CCAMLR

The integrated assessment of Patagonian toothfish, Dissostichus eleginoides, for the Heard and McDonald Islands (Division 58.5.2) was updated by replacing catch-at-length proportions from commercial catches with catch-at-age proportions using age length keys (ALKs) where the ALK for each combination of fishery and year had available at least 50 aged fish. For the trawl fisheries that were divided into periods within each year, the same ALK for the year was applied to the length frequency (LF) samples for each fishing period within that year. For years where insufficient fish were aged the catch-at-length proportions were retained but for a given fishery the same age-specific selectivity function and parameter values were logically applied to both types of data. For 2006 and 2007 random stratified trawl surveys, there were sufficient aged fish to convert abundance-at-length to abundance-at-age data. Effective sample sizes for the commercial catch-at-age proportions, assuming a multinomial distribution, and the coefficient of variation (CV) for the abundance-at-age, assuming a lognormal distribution, each took into account uncertainty due to haul-level variability in catch-at-length proportions, ALK sampling error (sampling fraction of the LF samples that were aged ranged from 0.8% to 18%) and random ageing error. CASAL allows a single ageing error matrix to be defined and applies this matrix to predictions of numbers-at-age and proportions-at-age. In other work, this matrix was found to depend on the readability score of the otoliths used for ageing, and sensitivity of the assessment results to the assumed readability score was investigated for readability scores of moderate (3), good (4), and excellent (5). The median score for all aged fish in the assessment was 3 but some fishery-by-year combinations had a higher value of 4. The output from the integrated assessment of most interest in this study is the CV of the estimated historical recruitment series, since this parameter strongly influences the effect of the depletion rule on the allowable catch. Compared to the assessment that did not incorporate catch-at-age or abundance-at-age data, the aged-based assessment dramatically lowered the CV for the recruitment series, from around 1.5 to 1.8 down to approximately 0.3 to 0.4, if a readability score 5 was assumed or if for a score of 4 the most stable subset (1986-2000) of the full historical series (1984-2006) was used to estimate the CV. There was no reduction in CV for either series if a score of 3 was assumed. The difference between a readability of 3 and 4 in ageing error is that zero ageing errors are relatively less prevalent (e.g. for age 8 the percentage of errors that were zero was estimated from previous work at 40% for score 3 and 48% for score 4, the corresponding +/- 1 yr errors had prevalence of 46% and 45%, respectively). A +/- 1 year error may seem minor relative to the complete age range modelled of 1 to 35 yr, however most fish caught are in a more restricted, younger age range. For example, the upper age of fish in the main survey that have an upper selectivity greater than 0.2 was approximately 12 yr while the corresponding values for the trawl and longline fisheries were 15 and 20 yr, respectively. The results presented suggest that future ageing work would give a greater improvement to the integrated assessment if otoliths with readability score of at least 4 can be obtained in sufficient numbers to allow ALKs to be constructed using only the ages obtained from these otoliths.

Catch-at-age proportions are generally incorporated into an integrated assessment as observations that contribute to the objective function via a multinomial likelihood. The multinomial likelihood requires a nominal sample size for each fishery and year combination. A method is described for estimating an effective sample size (ESS) that can be used as the nominal multinomial sample size. The method accounts for both the variation associated with sub-sampling of the random length frequency (LF) sample for ageing, and random reader error when ageing fish. The catch-at-age ESS is estimated by dividing the ESS for the LF sample, where this ESS is obtained from the haul-level LF data, by an over-dispersion parameter estimate obtained from simulated samples of age frequency data. These samples are obtained using Monte Carlo multinomial replicates of the observed age length key (ALK) with each ALK used to generate a replicate age frequency sample. For each replicate a random draw of the ageing error matrix is taken and applied to the age frequency sample, thus combining both sources of variation. The over-dispersion parameter is estimated from the fit of a log-linear Poisson generalized linear model to the replicated length frequency data. Using simulated data to include only sampling error, the over-dispersion parameter declined from a maximum of around 6 to 8 down to close to 1 as the aged sample fraction increased from 1% to 10%. When random ageing error was combined with sampling error the corresponding values were lower, with a corresponding range of around 4 down to 1. This reduction is due to the way the ageing error matrix ‘smooths-out’ peaks in the true (i.e. without ageing error) age frequency data.

