Researchers in Denmark have devised a way to accurately estimate the weight of free-living whales using only aerial images taken by drones.
By measuring the body length, width and height of southern right whales photographed by drones, the researchers say they have been able to develop a model that accurately calculates the body volume and mass of the whales.
Because of their large size and aquatic life, the only way up to now to obtain data on the body mass of whales has been to weigh dead or stranded individuals.
“Knowing the body mass of free-living whales opens up new avenues of research,” says assistant professor Fredrik Christiansen of the Aarhus Institute, and lead author of the study.
“We will now be able to look at the growth of known aged individuals to calculate their body mass increase over time and the energy requirements for growth.
“We will also be able to look at the daily energy requirements of whales and calculate how much prey they need to consume.”
The model is being used to assess the impact of kelp gull harassment on the health and survival of southern right whale calves.
By adjusting the parameters of the model, the approach can also be used to estimate the size of other marine mammals where alternative, more invasive, methods aren’t feasible or desirable.
Baleen whales, which include species like the blue whale, are the largest animals on this planet, with body mass being central to their success as an animal group.
However, data on their size has historically been limited to dead specimens, with most samples coming from whaling operations, accidental fisheries bycatch or beach strandings.
Collecting data on dead whales has limitations such as being unable to collect longitudinal data over a whale’s life span and inaccuracies from physical distortion of carcasses caused by bloating and deflation.
“The difficulty in measuring body mass reliably in free-living whales, has prevented the inclusion of body mass in many studies in ecology, physiology and bioenergetics,” adds Christiansen.
“This innovative new approach will make it possible to finally include this central variable into future studies of free-living whales.”
To calculate the body volume and mass of southern right whales the researchers first took aerial photos of 86 individuals off the coast of Argentina.
The clear waters and the large number of whales that gather there every winter for breeding made it an ideal place to collect high quality images of both the dorsal and lateral sides of the whales.
From these images they were able to obtain length, width and height measurements.
The measurements could then be used to accurately model the body shape and volume of the whales.
“We used this model to estimate the body volume of whales caught in scientific whaling operations, for which body girth and mass was known,” says Christiansen.
“From those estimates of body volume, we could then calculate the density of the whales, which we could in turn use to estimate the mass of free-living whales photographed by our drones.”