CCAMLR

We present the first differentiated information on the biology and ecological niche of Macrourus whitsoni and M. caml in the Ross Sea region of the Southern Ocean. Fish were caught by New Zealand vessels in the fishery for Antarctic toothfish (Dissostichus mawsoni) between December 2011 and February 2012 from CCAMLR subareas 88.1B, 88.1C, 88.1G, 88.1H, 88.1K and 88.2H. In total, 227 M. whitsoni and 636 M. caml were examined.

Macrourus caml grows larger than M. whitsoni and is about 19% heavier for a given length. The largest M. caml examined was 84 cm total length (TL) and 5.4 kg, whereas the largest M. whitsoni examined was 65 cm TL and 1.3 kg. The two main morphological characters (number of rays in the left pelvic fin; number of rows of teeth in the lower jaw) were very effective at distinguishing between the two species. Scientific observers on New Zealand vessels had an success rate of identification of the two species of 94% overall. We found that total length (TL) was no less precise a measurement than pre-anal length (PAL), even after catching fish on longlines, freezing and rethawing. On the broad scale, Macrourus whitsoni and M. caml seem to be almost completely sympatric by depth and area, with both appearing to be abundant between depths of 900 and 1900 m. There was a small but significant increase in the proportion of M. whitsoni relative to M. caml caught on baited autolines with increasing depth. Catches of females of both species exceeded that of males (especially for M. caml) and this sex-selectivity was not explained by size or age of fish.

Otolith aging data show that the two species have very different growth rates; M. whitsoni approaches full size at about 10–15 years of age and can live to at least 27 years; in M. caml, full size is attained at about 15–20 years of age and they can live in excess of 60 years. However, sexual maturity in female M. whitsoni is reached at longer length and older age than in female M. caml (length at maturity 52/46 cm TL; age at maturity 16/13 y for M. whitsoni/M. caml respectively). Our data were insufficient to estimate onset of sexual maturity for males of either species. Gonad staging data imply that the spawning period of both species may be extended, starting within or before December–January, and with the main part of the spawning occuring later than February. Gonad stage data did not reveal any substantial differences in spawning characteristics between areas.

Most stomachs from both species everted on capture, so we augmented stomach contents data with examination of material from intestines. Our diet data are preliminary but are consistent with previous feeding studies of Macrourus spp., suggesting that M. whitsoni and M. caml are euryphagous predators and scavengers. Both species were found to be partially piscivorous, and amphipods were probably one of the main crustaceans consumed. Our data suggest that M. caml may feed more benthically than M. whitsoni (evidence of feeding on coral, ophiuroid, echinoderm, benthic polychaetes), though both species had been recently feeding on both benthic and pelagic prey, and the sample size was small. Isotopic analysis of muscle samples led to an estimated trophic level of 4.4 for M. caml and 4.1–4.2 for M. whitsoni.

Two size classes of A. georgiana have been described in by-catch fishing data reported in the catch data from the Ross Sea fisheries. We extracted DNA from A. georgiana samples from both size classes collected by Scientific Observers in the Ross Sea. We determined the DNA sequences of five mitochondrial DNA (mtDNA) gene sequences and tested whether the two size classes represented genetically differentiated groups, which might have formed through a recent speciation event. We found no genetic differences among the samples of A. georgiana that were collected in the Ross Sea region, and the data did not support the hypothesis that the two size classes were reproductively isolated.

The diet of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region was examined based on stomach samples obtained from observers on New Zealand fishing vessels in 2003 and 2010. Overall, the diet of sub-adult toothfish was similar to that of adult toothfish, with a dominance of piscivory on benthic fishes and cephalopods, augmented by benthic invertebrates. Sub-adult toothfish ate a greater variety of smaller prey than adults, including smaller fishes (such as Trematomus sp., dragonfish, mainly Bathydraco spp.), and large decapod prawns (Nematocarcinus). By weight and index of relative importance (IRI), grenadiers (Macrourus spp.) were the most important fish and overall prey species. The study did not distinguish between M. whitsoni and M. caml, but this should occur in future. Over the Ross Sea continental shelf, icefish (probably mainly Chionobathyscus dewitti), and eel cods (probably mainly Muraenolepis evseenkoi) were important prey items. Glacial squid, P. glacialis, were the most important cephalopod prey species, especially over the northwest Iselin Bank. On oceanic seamounts, toothfish fed substantially on Macrourus spp. but also deep sea cod (Antimora rostrata), a variety of cephalopods and the occasional mesopelagic to epipelagic fish.

