Summary
To date humpback whales (Megaptera novaeangliae) that breed close to the west (stock D) and the east coast (stock E) of Australia are thought to restrict their feeding to IWC Management areas IV and V respectively, with perhaps only limited mixing of the two stocks at the boundary of the management areas at approximately 130°E. The most recent asse ssment of these stocks uses a population model that allows for mixing between feeding areas (Johnston and Butterworth 2002, 2005). This model makes a number of assumptions about whale movement in these feeding areas, the historic catch distribution across these feeding areas and the form of density regulation acting upon these populations. We tested the sensitivity of Johnston and Butterworth’s model to these assumptions using a simulation approach. Specifically we generated data from a population model where the assumptions were relaxed in a number of plausible ways and fitted the model to the data. Using this approach we explored whale movements over a finer spatial scale in the feeding areas, the catch distributions across this sub area scale and incorporated density regulation on the feeding areas. We found that Johnston and Butterworth’s model was robust to both whale movements on a fine scale and with catch distributed as per this scale. However, when density regulation was implemented in the form of density dependence on the feeding areas, Johnston and Butterworth’s model produced estimates that were quite different to the simulated population. We recommend that the inclusion of density dependence on feeding areas in models that allow for mixing of whales on these feeding grounds be investigated further.