Usually, the blogs we write relate to the three main species we research at Elding. Therefore, in this one we decided to mix it up a little bit and write about the smallest cetacean in Iceland: the harbour porpoise. They are the fourth most commonly sighted cetacean on our whale watching trips so they still deserve every bit of attention, just like the other three common species (the humpback whale, minke whale and white-beaked dolphin).
Photo credit: Peter Vergooh/Minden Pictures from Hakai Magazine
As previously mentioned they are quite small, only about 1-2 m in length and weighing approximately 70-80 kg. They also have a blunt, round head along with a triangular dorsal fin. These features make them easy to distinguish from dolphins. A fun fact about their dorsal fins is that they have tiny tubercles on them. Tubercules are tiny, pebble-like bumps only a couple millimetres in size. Their function is unknown but another regular visitor to Faxaflói Bay, the humpback whale, also has tubercles. On humpbacks tubercles are along their rostrum (face) and pectoral fins. Whilst on whale-watching trips it is fairly common to see harbour porpoises alone or in small groups of up to 4 individuals. We do see them all year round. There are thought to be about 30,000 individuals in Icelandic waters but due to their shy nature and being harder to spot, this is harder to estimate.
Graphic taken from: Bar Harbor Whale Watch Co.
Harbour porpoises are toothed whales with 40-60 spade-shaped teeth on each jaw. They will only dive for short periods of 2-6 minutes, but they are very fast so could reappear anywhere. Over the course of an hour they have been seen to hunt over 500 fish which then means over a day they can eat about 10% of their own body weight. They mostly eat two species of small schooling fish: capelin and sandeels. Eating schooling fish allows for the capture of more individuals. They have to eat so much because they are mammals and so just like us have to maintain their body temperature. As they have small bodies (so a large surface area to volume ratio) they lose heat rapidly in the cold Icelandic waters, making their almost constant feeding not only impressive but essential for their survival. When in groups it has been recorded that harbour porpoises will hunt cooperatively, with each individual playing a unique role in the hunt. This teamwork allows them to be more effective hunters despite their small size.
Photo credit: Elfyn Pugh from ORCA
This species is also one of the few toothed whales where females are larger than males (known as reverse sexual size dimorphism). This is thought to be due to their very fast metabolisms and the high energetic costs of pregnancy. By being bigger the females can better withstand the pregnancy and nursing a calf. Therefore, to get to this bigger size females will reach sexual maturity later (average 3.5 years old) than males (average 3.11 years old). This also allows males to mate with as many partners as possible, maximising the number of calves they will pass their genes on to. Usually, the gestation period will be just under a year before another 8-12 months of lactation. The females will not give birth every year either. One of the other benefits of females being bigger is that they can dive deeper. This means it is possible they could hunt different species of fish to the males so they wouldn’t be competing with each other.
Most toothed whales will whistle to communicate with other members of their pod. Harbour porpoises are one of a small group of toothed whales that do not do this. Along with them a few types of small dolphins and the pygmy sperm whale also do not. One thing all these species have in common is that they are sometimes hunted by orcas. Harbour porpoises can produce clicks at very high frequencies, up to 150 kHz, whereas orcas can only hear up to 120 kHz. This can be too high for the orcas to hear, meaning the porpoises can hide in plain sight. Luckily for the porpoises, even though the orcas cannot hear them, they can also hear when orcas are around so they can find safety too.
Unfortunately, just as with other cetaceans, this species does still face a number of threats. Namely, bycatch is a huge cause of concern for porpoises. This is when they are caught unintentionally whilst fishing is taking place. In 2018 alone, approximately 2,000 individuals were caught around Iceland through bycatch, this is over 5 every day! Considering that this is only a reported figure, the true number may be even higher which echoes the worldwide crisis where about 300,000 cetaceans die every year through bycatch, primarily due to gillnets. Gillnets are most commonly used to catch lumpfish and cod around Iceland. Hopefully these numbers can be reduced with developments of pingers, a device that can make a sound to alert the cetacean that there is a net.
Earlier this year, in April, the European Cetacean Society issued a letter urging for action to be taken regarding the bycatch of harbour porpoises across Europe. This highlights how drastic the situation is becoming.
Photo credit: ASCOBANS
Another threat to harbour porpoises' way of life is noise pollution. When sound is above ambient levels, individuals have been seen to either move away if it is daytime, or at night they will dive deeper. This could of course have big issues on their diet. As mentioned they need to feed almost constantly and if they are being forced to move it could also then reduce their prey intake. As their name suggests, harbour porpoises often remain close to shore, straying no more than 10km away from land usually. This brings them into contact with a lot of boat traffic, meaning the sound pollution is fairly constant and highly disrupts their foraging behaviours.
Overall, harbour porpoises are an integral yet often underappreciated part of Iceland’s marine ecosystem. Despite their small size and elusive nature, they contribute greatly to the biodiversity of Icelandic waters and serve as indicators of the health of their environment. Their fascinating adaptations, such as the very high-frequency clicks for stealthiness, highlight the unique challenges they face and the resilience they've developed. However, human activities leading to bycatch and noise pollution pose serious threats to their survival. By raising awareness and supporting conservation initiatives, we can help safeguard harbour porpoises.
By Rob Hyman
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