Project 0809/9

Project Title: A Comparison of Group IV Humpback Whale Population Estimates from Two Key Locations Along the Western Australian Coast - Implications for Future Survey Location and Methodology

Chief Investigators: Dr Chandra Salgado Kent and Mr Curt Jenner

The overall objective of this study was to assess the current status of the humpback whale population that migrates along the western Australian coastline each year. This project aimed to fill significant gaps in current knowledge by employing the latest statistical techniques to analyse seven years of survey data from North West Cape (NWC) to estimate sex ratio, population size; and to compare results to those from Shark Bay, ~400 km south of NWC. By addressing these needs the proposed project additionally aimed to elucidate on current survey design efficacy and practicality; which will have implications for future and ongoing locations and methodologies for monitoring humpback whales.

The results from the study indicated that the best humpback whale population size estimates from surveys at NWC were 7,276 (CI = 4,993-10,167) in 2000, 10,548 (CI = 6,502-48,622) in 2001, 18,692 (CI = 12,980-24,477) in 2006, 20,044 (CI = 13,815-31,646) in 2007, and 26,100 (CI = 20,152-33,272) in 2008. When combined with the best estimates for surveys conducted in 1999, 2005 and 2008 at Shark Bay (from Bannister and Hedley 2001, Paxton et al. 2005, Paxton et al., in press, and Hedley et al., 2009), the trend is of an over all annual increase rate of 12.35%, which is 0.5% greater than the most recent estimate of the maximum plausible increase rates for humpback whales (max 11.8%; Zerbini et al., 2010). While the population size estimates seem reasonable and comparable to previous increase rates of 10-12%, we recommend reducing uncertainty and increasing accuracy of future estimates by: 1) focusing surveys on the northern migration, 2) increasing survey effort, and 3) conducting effective double platform surveys. Finally, previous population size estimates assume that the entire population of humpback whales, with an expected composition of 50% males and 50% females (based on foetus sex ratios), migrates north to the Kimberley region. However, previous vessel based biopsy sampling has resulted in male biased sex ratios (>2 males to 1 female), which raises the question of whether a cohort of females over-winters in Antarctica.

To test for sampling biases associated with vessel biopsy data, predictive Generalized Additive Models to mathematically model the relationships between humpback whale sex ratio and physical, environmental and behavioural variables were developed from boat based biopsy surveys conducted in 2002 and 2003 at NWC. The best model which included group size as a predictor was then applied to predict the sex ratio of whales observed during the aerial surveys in the same study area (during 2000, 2001, 2006, 2007, and 2008). The raw boat based data resulted in a male biased sex ratio of approximately 2 to 1, while the predicted sex ratio of whales observed during aerial survey was approximately 1 to 1. While further validation of the model assumptions is required, the results suggest that equal numbers of male and female humpback whales migrate past NWC.