Cook Islands Humpback Whales – Part 2

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Gerald McCormack, CINHT

Soviet illegal whaling

The ‘Aluet’ (Aneyt), a Soviet Union factory ship. Photo by Yulia Ivashchenko.

The ‘Aluet’ (Aneyt), a Soviet Union factory ship, 1958. Photo by Yulia Ivashchenko.I came to Rarotonga in 1980 and over the next few years periodically saw a humpback or two. At that time Ron Powell and George Cowan told me that humpbacks were not 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.

Graph of the Oceanic Humpback Whales population.

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.

Humpback Whale MigrationIn 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.

Humpback whale migration

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% of the whales in Area VI were from Tonga; 80% in Area I were from Colombia; and Area IV was 30% from west Australia and surprisingly 30% 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% 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% recovered.

They categorised the Oceania sub-population (including the east Australia sub-stock) as Endangered because it had suffered a decline of  73% 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)

Recovery to 2014 – IWC

In 2014 IWC undertook a detailed assessment of the population trends of the Oceania sub-population, from which they excluded the east Australian sub-stock E1 (IWC 2014 Annex H). See Figure 4 for a graphical summary of the assessment.

The graph shows that before 1949 there were about 14,000 Oceania humpbacks; under intense whaling pressure the sub-population declined gradually to 1960 when it suddenly collapsed. In 1966 there was probably only about 140 humpbacks coming into Oceania. It is little wonder people rarely saw any whales off Rarotonga during the 1970s, although dive operator Greg Wilson (pers.comm.) reports that they usually saw three or four whales each year during the 1970s.

As of 2014 the sub-population had recovered to about 5,200 which was about 37% of the number in the 1940s, and it will probably take them another 20 years to recover their pre-Soviet whaling numbers. The Oceania recovery has been slower than west Australia and east Australia, 90% and 63% respectively.

It is not clear when IUCN will re-assess the Oceania sub-population. However, if the IWC estimates are correct the sub-population should reach 50% of its 1940s number around 2020 and then be categorized as of Least Concern.

In a separate assessment by NOAA in 2015 (Bettridge et al.) it was concluded that the east Australia sub-population is not at risk of extinction with high certainty (>80% votes), while the Oceania sub-population (New Caledonia to French Polynesia) is not at risk of extinction with moderate certainty (68% votes).

Now that IWC is accepting that the east Australian E1 sub-stock does not belong in Oceania with sub-stocks E2 and E3 it is probably time to revise the naïve model notation system. Most data also shows that the Cook Islands F1 sub-stock belongs with Tonga E3 and not with sub-stock F2 in French Polynesia.

 Feeding in the cold

During the southern Summer humpbacks are near Antarctica in near freezing water (about 2°C) feeding on the shrimp-like Antarctic Krill. Krill are only 3-5cm body-length, but they form immense swarms outside the sea ice around Antarctica; they are estimated to be one of the most abundant animals on earth.  The humpbacks lunge through the swarms of krill with their mouths wide to take an immense volume of water into their greatly extended mouth-cavity. They then expel the water through their comb-like baleen plates to retain the krill to swallow.

A detailed study of humpback feeding in May near Antarctica showed that during the daylight hours (8am-3pm) they rested at the surface with occasional dives to 50m. The krill migrated to the surface around 4pm (dusk) and the humpbacks dived up to 10 dives-per-hour throughout the night. They usually dived from 50-150m, although some dived to about 300m and one was recorded at 370m. In the dawn twilight the krill started to sink and feed-diving stopped around 7am.(Nowacek et al. 2011, Friedlaender et al. 2013) During each dive the whales made brief accelerated lunges after which they closed their mouths and expelled the water to sieve the prey to swallow. During feeding dives of less than 50m they typically did 2-3 lunges, while during 100m dives they often performed 6-8 lunges. (Ware et al. 2011)

Dietz et al. (2002) found a similar pattern diving for whales feeding near Greenland. The deepest accurately recorded dive was 395m, and there was some indirect evidence that a few dived to near 500m.

