Harbour Porpoise
Size: Females = 160 cm (63 in) and Males = 145 cm (57 in)
Weight: Females ≥ 60 kg (132 lb) and Males = 50 kg (110 lb)
Diet: Variety of fish depending on availability such as squids, herring, capelin and cod fishes
The harbour porpoise is the smallest cetacean found in Icelandic waters, and is actually one of the smallest marine mammals in the world. The harbour porpoise is not a dolphin but belongs to a family called Phocoenidae. Only seven species of porpoises exists in the world today and the harbour porpoise is the most widely distributed one, but the harbour porpoise is the only porpoise species living in the North Atlantic Ocean. Porpoises have in common a short and stocky body which enables them to limit heat loss in cold waters. Apart from their characteristic body shape, the porpoises can be recognized from their round and oval head which lacks the pronounced snout, the common characteristics of dolphins. The harbour porpoise is extremely fast and agile swimmer which can make them very challenging to spot out at sea. However, they are commonly spotted in whale watching tours from Reykjavík, usually not far out off the harbour.
Behaviour
Most harbour porpoise groups are small, usually less than 8 individuals. They do, at times, aggregate into large, loose groups of 50 to several hundred animals, mostly for feeding or migration. Harbour porpoises are usually not found in close association with other species of cetaceans and instead are observed to avoid dolphins such as killer whales and bottlenose dolphins due to aggressive and sometimes lethal interactions.
The harbour porpoises are fast sprinters and incredibly agile swimmers. Once spotted at the surface the harbour porpoises look like small, black wheels turning at the surface. This swimming behaviour is called porpoising. They can be very acrobatic at the surface and often sprint full body out of the water. They are not deep divers.
Reproduction
Most calves are born from spring through mid-summer but females can become pregnant each year. Once a female harbour porpoise reaches 3 years of age, she will enter an annual reproductive schedule, thus, she will be simultaneously pregnant and lactating for most of her life. The gestation lasts for about 10.5 months. The harbour porpoise is a promiscuous breeder, but the males will mate with as many females as possible during the breeding season. In order to swim faster the male harbour porpoises are particularly muscular, though being smaller than the females; that gives them an advantage when seeking as many females as possible within a short time frame.
Vocalization
Harbour porpoises are known to employ narrowband, high-frequency echolocation signals. These echolocation pulses are very strong (approx. 150 dB) but can be heard by very few animals since the pulses are produced at extremely high frequency or between 140 – 160 kHz. They use these pulses to navigate in the deep and find prey but also for communication, without being heard by larger predators, such as killer whales.
Life expectancy and population size
The life span is on average 8 to 10 years, the oldest documented individual was 23 years old. The global abundance of harbour porpoises is estimated to be at least 700,000 individuals. Harbour porpoises ranging the continental shelf seas from SW Norway, south to Atlantic Portugal, are estimated to be around 380,000. There is a considerable lack of information about the Icelandic population size.
Due to their habitat choice, i.e. productive coastal waters, harbour porpoises are frequently captured incidentally by commercial fisheries both in Icelandic waters and throughout their distribution range. There is some hope that acoustic deterrents may help to reduce by-catch rates in gillnets in certain fisheries. Harbour porpoises react very sensitively to anthropogenic noise. Consequently, shipping, marine exploration, construction and operation of noisy equipment such as sonar are likely to affect the behaviour and distribution of the species.
Edda Elísabet Magnúsdóttir, Marine Biologist