An updated model of the krill-predator dynamics of the Antarctic ecosystem

The objective of this thesis is to update the Mori-Butterworth (2006) model of the krill-predator dynamics of the Antarctic ecosystem. Their analysis aimed to determine whether predator-prey interactions alone could broadly explain the observed population trends of the species considered in their model. In this thesis, the Antarctic ecosystem is outlined brie y and details are given of the main krill-eating predators including whales, seals, fish and penguins, together with an historical record of the human harvesting in the region. The abundances and per capita krill consumption of the krill-predators are calculated and used to determine the main krill-predators to be used in the updated model developed. These predators are found to be the blue, fin, humpback and minke whales and crabeater and Antarctic fur seals. The three main ship surveys (IDCR/SOWER, JARPA and JSV) used to estimate whale abundance, and the abundance estimation method itself (called distance sampling), are summarised. Updated estimates of abundance and trends are listed for the main krill-predators. Updated estimates for the biological parameters needed for the ecosystem model are also reported, and include some differences in approaches to those adopted for the Mori-Butterworth model. The background to the hypothesis of a krill-surplus during the mid-20th century is discussed as well as the effects of environmental change in the context of possible causes of the population changes of the main krill-feeding predators over the last century. Key features of the results of the updated model are the inclusion of a depensatory effect for Antarctic fur seals in the krill and predator dynamics, and the imposition of bounds on Ka (the carrying capacity of krill in Region a, in the absence of its predators); these lead to a better fit overall. A particular difference in results compared to those from the Mori-Butterworth model is more oscillatory behaviour in the trajectories for krill and some of its main predators. This likely results from the different approach to modelling natural mortality for krill and warrants further investigation. That may in turn resolve a key mismatch in the model which predicts minke oscillations in the Indo-Pacific region to be out of phase with results from a SCAA assessment of these whales. A number of other areas for suggested future research are listed. The updated model presented in this thesis requires further development before it might be considered sufficiently reliable for providing advice for the regulation and implementation of suitable conservation and harvesting strategies in the Antarctic

Do catch-based indices provide a reliable index of annual recruitment for the South African anchovy population

The ability of catch-based measures (both catch itself and measures of catch-rate/CPUE such as catch-per-set) to provide reliable indices of abundance of the annual anchovy recruitment on the South African west coast is investigated. Ground truthing is provided by the annual acoustic surveys of this abundance and corresponding estimates provided when this information is utilised in population assessments. The measures considered for the fishery as a whole reflect at most a 30% increase across the turn of the century, way below the some three-fold increase from before to after that time indicated by the surveys and assessment. The same is true for similar measures from catches made close to Robben or to Dassen Island, except when catches within 10 nm of these islands are considered, though surprisingly then this result of a similar increase to the survey results does not extend to catches within 20 nm. Nevertheless the CVs about the catch within 10 nm – recruitment relationships are some 70%, which is hopelessly too large for such measures to provide meaningful and reliable predictions of anchovy recruitment. Thus this study has failed to reveal any evidence to support the suggestion that anchovy catch- related measures might provide meaningful indices of anchovy recruitment abundance