A key aim of this study was to see whether stomach contents of toothfish could be used to detect ecosystem change in the Ross Sea region, with a focus on the area of highest fishing intensity – the northern parts of the Mawson and Iselin Banks on the Ross Sea continental slope (70°–73°S, 175°E–175°W). Variation of stomach contents with toothfish length, weight, sex, month, depth and location was analysed to investigate whether a residual year effect was discernible in stomach contents in this region between 2003 and 2010. Although these factors explained some variation in stomach contents in the Mawson and Iselin Bank block, the proportion of variance explained was relatively small, and we found no statistically significant change in toothfish diet between 2003 and 2010. Holling type I and type II feeding relationships were determined for the Mawson and Iselin Bank on the continental slope using catch per unit effort (CPUE, fish per 1000 hooks) for grenadiers, ice fish and eel cods. Compared to CPUE, icefish and eel cods were over-represented in toothfish stomachs relative to grenadiers. The fitted feeding selectivity for toothfish relative to the selectivity of baited autoline fishing gear was found to be nearly 11 times higher for icefish than for grenadiers; the relative selectivity was more than 4 times higher for eel cods than that for grenadiers. It is not known whether this is due to lower catchability by longlines, greater availability to toothfish predation for a given abundance, or higher toothfish feeding preference for icefish and eel cods relative to grenadiers.

Reliable estimates of demersal fish community composition and population densities are essential for understanding the ecological effects of fisheries. Such estimates are conventionally derived from research trawl surveys but video techniques afford an alternative, or complementary, approach. Using data collected during a survey of demersal fish communities in the Ross Sea region of Antarctica, we compared measures of community composition, population density, and biomass derived from three sampling methods; a large demersal fish trawl, a beam trawl, and a towed camera system. Twenty-three sites spanning the continental shelf, northern continental slope, abyssal plain, and two seamounts were sampled using the towed camera and at least one of the trawl types, allowing direct comparisons between sampling methods. Patterns of species turnover between sites were similar across all methods. Estimates of fish population densities from the towed camera and beam trawl data were also comparable but those from the demersal trawl were consistently lower than for the other methods. Macrourus spp. grenadiers were ca. eight times less abundant in the demersal trawl than the video data but more large individuals were sampled by the trawl than the video and biomass estimates were similar. We hypothesise that this pattern could result from size-specific depth preferences in Macrourus spp. and conclude that video and trawl methods provide complementary information and if used together could be effective in routine assessments of demersal fish populations.

Muraenolepis species are caught in low numbers with bottom longline and trawl gears throughout the CCAMLR Convention Area. There are seven known species in the genus, but more work on the taxonomy of the group is needed and the number of species is uncertain. The few biological studies published suggest a relatively fast growing, semelparous, life history with a maximum age of 11 years. The estimate of K from the growth function for specimens assumed to be M. evseenkoi is much higher than for other Antarctic species. Individuals selected by longline gear are almost exclusively female, and a localised area of high catch rates occurs on Iselin Bank on Ross Sea slope. However, effects of fishing on this bycatch species depend on other factors, such as productivity, distribution pattern, and total biomass. Further directed sampling to determine species composition, life history attributes, reproductive strategy, and sex-specific distribution, and any trends in biomass is needed from the Ross Sea area and throughout the Convention Area.

Conservation Measure 41-01\Annex C requires participants in exploratory fisheries to only tag fish with a high probability of survival. Further it recommends that fish hooked only in the mouth should be tagged. A new data collection form was implemented in 2012 to allow evaluation of fish suitability for tagging from research hauls in exploratory fisheries in Subarea 58.4 and 48.6. We summarized these data and data from tag release forms to assess suitability of captured toothfish for tagging. Data were only available for trotline and experimental Spanish line - trotline gear. Fish suitability for tagging was most related to species, soak time, and fish size. For both gear types, an adequate number of suitable fish were available to tag. Further data collection from sets with different gear configurations and operational effects would be useful in a multivariate analysis to identify the factors affecting fish suitability for tagging. From paired sets in which Spanish and trotline gear types were deployed simultaneously, the trotline configuration had a higher catch rate overall for Patagonian toothfish. However, the relatively higher trotline catch rates degraded for small fish (less than 70 cm), and showed no difference in length selection over the rest of the sizes encountered.

