In the “The conservation of whales in the 21st century”
Population numbers for many whale species are poorly known and estimates are subject to disagreement. As a result, the IWC decided in 1989 to limit publication of whale population figures to species and stocks for which detailed and statistically sound assessments were available.
For the great whales, the IWC has agreed to abundance estimates for only some stocks of whales.
- Blue – fewer than 2000 in the Southern Hemisphere;
- Fin – 720,000 fins were taken from the Southern Hemisphere during the twentieth century. This population is now probably less than 20,000. There are no agreed estimates for fin whales in the North Atlantic or North Pacific;
- Sei – there are no agreed estimates for any ocean basin;
North Atlantic right – only 300–350, with at least 50 deaths recorded between 1970 and 2001. Attrition from ship strikes and entanglement in fishing gear may spell the end for this species;
- Southern right – between 7000 and 8000, probably at around 10 per cent of initial abundance;
- North Pacific right – eastern population critically endangered and likely consists of only tens of animals as a result of illegal whaling in the 1960s; western North Pacific population unknown but probably in the hundreds;
- Bowhead – between 7000 and 8000 for the Bering Sea stock, although the two other stocks are critically endangered – Atlantic bowheads probably number in the hundreds and the Spitsbergen stock is all but extinct;
- Humpback – some populations (eastern North Pacific, North Atlantic, west and east Australia) are recovering, while others (New Zealand, South Georgia) have shown little sign of recovery from near-extinction;
- Gray – eastern North Pacific gray whales are back from the brink of extinction in the 1920s to about their original abundance of 20–25,000. Western North Pacific grays are still on the edge of extinction, with an estimated 100-plus individuals;
- Bryde’s – no agreed population estimates for any ocean basin, but possibly in the tens of thousands worldwide;
- Sperm – no agreed abundance estimates, but were seriously depleted in the South Pacific by sail whalers of the nineteenth century;
- Minke – the IWC Scientific Committee estimates the North Atlantic population to be over 100,000. There are no agreed estimates for North Pacific or Southern Hemisphere.
20th century whaling in the Southern Hemisphere
Reported catches: 351,645
Unreported Soviet catches: 8,999
Total catch : 360,644
Reported catches: 735,087
Unreported Soviet catches: –9,971
Total catch : 725,116
Reported catches: 179,923
Unreported Soviet catches: 23,615
Total catch : 203,538
Reported catches: 6,310
Unreported Soviet catches: 1,447
Total catch : 7,757
Reported catches: 117,469
Unreported Soviet catches: –901
Total catch : 116,568
Reported catches: 162,528
Unreported Soviet catches: 45,831
Total catch : 208,359
Reported catches: 380,013
Unreported Soviet catches: 21,657
Total catch : 401,670
Reported catches: 988
Unreported Soviet catches: 3,350
Total catch : 4,338
Reported catches: 11,631
Unreported Soviet catches: –
Total catch : 11,631
Unreported Soviet catches: 94,027
Total catch : 2,039,621
Minke whale numbers
Even where population estimates are available, questions are frequently raised about their reliability. Minke whales are a case in point. Over the past two decades, more than US$50 million has been spent on sightings surveys, mainly for minke whales, in the Southern Ocean.
In the early 1990s the IWC Scientific Committee, after analysing the available data, agreed that minkes in the Southern Hemisphere numbered 760,000. In 2000, however, the Committee withdrew this advice in light of new survey data suggesting a much smaller population. It is unclear whether this is due to a real decline in population abundance or to inaccuracies in the survey methodology. It is unlikely that a new abundance estimate will be agreed before 2005.
Whales are slow breeders
A female sperm whale may produce one calf every five years, after reaching sexual maturity at nine years. Males reach breeding age in their late twenties. It is not known how many calves a female may bear before reaching menopause or the rate of natural sperm whale mortality. A young whale may suckle from her mother for up to 15 years.
Most baleen whale species produce one calf every three years. In years of poor environmental conditions, large whales (such as North Atlantic right whales) increase the time period between breeding.
Mortality rates in whales vary during their lives. Five to twenty per cent of young animals will die in their first few years of life. Once adulthood is reached, natural mortality drops to negligible levels. Orca females, for example, are very likely to survive their first 10-15 years of reproductive maturity. Some whales are known to live for more than 100 years, but, as in humans, that may only be a small percentage.
Scientists believe that it takes around 20 years on average for a female whale to replace itself with one mature female offspring. This does not account for the potential adverse impacts of new human-induced threats to whales such as bycatch, climate change, ozone depletion, marine pollution, ship strikes and underwater noise pollution.