Science - September 18, 2017

Small exotic fish turns up in North Sea

Tessa Louwerens

An unknown exotic fish was caught in the shrimp net of a Dutch trawler. Researcher Bram Couperus of Wageningen Marine Research dove straight into the mystery and was able to identify the little fish.

Crew members Peter Zaaijer and Hans Tap (with the fish) of the IJM8, together with researcher Bram Couperus (right). © Oscar Bos / Wageningen Marine Research

The fish was caught on the 8th of August, just off the Dutch coast, a little south of the Maasvlakte. Hans Tap, member of the crew on the IJM8, brought the 15-centimetre-long fish to Wageningen Marine Research. Couperus and a colleague subsequently dove into the field guides of the Northeast-Atlantic area and the Mediterranean Sea to identify the critter.

At first, it seemed futile. Colleagues in the United States and Australia where not able to help them either. Determined to find the name of the fish, Couperus systematically sifted through the Fishbase, a database that contains information on all fish species around the world. ‘It really was like looking for a needle in a haystack, especially as this little fish is not eye-catching. But I got very lucky: I eventually found the picture within thirty minutes!’ And suddenly, the animal had a name: the Indian driftfish, better known among scientists as Ariomma indicum, a genus of perciform fishes.

The Indian driftfish. © Oscar Bos / Wageningen Marine Research

It remains a mystery how the fish ended up in the North Sea. Couperus: ‘You would expect more reports if it had anything to do with climate change, along the African West Coast, for example. But this is the first sighting in the Atlantic area. In theory, the fish could have been brought along with ballast water. But the fish already measured 15 centimetres, which would mean that it either survived the winter in the North Sea or travelled with ballast water as an adult. Neither one seems plausible to me.’

As the fish did not have a Dutch name yet, Couperus was asked if he would like to provide one. ‘In Australia, they are called Eyebrow fish, and indeed: when one looks carefully, there is a kind of ridge above the eye. But the English name Indian driftfish is the most widely accepted, so I simply chose Indische driftvis [literal translation]. That makes this specific specimen “Drifty”.’

The fish has since been conserved and will be transported to the Naturalis Biodiversity Center in Leiden. A little piece of its pectoral fin was collected to confirm the finding through DNA analysis. This could also help determine its origin.