CCAMLR

By way of simulation, this study investigated how the bias and precision of biomass estimates from an integrated tag-based assessment are influenced by various aspects of a multi-year tagging program, in particular the effects of the size (tag size-overlap) and numbers of tagged fish, the duration of the tagging program, the type of auxiliary data used in the assessment, and the catch history. Biomass estimates generally improved with more and better quality tagging data, however important nuances emerged within this overall trend. The results show that in the early stages of the tagging program, a high tag size-overlap is imperative to maximise the likelihood of a robust assessment. Tagging fish with a low tag size-overlap, even with large numbers, is likely to result in overestimates of biomass and the resulting data should not be used in stock assessments with only few years of data available. In contrast, with a 100% tag size-overlap even low numbers of releases and subsequent recaptures collected in short tagging programs were sufficient for relatively robust assessments. Estimation errors of SSB0 and SSB status in the assessment year stabilized or were relatively small with a tag size-overlap over 60-70%. Biomass estimates were largely unaffected by the type of catch history. Therefore, creating ‘contrast’ in the data by strongly reducing the fish biomass is not needed for tag-based assessments, since data obtained from a tagging program in recent years only can be sufficiently informative as long as estimates of the catch history are available.

The analysis of the data on size composition of Antarctic toothfish (Dissostichus mawsoni) from catches in different statistical areas of the Ross Sea was given. Biological data for two fishery periods were selected for comparison: 2003-2005 and 2008-2012. It allowed estimating changes in the size composition of the fished part of Antarctic toothfish population. It was noted that during both periods of the fishery, in the southern statistical areas (SSRU H, I, K, L), the portion of small-sized immature fish had increased though the average size of fish in catches had not changed significantly. It may evidence to the good state of the whole population.

The current management strategy of the Ross Sea Antarctic toothfish fishery in an MSY mode is to target the oldest, largest, most fecund fish, theoretically to increase the number and growth rate of smaller fish. The literature is reviewed to reveal why this strategy has failed in the past for fisheries taking long-lived, late maturing benthic fish. Not only have there been ecological effects of removing  large fish but such fisheries, in a process called longevity overfishing, has selected against those fish that natural processes have selected for.

We report the analyses of a dataset spanning 39 years of near-annual fishing for Dissostichus mawsoni in McMurdo Sound, Antarctica, 1972–2011. Data on total length, condition and catch per unit effort (CPUE) were derived from the > 5500 fish caught, the large majority of which were measured, tagged and released. Contrary to expectation, the length frequency of the McMurdo Sound catch was dominated by fish in the upper two-thirds of the overall distribution exhibited in the industrial catch for the Ross Sea shelf. Fish length and condition increased from the early 1970s to the early 1990s and then decreased. Fish length positively correlated with Ross Sea ice extent in early spring, a relationship possibly caused by more ice encouraging larger fish to move farther south over the shelf and into the study area. Fish condition positively correlated with the amount of open water in the Ross Sea during the previous summer (Feb), perhaps reflecting greater availability of prey with the higher productivity that more open water brings. Decreasing fish size corresponds to the onset of the fishery, which targets the large individuals. CPUE was constant through 2001 and then decreased dramatically. We hypothesize that this decrease is related to the industrial fishery, which began in the 1996–97 austral summer, and concentrates effort over the ice-free Ross Sea continental slope. As a result of limited prey choices and close coupling among mesopredators of the region, Antarctic toothfish included, the fishery appears to be dramatically altering the trophic structure of the Ross Sea.

Foraging events and related trends in numbers of Type-B and -C killer whales (Orcinus orca) are reported for the vicinity of Ross Island, Ross Sea, Antarctica, between 2002 and 2010. Updating an earlier report, the frequency of sightings and the number of individuals per sighting of Ross Sea killer whales (Type-C; RSKWs), a fishing-eating ecotype, has continued to decrease in a pattern coincident with a decrease in the number and size of an important prey: Antarctic toothfish (Dissostichus mawsoni). Increasingly rare, large fish are much more energetically dense and may also be socially important to the whales, a relationship with potential parallels to that known between well-studied fish-eating killer whales and large Chinook salmon (Oncorhynchus tshawytscha) in the northeast Pacific. In contrast, the prevalence of the larger, mammal-eating Type-B killer whales has not changed in the southern Ross Sea study area. Predation events by Type-B killer whales involving Weddell seals (Leptonychotes weddellii), interest in large penguins, such as emperors (Aptenodytes forsteri), and lack of interest in small penguins, such as Adélies (Pygoscelis adeliae), are presented. In the case of both killer whale forms, the progressive seasonal breakup of fast ice in large bays bordering the Ross Sea likely provides reliable, enhanced foraging opportunities as prey are exposed one area at a time during summer. Given the apparent relationship between RSKW prevalence and the availability of large toothfish, we speculate that the current management strategy of Antarctic toothfish in the Ross Sea region threatens current population levels of RSKW.