We can now predict a potential crash in whale populations several decades in advance, and it looks like shrinking body size is the key indicator.

"It’s about finding some early warning signals that might indicate that a species has been over-harvested,” said Professor Mark Hindell from the University of Tasmania’s Institute for Marine and Antarctic Studies (IMAS).

Hindell and his colleagues have been studying the events that led to the collapse of whale populations in the 20th century.

Looking at historical whaling data, we found that the size of the whales reduced over time.

With the invention of explosive harpoons, steam-powered ships, and air compressors to keep fresh carcasses afloat, the late 1800s were a bad time to be a whale.

By 1900, the demand for fat to make soap and margarine had exceeded supplies, ramping up interest in whale oil and causing commercial whaling to experience a boom unlike anything the world had seen previously.

There was such an unmitigated onslaught on whales that, in terms of biomass, it’s become known as the greatest wildlife exploitation event in human history.

What commercial whaling did to whale populations around the world – particularly in the Southern Ocean, where huge numbers of whales were discovered in 1904 – was so disastrous, that an international moratorium was placed on commercial whaling in 1985.

But by that stage, the damage was already done. By 1970, the total number of blue whales had decreased to less than 6,000, and humpbacks weren’t faring much better. Populations of Pacific grey, sei, and sperm whales had been halved since the early 1900s.

While humpbacks have made an unexpectedly strong comeback, with scientists now suggesting that they could return to pre-whaling numbers by 2050, other species aren’t even expected to reach half their pre-whaling numbers by 2100.

Examining the historic whaling data on four species between 1900 and 1985, Hindell and his colleagues found that the average body size dropped rapidly during the mid to late 20th century.

Sperm whales (Physeter macrocephalus) were the worst off, ending up four metres shorter in the 1980s than in 1905.

Blue whales (Balaenoptera musculus) ended up losing about five metres, on average, between the 1920s and 1970s, and fin (Balaenoptera physalus) and sei whales (Balaenoptera borealis) lost between two and three metres.

The researchers found these shrinking sizes were detectable a full 40 years prior to the population collapse, and say they could have been used at the time to set up sustainable fishing practices.

It’s a signal if you’re a whaling manager now, that if there’s a sharp de-cline in the size, we need to slow down.

It’s not clear why whales shrink as they start to feel the pressure of exploitation, but it could be that larger whales are easier for whalers to spot, so the big spe-cies get picked off first, followed by the big individuals within each smaller species.

Professor Hindell and his team are now looking for other indicators of population declines by surveying seal, whale, and penguin numbers in the Southern Ocean.

Hopefully next time, we’ll be more prepared.

By Bec Crew.

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