Porpoises are often mistaken for dolphins, but they are a group of fully aquatic mammals that have a striking resemblance to a dolphin.
Both porpoises and dolphins are classified under the family known as Phocoenidae, parvorder Odontoceti (toothed whales).
However, porpoises are more closely related to belugas and narwhals than dolphins.
Scientific Classification
- Kingdom: Animalia
- Phylum: Chordata
- Class: Mammalia
- Order: Artiodactyla
- Infraorder: Cetacea
- Superfamily: Delphinoidea
- Family: Phocoenidae
Porpoises are one of the largest mammals in the sea. During the Middle Ages, these animals were called ‘sea pigs’ or mereswines.
In those days, people consumed them a lot. There was a substantial population of porpoises residing along the Dutch coast. Those were when creatures like anchovies and other little fatty fish were abundant.
The porpoises were known to follow the fish into harbors, earning them the title of harbor porpoises. Porpoises became scarcer halfway into the 20th century. However, there has been a tremendous increase in sightings since 1995.
The porpoise is now the easiest-to-spot cetacean in the North Sea.
Spotting Porpoises
It is not exactly easy to see porpoises at sea. Unlike other dolphins, porpoises hardly ever jump out of the water randomly.
You’re not likely to see more than the upper part of a porpoise’s back with its dorsal fin when coming up for a breath of fresh air. Porpoises live alone or in groups of three to five animals or even more.
If you see two animals together, there is a high chance that they are mother and calf. These mammals sometimes form large groups when they migrate.
From a boat or land, the most likely time to notice a porpoise is during the winter.
Distribution
The porpoises naturally live in all shallow and relatively cold coastal seas. There are about 250,000 porpoises that live in the entire North Sea, of which there are tens of thousands in the Dutch waters.
When there were a ton of anchovies around, porpoises were also in surplus in the Zuiderzee and the Wadden Sea. These days, they are a rarity.
Diet
Porpoises are voracious eaters, and they find their food underwater with the aid of sonar. The porpoises that live in the Netherlands’ coastal region and those in the Baltic Sea are known to eat mostly small benthic fish like guppies.
The open North Sea porpoises primarily feed on herring, mackerel, and sprat. Porpoises that live in the German Wadden Sea mainly eat small flatfish.
The large flatfish are known to be dangerous for porpoises. A good percentage of porpoises that have been washed ashore were discovered to have choked to death after feeding on large flatfish.
Two decades ago, the porpoises living in the North Sea fed mainly on whiting. However, this species of fish is not that common anymore. That is why porpoises have switched to gobies.
Gobies are a much smaller kind of fish than whiting, so they need to eat much more of those to satisfy their needs. Some scientists have discovered that porpoises stranded in the summer seem to have issues finding enough food.
Porpoises feed around five kilograms of fish daily– this is approximately 10% of their body weight. A theory also suggests that porpoises help each other search for food using their sonar system.
You can imagine how easy it is to find food in the sea when the porpoise group is spread out.
Mating Season for Porpoises
Porpoises are known to mate from the period of June to the early parts of August. Porpoise calves are birthed 11 months later, making July the birthing peak.
Females become sexually mature once they are between the ages of 5 to 6. Most porpoise females do not bear calves every year. Calves are usually nursed for about 8 months, after which they adopt a fish diet.
That is not always the easiest process: many young porpoises typically have trouble finding the right amount of food to eat. Most porpoises found stranded along the beach are often not much older than one year.
At the age of one, porpoises are around 100 centimeters in length.
Porpoise Calves
Porpoise calves are nursed for about eight months, although they must try a fish during that period. This time is vital for the calf to learn all the tricks of the sailing trade from its mother.
In September 2011, workers on a gas production platform noticed these lessons. The mother remained under the platform with her calf for a couple of weeks.
During this time, the workers noticed how the calf was nursed regularly and how the mother occasionally brought fish to the surface, which she would then release close to the fast-approaching young.
The calf will then attempt to catch the little fish. It was a successful event sometimes, but that isn’t always the case.
After the nursing period, the calf will have to switch to a fish diet. That is not always an easy process as many young porpoises have been noticed to have often trouble getting enough food to eat.