Fish cannot read a map. Yet salmon manage to find the river where they were born after thousands of kilometres of wandering at sea. Thick-lipped grey mullet know the channels of the Wadden Sea. Garfish swim northwards through the English Channel towards our coastal waters every spring. And mackerel that hunt here in large shoals in the shallow North Sea during summer overwinter in the deep fjords of Norway. They find the right routes because they have a built-in navigation system with storage space for refuelling locations, dangerous junctions and roadblocks. But sometimes they do lose their way, or deliberately swim off in a different direction.
Why fish sometimes know the way and sometimes do not is something I cannot fully explain myself. What I do know is that I am enormously curious about it. Last autumn, for that reason, I investigated the swimming behaviour of fifty female European eels in a canal in Friesland together with colleagues. A bubble screen had been installed in the canal to capture plastic from the water, close to the Wadden Sea. We wanted to know whether the eels were deterred by the bubbles and whether they could still find their way through Harlingen. That seemed to be no great obstacle: their transmitters were picked up before long off the Maasvlakte and even further south. They were probably heading for spawning grounds in the Sargasso Sea. We cannot be certain of that, because nobody has ever observed them reproducing there. So far, all adult European eels do swim in that direction, and eel larvae are found in the vicinity of the Sargasso Sea, so it seems a reasonable conclusion.
Why would you reverse your decision to head into salt water?
South is the right direction for European eels, but along the way there are quite a number of side roads offering opportunities to stray from the herd. We saw one of the eels swim into the Western Scheldt and continue upstream into the Upper Scheldt in Belgium – back into fresh water. That is strange, because why would you reverse your decision to head into salt water? That involves a great deal of osmoregulatory effort. Perhaps the eel sensed she was not fat enough to complete the long journey, caught a whiff of something appetising from Antwerp, or her navigation software crashed. It would be interesting to see whether male European eels suffer from this just as much. Unfortunately, we had only tagged females. I could already picture myself accepting the Ig Nobel Prize.
Other eels had meanwhile turned up neatly off the Belgian coast. Researchers there had shared observations from their receiver stations on an international data platform: the European Tracking Network. When I checked it again recently, I spotted a particularly remarkable entry. A transmitter number belonging to one of our eels had been detected in the Atlantic Ocean off the northern coast of Spain, close to Santiago de Compostela. Was this eel on pilgrimage? It soon became clear that God had nothing to do with it. When I asked the local fish researcher for additional data, the transmitter number appeared to have been generated by the equipment itself, caused by interference underwater. There went the eureka moment. Even in science, suffering is part of life.
Rob Kroes is a fish ecologist at ecological consultancy ATKB. He is also conducting doctoral research into migratory fish at the University of Amsterdam.