Antarctic toothfish (Dissostichus mawsoni) have a circumpolar distribution, and have been found associated with bathymetric features shallower than 2250 m and typically deeper than 500 m from the Antarctic continent north to approximately 57° S. Efforts to understand early life history and stock structure in the Ross Sea have parallels for populations in other regions of the Antarctic, and hypotheses about how stock structure is maintained may apply to each population.

We provide an update of the circumpolar models of larval dispersion for D. mawsoni developed by Hanchet et al. (2008) using the HiGEM circumpolar oceanographic model documented by Rickard et al. (2010). We used a finer resolution of simulated locations in the northern areas of Subarea 88.1 & 88.2 to further investigate the possible larval dispersal from spatially discrete spawning grounds in these Subareas. Further, we derive plausible spawning locations from Scientific Observer sampling of D. mawsoni in other ocean sectors to identify starting locations for modelling the potential egg and larval dispersal around Antarctica using the HadGEM (Rickard et al. 2010) oceanographic model. The results are currently too preliminary to make conclusions about possible stock structure within these sectors. Information on the depth of transport and the timing and velocity of any directed swimming will assist in improving these simulations.

A reliable commercial tagging program is critical to the successful management by CCAMLR of a number of toothfish fisheries in Antarctica, but the evaluation of tagging performance has been thus far inconclusive because the confounding effect of factors such as time and location of tagging, and size of fish tagged, makes meaningful comparisons between vessels difficult.

We propose that, by controlling for the spatial and temporal confounding factors using a case-control study design, we can derive meaningful indices of relative performance of groups of fishing effort (e.g. vessel, trip, etc). We developed indices of (i) the mortality (or loss of all tags) of released fish and (ii) the detection rate of recaptured fish.

This method was applied to the tagging data in CCAMLR Subareas 88.1 and 88.2. Results show that the indices developed can provide evidence of significant differences in performance between the different vessels or groups of fishing effort. Further investigation showed these indices are robust to the choice of the control group and the area included in the analysis, as well as variations to the 'space window' within which control hauls were selected and paired with each case haul. This method is a good candidate to investigate the relative performance of the CCAMLR tagging program across all fisheries, and more generally the relative performance of spatially and temporally heterogeneous data sets.

In the revision duplicate figures have been removed and Figures 2 to 7 now have ‘vessel number’ on the vertical axis (these numbers were allocated at random and are the same for each figure).

SPM (Spatial Population Model) is a generalised spatially explicit age-structured population dynamics and movement model. SPM can model population dynamics and movement parameters for an age-structured population using a range of observations, including tagging, relative abundance, and age frequency data. SPM implements an age-structured population within an arbitrary shaped spatial structure, which can have user defined categories (e.g., immature, mature, male, female, etc.), and age range. This manual describes how to use SPM, including how to run SPM, how to set up an input configuration file. Further, we describe the population dynamics and estimation methods, and describe how to specify and interpret output.

We present a method to evaluate potential biases and uncertainty in the tagging assumptions of the stock assessment for Antarctic toothfish in the Ross Sea region using spatially explicit operating models. The method allows investigation of potential biases and uncertainty in the assumptions of spatial distribution and fish mixing used in the standard stock assessments for the Ross Sea region (and potentially other CCAMLR areas).

We use the generalised Bayesian population dynamics model, the Spatial Population Model (SPM), to develop spatially explicit movement models of the Antarctic toothfish in the Ross Sea region as operating models in simulation experiments. Simulated observations from these models were then used in a single area stock assessment model derived from the stock assessment model of Antarctic toothfish in the Ross Sea region.

Results from preliminary case studies suggest that the standard single area stock assessment model for the Ross Sea was relatedly unbiased when we simulated from an operating model derived from the best-fitting coarse-scale model that restricted fish to areas inside the historical footprint of the fishery. However, the results when using a similar model that allowed for fish to be present in areas outside the area historically accessed by the fishery suggested the standard stock assessment may be biased low.

While we note that these results are preliminary and further analyses should be carried out, we consider that simulation experiments using spatially explicit models can provide a useful tool to evaluate potential bias and uncertainty in our understanding of the stock assessment in the CCAMLR region. We recommend the further development of this method at future meetings.