Starving in paradise

The South Pacific humpbacks migrate from their feeding grounds near Antarctica to the warmer subtropical waters to give birth and nurse their calves, court and mate. Ratios of males to females of 1.5-to-1 in French Polynesia and of 2.4-to-1 in east Australia indicate that many females do not migrate every year (Branch 2011). It is thought they migrate every two or three years.

During migration and in the subtropics there is not enough krill or small fish to cover the high energy cost of lunge feeding – so they starve for 6-8 months and live entirely off the fats in their blubber. This is especially true in places like Tonga and the Cook Islands which are in one of the most plankton poor areas of the South Pacific. The exception, is some opportunistic feeding off New South Wales where krill or small fish sometimes congregate when nutrient levels are higher than usual (Silva et al. 2011).

In the winter breeding grounds humpbacks spend most of their time in the shallow near-shore environment where the seabed is less than 100mbsl (metres below sea level). Research on male humpbacks in Hawai‘i (Baird et al. 2000) showed they spent 80% of their time within 30m of the surface, 15% between 30-100mbsl and about 5% deeper than 100mbsl with a maximum depth of 180mbsl involving dive times of up to 25 minutes. Of the 13 tagged whales, seven spent time in water greater than 100m depth and seven dove to more than 100mbsl. The authors used descent and ascent rates and dive duration to estimate that the whales could theoretically dive to about 650mbsl, although no such dives were recorded.

In the Cook Islands, humpbacks spend most of their time cruising, shallow diving and breaching on the leeward side of islands. Females make various sounds but not songs, while males spend a considerable amount of time singing their long and complex songs. The songs are stereotyped within a population but evolve over time. New “songs have been documented radiating repeatedly across the region from east to west, from east Australia to French Polynesia, usually over a period of two years” (Garland et al. 2013). While singing for hours on end is the norm in the breeding grounds, they sing only sometimes during migration and infrequently in the feeding grounds. Despite the infrequency of singing around Antarctica, Garland et al. showed that in one case it was there that New Caledonian whales acquired a song from east Australia whales.

People have an auditory range from a low of about 20Hz to a high of around 20,000Hz, being most sensitive from 1,000-5,000Hz.  Humpback males sing in the range 20-3,000Hz, which is all within the range of human hearing. It has been estimated that they can hear calls as quiet as 60dB (in water) which is nearly equivalent to the quietest sound we can hear in air.

Their songs are loud at around 170dB (in water at 1m) which is four times “louder” than a 12m fishing boat moving at 7kt or a typical motorboat.  Over what distance can they hear the songs?

It is known that Blue Whales and Fin Whales can send low frequency calls over distances of several thousand kilometres in the Deep Sound Channels (700-1,500mbsl), but there is no evidence that humpbacks use this channel.

I have not managed to find scientific information on how far humpbacks can communicate and can only offer a loose theoretical value. Outside the Deep Sound Channel sounds decrease by 6dB for each doubling of distance. A call of 170dB (at 1m) would decrease to 60dB by 500km and be inaudible to a whale. In reality the background noise from ships and boats would reduce the effective distance of communication.


Humpback populations have been recovering since Soviet whaling ceased in 1973. The Oceania sub-population, from New Caledonia to French Polynesia, is one of the slowest to recover. The IWC assessment of 2014 put it at 37% recovered and predicted around 50% recovery by 2020. What could prevent the continued recovery?

The most obvious direct threat would be a resumption of commercial whaling and we should all support the countries and organisations that oppose commercial whaling regardless of the level of recovery. A more subtle threat could come from the establishment of a krill fishery around Antarctica. Although such a fishery has so far proven to be uneconomic, a large krill fishery could seriously impact whale health by making feeding more difficult.

Author’s notes
First published CI News Oct 2015 
Photo from Google
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Gerald McCormack

Posted by Gerald

Gerald has worked on Cook Islands marine and terrestrial biodiversity since 1980. He was the foundation Director of the Natural Heritage Trust from 1990 to 2020.

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