Whales are the largest animals on the planet and important predators in the marine ecosystem. As a marine biologist, I was fortunate enough to see her up close. It is an amazing experience to watch a 14-meter-long and nearly 40-tonne whale mother gently push her "tiny" five-meter baby, which weighs almost a ton at birth, to the surface to breathe.
But how do I know that the whale weighed 40 tons? After all, we can not accurately capture an animal the size of a bus and simply place it on a scale. And swimming out to the sea and putting a measuring tape around your body is not a very smart thing.
This is a big problem for whale researchers. The sheer size of these animals is fundamental to their success, as they can store enough energy to travel long distances and find food in scattered locations. Although this should be taken into account, it is seldom possible to consider body weight as a variable in the study of wild whales, as it is so difficult to measure.
That's why we had to develop a new non-invasive method for weighing whales. The solution that colleagues and I found was to use a technique called photogrammetry, in which the body size of whales was measured using aerial images taken by drones.
We applied this technique to southern right whales in Península Valdés, Argentina. With two large round bays surrounding a central peninsula, Valdés has protected clear and shallow waters, attracting many whales that gather there to mate and give birth. This unusual geography also makes it a great place for explorers as we simply fly our drones offshore and photograph many whales in perfect coastal conditions.
With our drone we were able to measure the body length, width, girth and height of the whales. From these measurements, we were able to create an accurate 3D model of the whales that allowed us to predict their body volume.
Here you can play with the model whale:
Model 55B – 3D mesh Southern Right Whale by DigitalLife3D on Sketchfab
The weight of these whales can not be determined by the Determine volume – we would also have to determine their density. For this, we had to go through old records of closely related North Pacific right whales killed by scientific whaling operations. In these records, the length and circumference of each whale, but especially the weight were recorded.
Then, using our 3D model, we determined the volume of each dead whale by its length and width, which we refer to as its weight. Calculate a volume-weight factor – or a density. This meant that we could only calculate the weight of the living southern right whales by their drone measurements. The complete method is now presented in the journal Methods in Ecology and Evolution.
Of course, this is not the first time someone has measured the weight of a whale. Most of our early knowledge of the great-whale physiology comes from the whaling industry, where whales have often been measured and sometimes even weighed under the Industry Protocol. However, our method offers a way to weigh the whales without having to kill them.
Photogrammetry is not only more harmless, but also allows to measure the same whale over time, to study growth and changes in body condition such as fat stores. It also enables us to study the health of small vulnerable populations who can not sustain scientific whaling. Therefore, our approach opens up a new opportunity for the study of the ecology and physiology of the great whale, which we believe will greatly benefit both the science and the conservation of these amazing creatures.
Another study that I carried out a few years ago has shown that noise from the drone does not penetrate the water, so we can observe it calmly from the top in its natural behavior during the measurement. The whales do not even notice we're there.
This article was re-published under a Creative Understanding by Fredrik Christiansen, Assistant Professor of Marine Biology, University of Aarhus. Read the original article.