Toxic algae, stinging jellyfish and testicle-hungry pacu fish. Danish waters have become a swirling sea of scary monsters of late. 

Now, the latest addition to the list of immigrant aquatic life is killer whales. The animals were identified by Skagen bird watcher Rolf Christensen, who spotted fins rather than feathers when he looked through his telescope early Tuesday morning. What he saw was a pod of three orcas swimming in the Skagerrak just north of Grenen, mainland Denmark's northernmost point. Christensen said he spotted a large adult male and two smaller females swimming just 500 metres from shore.

“When I first saw them through the telescope, I thought they were a submarine,” Christensen told Nordjyske.dk.

Christensen continued to watch the as the whales swam close to the shore for eight minutes before disappearing to the north.

“The water in the Skagerrak is 200 metres deep very close to the coastline,” he said. “There are not many places in Denmark where the water is that deep so close to shore, so this is a good place to see whales.”

The last time killer whales were sighted in Denmark was in May when two anglers from Thy, in north-western Jutland, found their boat surrounded by a pod of seven whales.

While whales are not everyday sights along the Danish coastline, they do surface form time to time.

Whales are occasionally spotted near the Lillebælt Bridge between Funen and Jutland, particularly in the waters off the coast of the town of Fredericia.The are also commonly seen in Baltic Sea around the island of Bornholm and the reach between the coastal towns of Helsingør in Denmark and Helsingborg in Sweden.

Large numbers of whales have been known to beach themselves on the island of Rømø, off the coast of western Jutland.