CCAMLR

This paper presents bycatch information for the Australian fisheries in Divisions 58.5.2 for the 2003/2004 and 2004/2005 seasons. Bycatch in the trawl fisheries was low, generally less than 1% of the total catch (target plus bycatch). Higher percentage bycatch rates occurred in trawling grounds where the fishing effort and therefore target catch was low. Bycatch in the longline fisheries was higher, ranging from 6% to 13% of the total catch when only landed bycatch was considered and ranging between 11 to 26% when skate and grenadiers cut and lost from longlines were included. The main bycatch species were skates and grenadiers in the D. eleginoides fishery and skates and C. rhinoceratus in the C. gunnari fishery. The total skate catch in Division 58.5.2 (including those cut from longlines) was 126.6 tonnes in 2003/2004 and 91.1 tonnes in 2004/2005.

Southern Brazil is an important foraging ground for several albatross and petrel species during breeding and non-breeding seasons. We present information on seabirds attending longline fishing vessels on Brazilian Exclusive Economic Zone and international waters of the Southwestern Atlantic Ocean (SW), based on 262 censuses. Spectacled Petrel (Procellaria conspicillata) was the most frequent and abundant species all around the year. Other species with significant numbers found in both warm and cold months were Yellow-nosed Albatross (Thalassarche chlororhynchos) and Great Shearwater (Puffinus gravis). Abundant species in winter were Black-browed Albatross (Thalassarche melanophris), White-chinned Petrel (Procellaria aequinoctialis), Cape Petrel (Daption capense), and Southern Fulmar (Fulmarus glacialoides). Seabird Bycatch per Unit Effort obtained during 32 cruises (351 sets and 371,368 hooks) was 0.09 birds/1000 hooks, and affected Black-browed Albatross (70.6% of birds caught), White-chinned Petrel, Yellow-nosed Albatross, and Spectacled Petrel. Strong overlap between longline fishery and seabirds in southern Brazil during winter is a major cause for concern.

This paper presents methods to infer the rate of movement of fish that are marked, released, and subsequently captured. The information that is available is the location and date of the release and the recapture. In addition, the intensity of fishing effort (i.e. sampling) by location and time is known. The simplest approach is to make a frequency plot of the distance between each release and recapture. This is somewhat problematic, as recaptures 1) are biased by variable sampling between locations, and 2) do not consider the possibility of moving further than the maximum distance between any release and recapture point. An alternative approach presented here is to build an underlying model of movement, and then consider spatially variable sampling of marked fish moving according to this model. While this model-based approach requires assuming some description of movement, it has the advantage of being able to allow a much richer description of movement than is available with the frequency of distances approach. In addition, it also provides a way to generate a movement model that is parameterized, which includes uncertainty that can readily be integrated into a population simulation. If the goal of estimating movement distances and rates is to estimate mixing times for mark recapture studies, there are simpler approaches that might be useful. For instance, one alternative would be to ask how much time is required for the probability of recapture in a location to match the relative probability of catching an unmarked fish in that location. Clearly this may be related to the distance from the release point, but considering this distance and the time required, one could estimate the time required for mixing from any initial deployment scheme. Other issues surrounding the use of mark-recapture data for estimating movement patterns are considered.

Length-dependent fishing selectivity for trawlers fishing for the Patagonian toothfish (Dissostichus eleginoides) on the Heard Island Plateau (Division 58.5.2) was modelled as a 3-segment linear model, which has ascending lower and descending upper arms corresponding to lower and upper fishing selectivity and a plateau section in between for the length range where fish are assumed fully selected. Using random length frequency (LF) data collected from each of trawlers and longliners for fishing seasons 2003-2005 the upper arm of the gear selectivity function for trawlers is estimated using a two-stage process. In the first stage I estimate a parameter for each 40 mm wide length bin from the fit of a binomial generalised linear mixed model (GLMM) to the number of fish in the bin using a logit link function and binomial sample size defined as the number of fish in that bin or greater. I combine the data for length-bins and fit the GLMM to this combined dataset since the binomial counts are conditionally independent. A forward-calculated continuation ratio (FCR) is the probability of a fish being in a length bin compared to that of being in a greater length bin and the logit link function allows the predicted log of these ratios or odds to be obtained directly from the GLMM. The ratio of the odds for trawl to that of longliners when this odds ratio is less than one gives the trawl gear selectivity for that length bin assuming fish in this bin are fully selected by longlines. To account for different availabilities of length classes with fishing depth I simultaneously fit length bin by depth interaction terms in the GLMM. In the second stage, the upper arm of the selectivity function was obtained as a linear fit through these ratios for the subset of bins where this ratio is less than one. The fitted linear decline in selectivity begins from near 800 mm and declines from a selectivity of 1 to 0 at 1731 mm length. Adjusting for gear type, the GLMM predicts the availability of large fish increases with increasing fishing depth.

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.