How do you weigh a blue whale?

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Explaining exactly how much a whale weighs is quite a difficult question to answer because there are currently between 80 and 90 different species of cetaceans and each species has a different size and weight. 

Cetacean species are divided into two groups known as suborders of toothed whales and baleen whales. The blue whale is the largest known animal on the planet. 

How do you weigh the largest animals on the planet?

Up until now, it has only possible to weigh whales once they were found dead on beaches, as explained in an article by Nobbot. Scientists have solved this problem thanks to a method called photogrammetry, a method that could also contribute to the conservation of these magical species.

Photogrammetry?

With the help of aerial photographs taken by drones, an international team led by Danish marine biologist and Aarhus University professor Frederik Christiansen used images taken by drones to create a mathematical model that allowed them to calculate the dimensions and mass of a total of 86 wild southern right whales (Eubalaena australis). In fact, this technique is already used to assess calf survival and has many potential uses in species conservation.

Body mass is a key factor in the success of whales as a group, determining their energy use, food requirement and growth rate. However, most of what we knew about whale body size came from ancient literature about hunting whales or animals that end up stranded on the beach or caught in fishing gear.

"It's very difficult to measure a whale on a scale, I mean you have to kill it to do it, and that's exactly what we're avoiding here," said Fredrik Christiansen of the Aarhus Institute for Advanced Studies.

Researchers flew a drone over whales that swim in clear waters, capturing photographs as adults and calves surfaced to breathe, including backs and sides as the whales turned around. From all these photographs, they were able to obtain measurements of length, width and height for 86 individuals.

The experts, who detail their study in the journal Methods in Ecology and Evolution, discovered that they could obtain a good representation of the shape of the whales' bodies. Then they converted the body shape, or volume, into the mass.

"The ability to predict the body mass of wild whales opens up the opportunity for us to look at animals over time and see how they change, how they grow," said Professor Christiansen.

Drone studies could help in conservation by monitoring the health of different populations of whales in the oceans.

The approach could also be used to estimate the size of other marine mammals by adjusting model parameters.

The research was conducted in collaboration with the Southern Right Whale Health Monitoring Program in Argentina and the Woods Hole Oceanographic Institution in the United States.

Reference: 'Estimating body mass of free-living whales using aerial photogrammetry and 3D volumetrics' Fredrik Christiansen, Mariano Sironi, Michael J. Moore, Matías Di Martino, Marcos Ricciardi, Hunter A. Warick, Duncan J. Irschick, Robert Gutierrez and Marcela M. Uhart. Methods in Ecology and Evolution 2019. https://doi.org/10.1111/2041-210X.13298

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