Stock study

What has happened to the three-spined stickleback?

Miraculously, the stickleback population in Lake Constance seems to have collapsed within a very short space of time.

(Bild: Rostislav Stefanek/Shutterstock.com)

The three-spined stickleback is well known on Lake Constance - and its image is anything but good. Among other things, the invasive species is blamed for the decline in whitefish stocks. In this respect, it is good news that the number of sticklebacks has apparently fallen massively.

Every five years, the fisheries research center based in Langenargen (Germany) carries out a stocktake in Lake Constance on behalf of the public authorities. This very special form of inventory was on the agenda again this year. On the shores of Lake Constance in Vorarlberg, where many fish species have their most important "nursery" thanks to the extensive reed belt, researchers and fishermen were recently out and about again to make a big catch.

The cause of the decline is still unclear
The nets caught everything Lake Constance has to offer in terms of fish - pike, zander, perch, whitefish, roach, catfish, lake trout, etc. - but the dominant species was the common perch. -However, the dominant species of recent years was only found sporadically: the stickleback. "We caught hundreds of sticklebacks during the last fishing trips, now there are less than 50 in total," says Alexander Brinker, Head of the Fisheries Research Center. In view of the fact that sticklebacks made up more than 90 percent of the fish stock in open water at the beginning of the year, this is a veritable sensation.

We have caught hundreds of sticklebacks in previous fishing trips, but now there are no longer 50 specimens in total.

Alexander Brinker, Fischereiforschungsstelle Langenargen

The neozoa were first detected in the 1950s, presumably because aquarium fish had been "disposed of" in Lake Constance. The species then multiplied almost explosively from 2012 onwards. The researchers are now faced with a major puzzle, as they cannot explain the disappearance of the invasive species. According to Brinker, there were no signs of such a population decline. At the moment, he and his colleagues have no choice but to make assumptions: "A parasite or a disease could be the possible cause of the decimation."

For the whitefish, the collapse in the stickleback population is very good news: although the sticklebacks are quite small, they are nevertheless very voracious and feed on whitefish eggs and larvae, among other things.

(Bild: stock.adobe.com)

This is good news for the whitefish, once the bread and butter of Lake Constance. Because sticklebacks feed on their spawn and larvae, among other things, and were therefore mainly responsible for the widespread disappearance of this excellent food fish. Recently, so few whitefish were caught that a fishing moratorium was even imposed on Lake Constance. There are already initial signs that the whitefish population could actually recover - "The whitefish we caught no longer looked as emaciated as in previous years."

Recovery of the whitefish population
Of course, it is too early to be jubilant, especially as many of the reasons behind this are still unclear. "But if the mass disappearance of sticklebacks is confirmed, it could be a real game changer for a recovery in whitefish stocks," says Brinker, giving hope to all professional fishermen on Lake Constance.