During February and March 2008 New Zealand carried out a major research voyage into the Ross Sea region in support of the International Polar Year Census of Antarctic Marine Life (IPY-CAML). The 50 day voyage on the research vessel Tangaroa involved an extensive survey of marine organisms from viruses to pelagic and demersal fish and cephalopods in depths from the surface down to 3500 m, and from the continental shelf and slope of the Ross Sea to unexplored seamounts and abyssal plains immediately to the north. Multifrequency acoustic data (12, 38, 70, and 120 kHz) were collected throughout the survey. Mark identification was achieved using targeted midwater trawls. Additional midwater and demersal trawls were carried out at randomly selected locations over the shelf as part of the core biodiversity survey. The main target species of the acoustic survey work was Antarctic silverfish (Pleuragramma antarcticum). Silverfish are a key link between plankton and the community of top predators in the shelf waters of the Ross Sea, but little is known of many ecological and biological aspects of this species. Silverfish were widely distributed over the Ross Sea shelf. Adult silverfish tended to form layers at 150–450 m depth and were sometimes present close to the bottom, where they were frequently caught in demersal trawls shallower than 500 m. A weak layer at about 80 m depth was found to be associated with juvenile silverfish of 40–80 mm standard length. The other major source of acoustic backscatter on the Ross Sea shelf was ice krill (Euphausia crystallorophias). Further north, over the slope and abyssal plain, acoustic marks were associated with myctophids (Electrona spp) and Antarctic krill (Euphausia superba). Acoustic backscatter from both silverfish and krill marks increased with increasing frequency (i.e., was highest at 120 kHz), which is characteristic of species without an air-filled swimbladder. Acoustic target strength (TS) estimates for silverfish at 38 kHz were estimated from anatomically detailed scattering models based on CT (computed tomography) scans of thawed specimens. The derived relationship between TS and fish length was unusual, having an extremely high slope for fish less than 11 cm. Preliminary estimates of acoustic biomass of silverfish in the Ross Sea based on this TS-length relationship were probably not credible, suggesting a very high biomass of juveniles (3 809 000 t) and much lower biomass of adults (118 000 t). Biomass estimates were also calculated for krill, with preliminary estimates (based on the krill TS of Greene et al. 1991) of 517 000 t (mainly E. crystallorophias) on the shelf, and 471 000 t (mainly E. superba) further north.

Acoustic target classification protocols recently adopted by utilising the Stochastic Distorted Wave Approximation (SDWBA) krill target strength model, are assessed. Three frequency acoustic data and concurrent net information, from 16 net hauls through krill aggregations, were collected during two cruises to the South Georgia region in 1996. For each net-sampled aggregation the differences between acoustic backscatter at and 38 kHz (SV120-3B) and at 200 and 120 kHz (Sv200-120) were calculated. Using the SDWBA model, dB difference identification ranges based on the length-frequencies of krill in each aggregation were calculated Four acoustic target classification algorithms were assessed: (i) '3 freq model' -using SDWBA-derived ranges for Sv120-38 and Sv200-120, (ii) '2 freq model' using an SDWBA-derived range for Sv120-3 , (iii) '2 freq 2-16' - with an Sv120-38 range of 2-16 dB and (iv) '2 freq 2-12' -with an Sv120-38 range of 2-12 dB The '3 freq model' algorithm parameterised using the standard CCAMLR values for orientation distribution correctly identified only 6 of the 16 krill aggregations sampled the nets. contrast the '2 freq model' and freq 2-16' algorithms correctly identified all the aggregations as krill. However, different krill orientation distributions for the two cruises were estimated with the SDWBA model, with cruise specific orientation distributions applied the performance of the '3 freq model' was comparable with the two frequency techniques. Such results indicate that use of the present SDWBA-derived ranges in the CCAMLR 3 frequency target classification protocol without taking account of krill orientation distribution is likely to substantially underestimate krill biomass

The Agreement on the Conservation of Albatrosses and Petrels held the Third Session of its Meeting of the Parties from 27 April – 1 May 2009. Key outcomes of relevance to the Ad Hoc WG-IMAF were the adoption of the Advisory Committee’s Work Programme for 2010-2012 and the granting of approval for the ACAP Secretariat to enter into a Memorandum of Understanding with CCAMLR. The proposed MoU has been submitted as a background document for consideration at CCAMLR XXVIII. There were no meetings of ACAP’s Advisory Committee or its Seabird Bycatch Working Group held since the last meeting of the meeting of the Ad-hoc WG IMAF. However, the Agreement was represented at the Second Joint Meeting of Tuna RFMOs and the outcomes of this meeting of relevance to the work of the Ad-hoc WG IMAF are reported on.

This paper documents recent and ongoing developments in New Zealand’s Exclusive Economic Zone that are relevant to the work of the IMAF working group. The bycatch of seabirds by fisheries within New Zealand’s EEZ in recent years that either breed or forage within the CCAMLR convention area (Convention Area seabirds) is detailed. This paper contains description of the recent and ongoing seabird mitigation trials that are underway in New Zealand. We also briefly report on the ongoing revision of the NPOA-Seabirds which will utilise both Level 1 and Level 2 risk assessment methodologies.

Seabird mortality levels caused by net entanglement in the icefish trawl fishery in Subarea 48.3 became a concern in the late 1990s. At this time there was little knowledge on how to mitigate such mortality. As an interim measures a vessel specific 20‐bird mortality threshold was introduced for Subarea 48.3 in 2001. This provided a strong commercial incentive for industry to develop measures to reduce seabird bycatch levels. This resulted in the development and operational trialing of several measures, including net‐binding and net weighting. Net binding has proved to be a highly effective and simply applied mitigation measure and is thought to be largely responsible for the continued reduction in incidental mortality in the icefish trawl fishery in Sub area 48.3 from 0.26 birds/trawl in 2001 to 0.01 birds/trawl in 2008. Evidence suggests that net weighting and good deck practices to minimise the time that the net is on the water’s surface have been the key factors in reducing seabird entanglements during the haul down from 132 birds in 2005 to single figures in 2007 and 2008.