Humpback whales (Megaptera novaeangliae) are a moderately large Baleen whale, that belong to the Order Cetacea and the Family Balaenopteridae. Adults range in length from 12 – 16 m, with females generally 1 – 1.5 m longer than males (Chittleborough, 1965). Humpback whales are made distinct by their long pectoral fins, the hump on their dorsal fins and the individual markings on the underside and trailing edge of their fluke and tail (which can be used for individual identification of whales; Katona et al., 1979.
Humpback whales are also known to produce an array of complex sounds and songs (this is particularly common among solitary males). The sounds vary in frequency from 20 Hz to 24 KHz (Tyack, 1981) and are used for a variety of functions, including foraging, breeding and other social events (Clapham, 2000).
Male whales reach maturity between 3 – 6 years, and females between 4 – 5 years (Chittleborough, 1965). Humpback whales are conceived in their winter breeding grounds. The gestation takes approximately 11 months, with calves being born in the breeding grounds the following year. At birth, calves are approximately 4 – 5 m in length and weigh approximately 2 tonnes. Mothers produce nearly 240 litres of milk for their calves each day. Calves are weaned at 11 months, as they return to their winter breeding grounds as yearlings, often with their mothers (Valsecchi et al., 2001).
There are two distinct humpback whale populations in Australian waters; those that migrate along the east coast, known as the Breeding Stock E population, and those that migrate along the west coast, known as the Breeding Stock D population (Chittleborough, 1965).Our study is concerned with this west coast population of humpback whales.
Both groups make the annual migration from their feeding grounds in the Antarctic to their northern breeding and calving grounds (Chittleborough, 1965). This migration generally takes place between June and November each year (Jenner et al., 2001).
Humpback whales are protected under both State and Federal legislation. They are listed as a ‘vulnerable’ and ‘migratory species’ under the Commonwealth Environmental Protection and Biodiversity Conservation Act 1999, and as ‘rare and likely to become extinct’ under the Western Australian Wildlife Conservation Act 1950.
Historically, the size of the Breeding Stock D population was estimated to lie between 12, 000 – 16, 000 whales. However, as a result of commercial whaling, by 1962, the population was thought to have declined to less than 800 whales (Chittleborough, 1965).
Since the cease of commercial whaling in 1963, whale numbers have increased exponentially off the west coast of Australia (Jenner and Jenner, 1996; Bannister and Hedley, 2001). Current abundance estimates suggest that the Breeding Stock D population lies somewhere between 26, 000 – 34, 000 individual whales (Hedley et al., 2009; Kent et al., 2012).
The National Humpback Whale Recovery Plan 2005-2010 (DEH, 2010), lists the area between Broome and Camden Sound as the calving area for the Breeding Stock D population. In contrast, the RPS study (2010), which was conducted as part of the environmental impact assessment process, maintained that the main calving ground stretched from Pender Bay to Camden Sound. A recent study conducted by Double et al. (2012), found that many whales calve and terminate their migration south of these recognised calving areas. This led the authors to conclude that there is a broad spatial distribution of calving events within the population and that the area from the Lacepede Islands to Camden Sound should not be seen as an exclusive calving area.