Browsing by Subject "Antarctic"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
- ItemOpen AccessAbundance of Antarctic blue whales south of 60°S from three complete circumpolar sets of surveys(International Whaling Commission, 2007) Branch, Trevor ASightings from the IDCR/SOWER austral summer surveys are analysed to provide abundance estimates for Antarctic (true) blue whales (Balaenoptera musculus intermedia) south of 60°S. The IDCR/SOWER ship-borne surveys have completely circled the Antarctic three times: 1978/79-1983/84 (CPI); 1985/86-1990/91 (CPII); and 1991/92-2003/04 (CPIII), covering strata totalling 64.3%, 79.5% and 99.7% of the ocean surface between the pack ice and 60°S. During the surveys, blue whale sightings were rare but were recorded in all regions. Raw sighting rates (schools per 1,000 n.mile of primary search effort) were 0.44 (CPI), 0.67 (CPII) and 1.48 (CPIII). Respective circumpolar abundance estimates were 453 (CV=0.40), 559 (CV=0.47) and 2,280 (CV=0.36), with corresponding mid-years of 1981, 1988 and 1998. The CPIII estimates are the most complete and recent for this subspecies. When adjusted for unsurveyed regions in a simple way, the estimated circumpolar rate of increase is 8.2% (95% CI=1.6–14.8%) per year; nevertheless, Antarctic blue whales still number far less than the estimated 202,000-311,000 that existed before exploitation. These abundance estimates are negatively biased because some Antarctic blue whales may have been north of 60°S or in the pack ice at the time of the surveys and because a small number of blue whales on the trackline were probably missed. Furthermore, a small proportion of pygmy blue whales, probably less than 1%, may have been included in the sightings.
- ItemRestrictedConsideration of multi-species interactions in the Antarctic—An initial model of the minke whale–blue whale–krill interaction(National Inquiry Services Centre, 2004) Mori, M; Butterworth, Doug SAs a first step in investigating the major predator–prey interactions in the Antarctic, a model describing blue whales Balaenoptera musculus, minke whales Balaenoptera acutorostrata and krill Euphausia superba is developed. Blue and minke whales feed mainly on krill, and they share a similar feeding area near the Antarctic ice edge. In the early 20th century, the large baleen whales in the Antarctic were heavily harvested, some to near extinction. Blue whales were taken for almost 60 years, before being officially protected in 1964. Harvesting of the smaller minke whales commenced only in the 1970s, and the population probably increased during the mid 20th century, likely in response to increased krill abundance following the depletion of the large baleen whales. Recent studies show recoveries of some of these large baleen whale species in response to protection, and also a possible recent decrease in the stock of minke whales as the larger whales recover. This work investigates whether the abundance trends indicated by surveys and other information for these species can be explained by considering only harvesting and the predator–prey interactions between the two whale species and krill. Using historical catch data for blue and minke whales, a simple age-aggregated model including species interactions is fitted to survey abundance estimates. Uncertainties in the abundance estimates and the biological parameters are taken into account in the process by considering plausible ranges for their values. Abundance trends for the species can broadly be replicated by the model, provided the parameter values show certain features, including (i) that blue whales are able to maintain their birth and krill consumption rates until krill abundance drops to relatively low levels, and (ii) that both minke and blue whales show relatively fast rates of growth if krill is abundant, but that minke growth rate falls more rapidly as krill abundance drops. The model suggests two interesting features of the dynamics of these species. First, a substantial decrease in krill biomass from the 1970s to the 1990s as a result of the preceding rapid increase in minke whale abundance, and hence krill consumption, following the depletion of the larger baleen whales. Second, a recovery of blue whales despite the impact of minke whales on krill abundance and its resultant decrease, because blue whales are better able to tolerate decreased krill abundance. Future projections show a gradual increasing trend in blue whale abundance and a gradual decrease in minke abundance, with large amplitude oscillations superimposed. Long-term monitoring of biological parameters and abundance are essential to provide a basis for verification or otherwise of such predictions. Results presented here should be viewed qualitatively rather than quantitatively. However, for the future, refinement of the model structure and incorporation of age structure, data on some other major predator species that feed on krill and some spatial structure, is under consideration.
