The beach on the river Weser in Sandstedt, a Lower Saxon community between the cities of Bremen and Bremerhaven, is Christian Langenstein's favorite spot. The zealous angler has spent countless hours here waiting for the eels to bite.
Lately, however, his patience has been rewarded not just with fish, but with a far more exciting experience - harbour porpoises, swimming just 30 metres from the river bank.
“The first time, it was two mother porpoises swimming with their calves,” the 47-year-old said, “and then a few days later I spotted three others.”
The porpoises, one of the smallest species of toothed whale, are unmistakable with their triangular dorsal fin.
“We have observed porpoises each spring for the past several years, swimming upstream either alone or in small groups, sometimes all 65 kilometres to Bremen,” said Denise Wenger, a biologist with the Society for Dolphin Conservation (GRD), based in Munich.
Their numbers are strong in the North Sea – an estimated 231,000 individuals live there – and it is thought the migration of fish such as herring and smelt may have attracted the porpoises to the Weser - 125 have been reported in the river since 2007.
“The porpoises are presumably following schools of fish coming from the North Sea to the river to spawn,” Wenger said.
But the exact reason for the porpoises' reappearance, after a decades-long absence from the North Sea coast and surrounding rivers, remains unclear.
The marine mammals, which can grow to up to 1.8 metres long and weigh up to 65 kilograms, have been sighted with increasing frequency also in the Ems, Jade and Elbe rivers.
“That's why we ask sailors, boat drivers, fishermen, residents and holidaymakers on the Weser to keep an eye out for the protected animals, and to share their observations with us,” said the 47-year-old biologist.
Langenstein is happy to cooperate with the animal authorities, admitting relief that his experiences haven't been dismissed as just a fisherman's tale.
Boat captain Holger Jureczko is another porpoise spotter. He saw two of them hunting on the river just 15 kilometres from Bremerhaven at the end of March.
“The water was smooth as a mirror and I could see very clearly how the animals would dive down for two or three minutes at a time, then come up every now and then for a breath of air,” said the 48-year-old skipper.
Jureczko, a member of the GRD, finds the marine mammals fascinating because “they are markedly intelligent and have extraordinary communication abilities,” he said.
The porpoises are also excellent swimmers, able to reach a top speed of 20 kilometres per hour and dive to a depth of more than 200 metres, while holding their breath for over six minutes.
To learn more about the Weser porpoises, two under-water microphones – known as click detectors – were installed in the waterway last year. The microphones pick up the noises used by the porpoises to navigate, hunt and communicate.
“Using this method, in the past year we've detected 34 sounds related to their food search, and 29 for regular communication,” said Sven Koschinski, one of a group of experts responsible for the interpretation of the sounds.
The data indicate that the porpoises can be found in the Weser primarily between mid-March and late-May, after which they return to the North Sea.
“Through this project, we want to raise awareness and affection for the little porpoises, so we can better protect them,” said Wenger.
“Only when we know what kind of activity takes place in a region can we take the necessary precautions.
"For example, we will sometimes ask that pleasure boats reduce their speed at certain times, or that construction work be rescheduled.”
Currently, the only area in Germany protected for whales is west of the Friesian island of Sylt. Porpoises are susceptible to being caught in fishing nets, while overfishing and climate change have also had a negative affect on their diet.
Porpoise sightings can be reported to the GRD at 089-74160410 or by email at firstname.lastname@example.org