Orca behaviour can be generally split into foraging, travelling, socialising and resting. These behaviours are relatively easy to tell apart; travelling whales move consistently in a specific direction; socialising whales tend to be milling and there is usually a lot of surface activity; while resting whales lay stationary at the surface for a few minutes, all close together and in line (see the picture below).
When feeding, Icelandic orcas encircle herring schools to force the fish to school tighter and then use their tails to slap it and feed on each fish one by one. This behaviour has been extensively studied in Norwegian orcas and was named ‘carousel feeding’. In Norway this is clearly seen because the whales force the herring close to the surface and herring is often seen jumping out of the water. However, when we see orcas feeding in Vestmannaeyjar, herring is not seen jumping at the surface, suggesting the feeding behaviour might be a bit different.
We deployed Dtags on 4 whales in 2009 to investigate this and we were able to see that the whales seem to be feeding on herring schools at the bottom of their dives, instead of bringing the fish to the surface. This is probably because the differences in the herring behaviour between winter and summer and in the environment between the deep waters of the Norwegian fjords and the shallow waters of Icelandic herring spawning grounds affect the balance of benefits of bringing the fish to the surface.
This figure shows the dive profile of a tagged whale showing tail slaps produced at depth, suggesting feeding does not take place near the surface.
(Adapted from Samarra & Miller, 2015 – available here).
Although we understand more about the feeding behaviour of Icelandic orcas, many aspects remain poorly understood. For example, herding calls are only produced in some feeding events and the reasons why are still unknown. Other feeding behaviours, such as attacks on marine mammals, have also been observed in Iceland however we do not know which whales feed on marine mammals for the majority of these events.
Studies on the acoustic behaviour of Icelandic orcas have been underway since the 1980’s and have shown that, like other orca populations, orcas in Iceland also produce clicks, pulsed calls and whistles. However, some of their vocal behaviour is quite unique.
Icelandic orcas produce high-frequency whistles, most commonly at ultrasonic frequencies (>20 kHz), which means we cannot hear them.
Unlike the audible whistles produced during socialising, high-frequency whistles are short and have very simple contours. These whistles are produced by orcas in Norway and Shetland and also by some orcas in the Pacific. However, so far there is no evidence that the well-known resident and transient orcas produce these sounds too.
But not all whistles are the same. Orcas in the Pacific produce high-frequency whistles of lower frequency and predominantly with downsweep contours whereas in the Atlantic whistles can go up to 75 kHz and have a variety of contour shapes.
Icelandic killer whales also produce a very unique call: the herding call. It is such a long and low-frequency call that it stands out from others very clearly and can be heard when the whales are feeding. This call is thought to function in manipulating the herring, by making it school tighter and consequently an easier target for the whale’s tail slap.
Icelandic orcas feeding on herring are very vocal, producing a lot of calls, echolocation clicks, herding calls and tail slaps. In striking contrast, when the whales are travelling, they usually produce no sounds and can be silent for a long time. Below you can listen to recordings of Icelandic orcas showing the variety of sounds they produce.
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