Right whales born in 1981 grew a metre longer than they do today

A North Atlantic right whale photographed using a drone

John Durban (NOAA) Holly Fearnbach (SR3), NMFS research permit #17355

Surveillance of right whales in the North Atlantic show that individuals born today will grow to be 1 metre shorter, on average, than whales born in the early 1980s.

Joshua Stewart at the National Oceanic Atmospheric Administration in San Diego, California, his colleagues have used surveillance data collected from aircraft drones to investigate how North Atlantic right whales (Eubalaena glacialis) have changed over time.

The whales have been monitored consistently for decades, researchers can identify individuals know when each was born. Stewart’s team collected 202 length measurements of 129 of the whales born between 1981 2019: 133 measurements were taken from aircraft between 2000 2002, 69 measurements were obtained using remotely operated drones between 2016 2019.

The researchers then paired these length measurements with each whale’s birth year whether or not the whale its mother had a history of being entangled with fishing gear.

Whales born in 1981 were typically longer as fully grown adults than whales born more recently. The measurements suggest that, on average, for every year after 1981 a given whale was born, its body length was 2.5 centimetres shorter. This corresponds to a 7.3 per cent decline in maximum body length – means a whale born this year would be expected to reach an adult size a metre shorter than that of a right whale born in 1981.

“The whales that are born more recently are growing to sizes that are shorter than we would expect,” says Stewart.

Given that North Atlantic right whales are among the most endangered whales in the world, there is detailed individual-level information available on each whale their entanglement history with fishing gear. “We’re interested in this species in particular because they’re a critically endangered species – there are fewer than 400 left,” says Stewart.

“[The stunted growth] could be due to cumulative impacts as there’s so much going on in the North Atlantic,” says Stewart. Although North Atlantic right whales are protected from direct catch, they are susceptible to fishing gear entanglements. More than 80 per cent of individual whales have scars from rope entanglements, says Clay George at the Georgia Department of Natural Resources, who wasn’t involved in the study.

Increased drag from swimming while pulling fishing gear behind them could cause the right whales to expend energy that would have otherwise been used for growth.

Stewart his colleagues also found that whales whose mothers were entangled during the nursing period were significantly shorter.

“Rapidly changing ocean conditions as a result of climate change is affecting their prey availability, which could be another contributor,” says Stewart. “They also get lots of vessel traffic which disrupts them on their feeding grounds where they can even get hit by boats.”

Journal reference: Current Biology, DOI: 10.1016/j.cub.2021.04.067

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