Modelling the krill-predator dynamics of the Antarctic ecosystem

Doctoral Thesis


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University of Cape Town

The main objective of this thesis is to model the krill-predator dynamics of the Antarctic ecosystem so as to determine whether predator-prey interactions alone can broadly explain observed population trends of the species considered in the model without any appeal to systematic effects possibly caused by environmental change. The history of human harvesting in the Antarctic is summarized briefly, and the central role played by krill is emphasized. The background to the hypothesis of a krill surplus in the mid 20th Century is described, and the information, particularly regarding population trends, that has become available since the postulate was first advanced is discussed. By reviewing the consumption and abundance estimates for various species in the Antarctic, it is evident that among the baleen whales, blue, fin, humpback and minke whales feed mainly on krill, and could collectively be consuming up to 120 million tons of krill in this region for each of the years around 1990. Of the seals, the Antarctic fur seals and crab-eater seals also feed mainly on krill, and these two species could be consuming up to 70 million tons of krill each year. Consumption estimates for other krill predators (birds, fish and cephalopods) are relatively poorly determined by comparison. Of these four baleen whale species, minke whales currently make the greatest impact on krill due to their large number at present compared to the other larger whale populations which are still depleted. Trend information suggests that the large baleen whales that were heavily depleted during the commercial whaling period are now recovering at rates in the vicinity of 10% per year, but there are some indications of a recent decrease in minke whale numbers. Thus, the consumption of krill by these large baleen whales has probably been increasing over recent years, though decreasing for minke whales. Updated and refined catch-at-age analyses of minke whales for the International Whaling Commission (IWC) Management Areas IV and V suggest an increase in abundance of this species in the middle decades of the 20th Century to peak at about 1970, followed by a decline for the next three decades. Fitting the recruitment time trend obtained from these analyses to a stock-recruitment model suggests that minke whale carrying capacity first increased from about 1940 to 1960 followed by a 60% decrease from the 1960s to the present. General trends in the biological parameters of this species are consistent with such a decline. A predator-prey interaction model is developed including krill, four baleen whale (blue, fin, humpback and minke) and two seal (Antarctic fur and crab-eater) species. The model commences in 1780 (the onset of fur seal harvests) and distinguishes the Atlantic/Indian and Pacific sectors in view of the much larger past harvests in the former. A reference case and six sensitivities are fit 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 areas for future 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, with crab-eater seals seemingly playing an important role and constituting a particular priority for improved abundance and trend information; iii the Atlantic/Indian region shows major changes in species abundances, in contrast to the Pacific which is much more stable; iv) baleen whales have to be able to achieve relatively high growth rates to explain observed trends; v) species interaction effects impact the dynamics of these predators in ways that differ from what might be anticipated in a conventional single-species harvesting context, and they need to be better understood and taken into account in management decisions, and vi) Laws' (1977) estimate of some 150 million tons for the krill surplus may be appreciably too high as a result of his calculations omitting consideration of density dependent effects in feeding rates. . A priority for future work is to obtain improved estimates of the amount of krill consumed by other species, such as birds, cephalopods and fish as well as to obtain consensus on current abundance estimates for crab-eater seals and baleen whales (especially minke whales and also the associated abundance trend). Once such information is improved, more thorough sensitivity tests to the assumptions of the model and uncertainties in the abundance estimates of the species considered need to be explored. With such further development, it is hoped that such a model may ultimately assist in providing scientific advice for appropriate sustainable harvesting strategies for the Antarctic marine ecosystem taking species interactions into account, as this is a matter of key importance for the IWC and for the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR).

Includes bibliographical references (p. 281-303).