Travelling at whale speed

Killer whales are fast and powerful swimmers, exceeding 20-30 km/h in short bursts. But like us, or any animal, they cannot maintain maximum speeds all the time. This is especially true for group-living species that travel together; think of the #27s pod familiar from Shetland for example, which has a new calf since this summer and was spotted in Fair Isle the other day (4th Oct). They must travel hundreds of kilometres every year: the distance between Sumburgh head and Fair Isle is about 40 km, and one round trip around Shetland mainland 200 km.

Land-based visual monitoring can be used to estimate the speeds that killer whales swim when transiting from A to B (see for example this 2009 paper by Williams & Noren). I thought I would share with you a couple of rough estimates of transit speed that I made for the #27s pod when we tracked it in Shetland this summer, just based on sighting data – I didn’t have a theodolite like Williams & Noren though, so my precision is not going to be the same!

I calculated ~1.5-1.8 m/s (5.5-6.5 km/h) across three different tracks: crossing from Mousa to Cunningsburgh, between leaving the Cunningsburgh skerries to arriving at the nearby headland (Helli Ness; footage above), and moving further offshore outside Aithsetter and Fladdabister. The values average over brief variations in speed and likely different behaviour modes, but they do give you something to compare to the next day when the pod moved visibly faster (see this video). Keeping in mind how tricky it is to get to the whales and record exact sighting location and times, I arrived at an approximate range of 2.5-2.7 m/s (9-10 km/h) between Mossbank and Toft. Although anecdotal, the 1.5 vs. 2.7 m/s match with the average vs. max travel speeds reported in the literature. Interestingly, the optimal cruising speed that minimizes energy expenditure per unit of distance (‘cost of transport’) is thought to be at the higher end of this range for killer whales.

So, at a travel speed of 5.5 km/h, the #27s pod could have made the crossing from Shetland to Fair Isle in just over 7 hours.

In the future I’m hoping we can get at more precise and accurate swim speed measurements, with concurrent breathing rates, using drone footage in collaboration with local videographers. This would help us understand the whales’ energetic cost of foraging in the coastal waters of Shetland. Watch this space 😊

The whales at Mossbank.
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