New solution to weighty issue

There’s always been a problem with checking the mass, length and various other characteristics of wild whales.

Problem is – given that whales are very large and live in the ocean – the only way to get physiological data on them was if they were dead or stranded.

And as the authors of a study just published in Methods in Ecology and Evolution observe, examining lifeless, or nearly so, whales “added all sorts of uncertainties to our studies”.

Fortunately, innovative help is at hand. Using photographs taken by drones of free-swimming whales and historical data from whales derived from “scientific” whaling operations, a research team led by Fredrik Christiansen, from Aarhus Institute of Advanced Studies, Denmark, has developed a model that accurately calculates their body volume and mass.

“Knowing the body mass of free-living whales opens up new avenues of research,” says Christiansen.

“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.”

To create their model the researchers first took aerial photos of 86 southern right whales in clear waters off the coast of Península Valdés, Argentina, where large numbers gather every winter for breeding.

The study team was able to collect high-quality images of both the dorsal and lateral sides of the whales, from which they were able to calculate length, width and height measurements, and thus accurately model the body shape and volume of the mammals.

“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 these estimates of body volume, we could then calculate the density of the whales, which we in turn, could use to estimate the mass of free-living whales photographed by our drones.”

By adjusting the model’s parameters, the approach could also be used to estimate the size of other marine mammals, instead of gathering these data by more invasive methods.

Christiansen concedes that, although the model yields highly accurate body mass estimates, it does have some limitations due to the relative proportion of different tissues in baleen whales.

“We had to assume a constant body density of the whales, which is not realistic as the proportion of different body tissues (fat, muscle, etc) changes seasonally as the whales deposit or lose body condition,” he says.

But the model’s usefulness can’t be underestimated.

Co-authors Mariano Sironi and Marcela Uhart, from the Southern Right Whale Health Monitoring Program, are already using it to assess the impacts of kelp gull harassment on the health and survival of southern right whale calves.

“The use of drones to estimate whale weight and condition, as well as to individually track calves while they grow beside their mothers, has been a real breakthrough in our investigation,” say Sironi and Uhart.

Using the model, researchers have also collaborated with the Digital Life Project at the University of Massachusetts, USA, to build a 3D mesh of the right whale, and then – with computer graphic artist Robert Gutierrez – to recreate a full-colour 3D model of the whale.

The 3D model can be used for both scientific purposes, such as studying movement, and for educational uses.

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