At that time Ron Powell and George Cowan told me that humpbacks had not been seen for many years and they were keen to make a documentary to tell people that whales used to visit the Cook Islands.
During the 1980s, whale sightings slowly became more frequent and this trend continues. Why did they disappear?
The Soviet Union formally collapsed in 1991. In 1993 the Russian biologist, Alexey Yablokov revealed that Soviet whalers had implemented an immense global campaign of illegal and unreported whaling from 1948 to 1973.
This was despite the legal protection given whales in 1966 by the International Whaling Commission (IWC) of which the Soviet Union was a foundation member. The 1966 protection evolved into an international moratorium on commercial whaling in 1985 which was binding on all signatory nations.
Since its discovery in 1993, the details of Soviet whaling have been clarified and show that the populations of several species of whale around Antarctica were decimated during the 1950s and ’60s (Rocha et al. 2014).
To understand the past and predicted future of Cook Islands whales we need to look at the Oceania sub-population to which they belong.
The Oceania sub-population
The whales that breed in the subtropics of the South Pacific migrate about 5,000km southward to feed on krill around Antarctica in a band about 500-1,000km wide north of the sea ice.
In 1998 the IWC identified seven breeding stocks (A-G) in the Southern Hemisphere and linked them directly southward to six feeding areas (I-VI) around Antarctica. This simple linkage of breeding stocks to feeding areas is known as the Naïve Population Model and it provided an initial hypothesis to start assessing discrete populations and their recovery.
In the South Pacific the three IWC breeding stocks were E, F and G. The complex and widely dispersed nature of the breeding areas lead to Stock E being divided into three sub-stocks: east Australia (E1); New Caledonia (E2); and Tonga (E3). Stock F was divided into two sub-stocks: Cook Islands (F1); and French Polynesia (F2). The last stock was the Southeast Pacific stock (Stock G) breeding mainly off Colombia, but extending from 6°S (north Peru) across the Equator to 12°N (Costa Rica). See figure 3.
To assess population recovery of sub-stocks it is necessary to discover where each feeds around Antarctic so the impacts of Soviet whaling can be accurately assigned and original populations estimated.
Genetic analysis to 2008 (Albertson-Gibb et al.) indicated that 80 per cent of the whales in Area VI were from Tonga; 80 per cent in Area I were from Colombia; and Area IV was 30 per cent from west Australia and surprisingly 30 per cent from New Caledonia; they did not include data for east Australia whales. The overlapping of sub-stocks in the feeding area shows that the Naïve Population Model is in need of further development.
In general terms the Soviet whalers killed 22,570 in Area V, 7,195 in Area VI, and 649 in Area I (Clapham et al. 2009). Scientists break this data into smaller areas to better link it to the subtropical sub-stocks.
Genetic analysis in 2011 (Pastene et al.) gave further support for putting Tonga and Cook Islands whales together and feeding in Area VI. Although there was also evidence that whales from New Caledonia were mainly east of 135°E in Area V and might form a single stock with the Tonga and Cook Islands whales in Area VI. The only tagged Cook Islands whale tracked to Antarctic waters was in 2007 and it ended up in the far east section of Area VI (Hauser et al. 2010).
Pastene et al. found no evidence of French Polynesia whales in Area VI. It is surprising that there are more than 1,000 humpbacks nearby east of the Cook Islands that rarely interact during the breeding season or during the feeding season – is there a language barrier? The feeding area of the French Polynesia whales remains a mystery with very few being found in Area I. (Pastene et al. 2011, Poole pers.comm).
Some whales undertake unexpected migrations. One remarkable migration involved two whales that bred in American Samoa and fed at the Antarctic Peninsula (eastern Area I), a distance of 9,400km apart. One made a return journey to the same area of Antarctica: Area 1 in Jan 2002; American Samoa in Oct 2005; and Area 1 in May 2009 (31km from its 2005 position!). (Robbins et al. 2011).
Areas around Antarctica are not equally good feeding areas. Extensive surveys under the IWC IDCR/SOWER programmes around Antarctica reported in 2011 (Branch) showed high concentrations of humpbacks south of NZ in Area V and around the Antarctic Peninsula south of South America in the eastern section of Area I. There were only moderate densities through Area VI and most of Area I. See Figure 3. If one compares this distribution with the distribution of Antarctic Krill on KRILLBASE it can be seen that whales have higher densities in areas with high densities of krill.
Recovery to 2008 - IUCN
The International Union for the Conservation of Nature (IUCN) has elaborate criteria to assign species to categories of threat or risk of extinction for their famous Red List. Extinct (EX) is totally gone, Extinct in the wild (EW) means it survives only in captivity; and, in descending order of threat, there is Critically Endangered (CR), Endangered (EN), Vulnerable (VU), Near Threatened (NT) and Least Concern (LC).
The Humpback Whale was listed as Endangered in 1986 and 1988, and downgraded to Vulnerable for 1990, 1994 and 1996, and in 2008 it was further downgraded to Least Concern. Least Concern was based on evidence that the population continued to increase and that it was more than 50 per cent of the 1940s population, which was the threshold to be considered Vulnerable.
While assessing the humpback as a species to be of Least Concern, IUCN singled out the Oceania sub-population as being at a greater risk because it was only 27 per cent recovered.
They categorised the Oceania sub-population (including the east Australia sub-stock) as Endangered because it had suffered a decline of 73 per cent over the last three generations of 21.5 years each. This decline spanned criteria for Vulnerable and Endangered, and the latter was selected based on a balance between “precaution and credibility.” (Childerhouse et al. 2008).
Next week: Recovery to 2014