Humpback whale (Megaptera novaeangliae) populations in the Southern Hemisphere (SH) were taken to the edge of extinction from over exploitation by whaling in the 19th and 20th centuries. Commercial whaling on SH humpback whales ended in 1963 (apart from illegal catches) and a moratorium on all commercial whaling was implemented in 1986. Since this time, many but not all, humpback whale populations in the SH have shown signs of recovery. The International Whaling Commission (IWC) is currently undertaking a review of the status of all SH humpback whale populations.
The aim of this thesis is to investigate the population status, structure and migratory interchange of Southern Hemisphere humpback whales (SHH) with a specific focus on east Australian humpback whales (EAH). This population has shown a strong recovery over the last 20 years. However, the migratory interchange and linkages among SHH populations, including EAH and neighbouring populations in Oceania, is not well understood. This uncertainty presents a potential challenge for the accurate assessment of stock structure and also makes it difficult to estimate pre-exploitation population size accurately and therefore determine the present status of their recovery.
This thesis documents the movement patterns of EAH based on existing and previously unpublished Discovery mark data. These data indicate that SHH form relatively discrete groups with strong linkages between tropical breeding grounds and specific Antarctic feeding areas. There is a relatively low incidence of large-scale movement between feeding areas, perhaps with the exception of Breeding Stock E (comprised of EAH, New Caledonia and Tonga breeding stocks), which appears to have a more broadly dispersed system of interchange between multiple breeding and feeding areas. These findings are consistent with those of similar recent studies using molecular and photo-identification techniques. It now appears that the dispersal of whales from Breeding Stock E is considerably wider than was originally considered to be their feeding area (between 130° E and 170° W), extending from 95° W to 87° E and covering a range of approximately 175° of longitude – nearly half the globe.
The IWC review involves an in-depth evaluation of the status of whale stocks. It includes the examination of issues such as current stock size, recent population trends, carrying capacity and productivity. This thesis provides valuable information on the recent population trend for EAH during their northward migration between 1998 and 2004. The annual rate of increase observed for humpback whales migrating past Cape Byron between 1998 and 2004 is calculated to be 11.0% (95% CI 2.3–20.5).
This research also derived two population estimates based on photo-identification capture recapture methods. The first estimate of 7,041 (95% CI: 4,075-10,008) is based on a multi-point single year (2005) population estimate using photo-identification data collected at Byron Bay, Hervey Bay and Ballina. The second estimate of 7,390 (95% CI: 4,040-10,739) is a multi-year population estimate for the population in 2005 using photo identification data collected at Byron Bay between 1999 and 2005. These are the first and only population estimates available for these sites. The results from this work are consistent with results for other survey areas on the east coast of Australia using different methodology. While the population estimates presented here utilise different assumptions and are potentially subject to different biases, the fact these studies obtained comparable results provides good confidence in the estimates.
Already data from this thesis have been applied for the purposes of conservation management. They were presented by the author to the IWC Comprehensive Assessment of Southern Hemisphere Humpback Whales (CASH) process and were a critical source of information in that process. Specifically, they were used to reduce the number of plausible stock structure hypotheses, to develop scenarios for the allocation of historic catch from feeding grounds to Breeding Stocks, and made a vital contribution to the identification of stock structure itself.
The thesis also reviews the management of EAH in Australia, their status under the Environmental Protection and Biodiversity Conservation Act 1999 (EPBC Act), and identifies further research required in order to address unresolved management and conservation questions. A review of the current status of the EAH population determined that they no longer meet the Threatened Species criteria under the EPBC Act and should be considered for delisting.
Overall, this thesis provides new insights into EAH population structure, status and management issues, and summarises an extensive body of work by the author with the support of a range of collaborators. This work will allow the IWC and Australian management authorities to better manage and conserve EAH into the future.