The commercial hunting of whales during most of the 20th century caused the largest animals ever to exist to shrink in size. This decrease in weight and length became noticeable at least 40 years before the population crashed, suggesting that body size could be an early marker of when animal communities are about to collapse.
The researchers, who have published their results in Nature Ecology & Evolution, used data collected on whale abundance, weight, and length between 1900 and 1985, after which the commercial hunting of the animals became illegal. They found that during this period, not only did whale abundance decline, but the weights and lengths of the animals also dropped, in some cased up to four decades before it became apparent that the population was also crashing.
“We looked at data on blue, fin, sei and sperm whales and found significant declines in body size, with sperm whales taken in the 1980s four meters [13 feet] shorter on average than those in 1905,” explains the University of Zurich’s Christopher Clements in a statement. “This means that warning signals were detectable up to 40 years before a population collapse.”
When assessing how an animal community is responding to harvesting, it is standard for researchers and managers to use abundance data. From this, they can then make a judgement on how sustainable current quotas are and whether particular groups are at risk. However, there are issues with using these figures, as they are often incomplete and fail to sample all of a population, meaning they can be fairly inaccurate.
It is now thought that by the time the memorandum came into play, the population of blue whales had been reduced to just 6 percent of pre-hunting levels. The team of scientists looking at the body size data from the whales found that they could predict the imminent crash of the animals way before it was clear their numbers were on the edge of a cliff.
They think that as the larger animals were targeted by whaling ships first, it added a selection pressure to the marine mammals that pushed for smaller individuals. This took decades to manifest itself in terms of reduced body size due to the long generation time of whales, but can be seen more easily for other species.
It is also known, for example, that the average size of some species of commercially important fish have been declining, as larger individuals and those that mature at younger ages are picked off first. This new information may now be applied to other species that are at risk of over-harvesting.