yet to be confirmed. The proposal is to tag 500+ fish taking 100 tonnes of catch each year. The target species will be D. eleginoides but any D. mawsoni caught will also be tagged or retained for analysis as appropriate.

The Patagonian toothfish longline fishery set approximately 7.5 million hooks and 9.6 million hooks in 2002/03 and 2003/04, respectively. Observers were present during the setting of 59% of all lines set during 2002 – 2004 to record adherence to the above provisions and record seabird mortality. Only Black-browed albatross (Thalassarche melanophris) mortalities were recorded. Estimated total mortality for 2002/03 was 80 Black-browed albatross and for 2003/04, 45 Black-browed albatross. The rates for 2002/03 & 2003/04 were 0.011 and 0.005 birds/1, 000 hooks, respectively, indicating that the toothfish longline vessels are meeting the requirements of the relevant National Plan of Action.

Information on Australia’s national albatross and petrel research programs is provided to CCAMLR as requested in IMAF Intersessional Workplan Item 2.1.