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National Marine Mammal Laboratory (NMML)

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April-June 2006
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Cetacean Assessment & Ecology Program

International Whaling Commission Comprehensive Assessment of Southern Hemisphere Humpback Whales

Two NMML scientists, Phil Clapham and Paul Wade, flew to Tasmania in April 2006 to attend the International Whaling Commission’s (IWC) Workshop on the Comprehensive Assessment of Southern Hemisphere Humpback Whales. An assessment reviews all available information for a population of whales; by analyzing current numbers and growth rates together with the population’s catch history, the assessment attempts to determine the original (prewhaling) size of the stock and its current status relative to environmental carrying capacity. More than 200,000 humpbacks were killed by whaling south of the equator in the 20th century, representing probably more than 90% of the pristine population. The catch included more than 48,000 whales taken illegally by the former Soviet Union, which in just two seasons (1959-60 and 1960-61) killed 25,000 humpbacks in the Antarctic south of Australia and New Zealand.

There are at least seven populations of humpback whales in the Southern Hemisphere, which all feed in Antarctic waters in the austral summer and undertake long annual migrations to winter breeding areas in the tropics. For example, whales feeding off the Antarctic Peninsula migrate thousands of miles up the western coast of South America to breed off Colombia and Ecuador, and even north of the equator as far up as Central America. Similarly, humpbacks migrating up the eastern coast of Australia (probably to breeding grounds in the Great Barrier Reef and other areas) feed in the productive polar waters to the south of that continent. However, these feeding grounds also appear to be used by humpbacks from other regions. While the eastern Australia group is large and clearly increasing, other populations (e.g., one which was once substantial off New Zealand and Fiji) show little sign of recovery from whaling. With Japan set to begin scientific whaling on humpbacks in the 2007-08 whaling season, the concern is that the catch may involve whales from one of these small, struggling stocks.

The Tasmania workshop decided that only three of the seven humpback populations could reasonably be assessed—those breeding off western South America, eastern South America, and western Australia—and this analysis was continued at the IWC’s annual Scientific Committee meeting in St Kitts in May-June 2006. Of these three assessments, that for the population off eastern South America was considered to be the most certain. It was estimated that this population is currently at 25%-34% of its pre-exploitation level, and that while there has been an observed increase in abundance in recent decades, the stock remains well below its pristine levels. Assessment modeling for the western South America stock was rather more complicated; the assessment suggested that current population size was at 30%-70%, but it was agreed that no conclusion could be drawn about the status of this stock, given the substantial variation in existing estimates for current abundance.

The breeding stock off western Australia was the most problematic of the three, particularly because of a concern regarding the potential for exchange on the Antarctic feeding grounds with the population that breeds off eastern Australia. As a result, the assessment for the western Australian stock was considered preliminary and will be reevaluated in the future; this will require clarifying stock structure of the humpback whale population(s) in Oceania and the potential for mixing in high latitudes. Nonetheless, it was agreed that there has been a substantial increase in the abundance of this population since protection.

The IWC briefly considered additional information on the remaining four Southern Hemisphere breeding stocks and will reevaluate the situation with regard to these populations in the future as more information becomes available. There was no further discussion on the issue of whether animals from small unrecovered populations in Oceania (e.g., New Zealand and Fiji) were feeding in the Antarctic area which is the focus of the Japanese scientific whaling program (JARPA II), but this remains a concern in light of Japan’s intention to kill 50 humpback whales per annum beginning in 2007-08.

By Phil Clapham
 

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