David @ Tokyo

A paper submitted to the IWC Scientific Committee provides further confirmation of the recovery of the Antarctic blue whale:

ABSTRACT
Sightings from the IDCR and SOWER austral summer surveys were analysed to provide abundance estimates for Antarctic (true) blue whales (Balaenoptera musculus intermedia) south of 60°S. The IDCR/SOWER ship-borne surveys have completely circled the Antarctic three times: 1978/79–1983/84 (CPI), 1985/86–1990/91 (CPII) and 1991/92–2003/04 (CPIII), covering strata totalling 64.3%, 79.5% and 99.7% of the ocean surface between the pack ice and 60°S. During the surveys, blue whales were only rarely sighted but were present around the Antarctic. Average sighting rates (schools per 1,000 km of primary search effort) were 0.24 (CPI), 0.36 (CPII) and 0.78 (CPIII). Respective circumpolar abundance estimates were 453 (CV=0.40), 559 (CV=0.47) and 2,280 (CV=0.36), with mid-years of 1980/81, 1987/88 and 1997/98. When adjusted simply for unsurveyed regions, the circumpolar rate of increase was 8.2% (95% CI 3.8–12.5%) per year, although they are still under 1% of their pre-exploitation abundance. These abundance estimates are negatively biased because they exclude some Antarctic blue whales that are north of 60°S, and because a low number of blue whales on the trackline may be missed. Additionally, estimates may include a small proportion of pygmy blue whales, probably less than 1%. Abundance estimates were also provided for each IWC Management Area and for each individual survey, but these have high associated uncertainty.

Here's a reproduction of "Figure 1" from the document, showing blue whale sightings around Antarctica:
From the paper:

Antarctic blue whales were sighted around the Antarctic, thus it is not surprising that the abundance estimates are spread among all of the IWC Management Areas. Highest historical catches were taken from Areas I–III, which have lower current abundances of Antarctic blue whales than Areas IV–VI. However, estimates are highly variable from year-to-year because of the low numbers of sightings, a feature also apparent in JARPA estimates for Areas IV and V...

Assuming this pace of recovery is maintained, Antarctic blue whale abundance will still only recover to a level of 10,000 in 20 years time. Under such a scenario, ongoing (and indeed consensus) protection seems inevitable for the next 2 decades, at least.

The paper also notes that the rate of increase (albeit with the 95% confidence interval of 3.8–12.5%) is close to the theoretical biological maximum. It's fascinating given how scarce blue whales are that they are still able to find breeding opportunities to be able to exhibit such a recovery.

It will be interesting to follow developments as hopefully a clearer picture about the Antarctic blue whale recovery emerges, and any differences in rate of increase in different parts of the Antarctic become apparent.


The paper was authored by Trevor Branch, who is a leading player in the IWC SC's Antarctic baleen whale abundance estimate work.

Labels: blue whales, Trevor Branch