- ItemRestrictedA first step towards modeling the krill-predator dynamics of the Antarctic ecosystem(Ccamlr Science, 2006) Mori, M; Butterworth, Doug SThe history of human harvests of seals, whales, fish and krill in the Antarctic is summarised briefly, and the central role played by krill emphasised. The background to the hypothesis of a krill surplus in the mid-20th century is described, and the information on population and trend levels that has become available since the postulate was first advanced is discussed. The objective of the study is to determine whether predator–prey interactions alone can broadly explain observed population trends without the need for recourse to environmental change hypotheses. A model is developed including krill, four baleen whale (blue, fin, humpback and minke) and two seal (Antarctic fur and crabeater) species. The model commences in 1780 (the onset of fur seal harvests) and distinguishes the Atlantic/ Indian and Pacific Ocean sectors of the Southern Ocean in view of the much larger past harvests in the former. A reference case and six sensitivities are fitted to available data on predator abundances and trends, and the plausibility of the results and the assumptions on which they are based is discussed, together with suggested further areas for investigation. Amongst the key inferences of the study are that: (i) species interaction effects alone can explain observed predator abundance trends, though not without some difficulty; (ii) it is necessary to consider other species, in addition to baleen whales and krill, to explain observed trends – crabeater seals seemingly play an important role and constitute a particular priority for improved abundance and trend information; (iii) the Atlantic/ Indian Ocean sector shows major changes in species abundances, in contrast to the Pacific Ocean sector, which is much more stable; (iv) baleen whales have to be able to achieve relatively high growth rates to explain observed trends; and (v) Laws’ (1977) estimate of some 150 million tonnes for the krill surplus may be appreciably too high as a result of his calculations omitting consideration of density-dependent effects in feeding rates.
- ItemOpen AccessHumpback whale abundance south of 60°S from three complete circumpolar sets of surveys(International Whaling Commission, 2011) Branch, Trevor AAustral summer estimates of abundance are obtained for humpback whales (Megaptera novaeangliae) in the Southern Ocean from the IWC’s IDCR and SOWER circumpolar programmes. These surveys have encircled the Antarctic three times: 1978/79–1983/84 (CPI), 1985/86–1990/91 (CPII) and 1991/92–2003/04 (CPIII), criss-crossing strata totalling respectively 64.3%, 79.5% and 99.7% of the open-ocean area south of 60°S. Humpback whales were absent from the Ross Sea, but were sighted in all other regions, and in particularly high densities around the Antarctic Peninsula, in Management Area IV and north of the Ross Sea. Abundance estimates are presented for each CP, for Management Areas, and for assumed summer feeding regions of each Breeding Stock. Abundance estimates are negatively biased because some whales on the trackline are missed and because some humpback whales are outside the survey region. Circumpolar estimates with approximate midpoints of 1980/81, 1987/88 and 1997/98 are 7,100 (CV = 0.36), 10,200 (CV = 0.30) and 41,500 (CV = 0.11). When these are adjusted simply for unsurveyed northern areas, the estimated annual rate of increase is 9.6% (95% CI 5.8–13.4%). All Breeding Stocks are estimated to be increasing but increase rates are significantly greater than zero only for those on the eastern and western coasts of Australia. Given the observed rates of increase, the current total Southern Hemisphere abundance is greater than 55,000, which is similar to the summed northern breeding ground estimates (~60,000 from 1999–2008). Some breeding ground abundance estimates are far greater, and others far lower, than the corresponding IDCR/SOWER estimates, in a pattern apparently related to the latitudinal position of the Antarctic Polar Front.
- ItemRestrictedProgress on multi-species modeling in the Antarctic(2005) Mori, M; Butterworth, Doug SThis paper introduces a multi-species predator-prey model of whales, seals and krill in the Antarctic that is currently in development, and illustrates some example results that are obtained from the model. Due to limited time, sensitivities of the output of the model to various input parameter values and functional response forms have not yet been investigated, and are planned for future work. Preliminary results show the possible role of predator-prey interactions in influencing the dynamics of the Antarctic species considered in the model.