Stickleback explosion in the Baltic Sea

10 March, 2020

The big stickleback has become one of the most serious environmental problems in the Baltic Sea. Something is wrong with the ecosystem and the small fish has quickly become one of the sea’s dominant inhabitants. The bays and seas are teeming with sticklebacks.

When describing the situation, marine ecologist Ulf Bergström at SLU Aqua uses similes that bring to mind biblical hauntings.

– It’s like swarms of locusts coming in from the open sea and devastating the shallow bays of the Baltic coastal area,” he says.

A small fish, rarely more than eight centimetres long, with a major impact on the marine plant and animal community. When they come in to spawn in the spring, they feed on the fry and eggs of other coastal spawning fish, such as perch and pike.

– The sticklebacks can wipe out entire stands in some areas. The decline of pike and perch in the Baltic Sea can at least partly be explained by the increase in sticklebacks, says Ulf Bergström.

And there is a power struggle between stickleback and other predatory fish in the Baltic Sea. An adult perch likes to eat stickleback, as do pike. However, the spawning of the sticklebacks in shallow water coincides with the rejuvenation of other predatory fish. The large amount, the swarms of small fish, means that the fight has turned over in the stickleback’s favor. Of the total biomass of fish in the Baltic Sea, the stickleback currently accounts for about ten percent. A large and rapid increase in just over thirty years

– There are fifty times more stickleback in the Baltic Sea today than in the late 1980s.

The large numbers of sticklebacks also do other damage. A large part of the sticklebackt’s diet is the small crustaceans that live among the vegetation in the shallow areas of the Baltic Sea. Crustaceans themselves are an important part of the marine ecosystem, as they feed on filamentous algae, and as crustaceans decline in numbers, there is more filamentous algae in shallow coastal areas. We get more of the greenish slime that grows on rocks and bladderwrack.

Big stickleback in the Baltic Sea Photo: Ulf Bergström

Ulf Bergström and his colleagues at SLU are currently investigating the reasons behind the increase. Some is known, but much is still unknown. The stickleback has partly moved under the radar of scientists. It is so small that it does not get caught in the nets used to study the evolution of stocks.

– We know that human activity is behind the stickleback explosion. Climate change, eutrophication and fishing for pike and perch are at least part of the explanation for the rapid increase in sticklebacks. But research is ongoing to investigate the causes in detail, Ulf continues.

The rapid growth of the big stickleback came as a surprise to scientists. In the early 80s, few people were interested in stickleback. But when scientists became aware of the fish’s rapid rise, they began searching the archives to possibly understand the stickleback’s peaks and troughs.

– Newspaper articles from the 19th century show that the spigot was quite well known at the time. They knew how much stickleback there was, where to catch it and how. And they also knew that it was a fish that had a big impact on other species. It was known to have a strong negative impact on perch and pike. But we have forgotten that knowledge along the way.

There have been times when the spigot was sought after and desirable. It was one of the most important species at certain times of the year for fishing, especially in late autumn and winter.

At the beginning of the last century, Aktiebolaget Fettindustri in Karlshamn started its operations. Stickleback was the main raw material. It was caught in large quantities along the coast of Blekinge, dried and boiled to extract oil. Stickleback oil was popular and known for centuries. From the mid-18th century, for example, lamp oil was the most common fuel for Stockholm’s many oil lamps.

In fishing villages all along the coast of the Baltic Sea, stickleback was boiled for oil production for most of the 19th century. At times when normal fishing failed, the meagre income could be supplemented by the production of stickleback oil.

The cooking oil was more expensive than other oils and therefore it was usually the better-off in society who could afford to burn their lamps with this high-quality product.

– The stickleback is an incredibly fat and energetic fish,” says Ulf Bergström. It contains a very high oil content. It is also a fine oil that does not soot.

There were large numbers of sticklebacks along the coast of Bledking. In one year, no less than 2,500,000 litres of spit were delivered to the factory in Karlshamn. Fishing was so efficient that it has been likened to predatory fishing, and the stickleback was even on the verge of extinction from the area’s coastal waters.

In Stockholm at the same time, it is estimated that about 1,200 barrels of sprat were caught per year. According to a rough estimate, each barrel held 50,000 sticklebacks, so the annual catch in this area alone would amount to 60 million large spigots.

The year was 1919 and the First World War had ended in Europe. For Sweden, imports were still strictly regulated and, in particular, imports of oil products were scarce. Domestic oil production was therefore of great value.

Although the modern plant in Karlshamn met all efficiency requirements, it went bankrupt after only four years. The push for this type of oil production had come too late. The end of peace was a fact even before the factory in Karlshamn was put into operation, and at the same time as import restrictions began to be relaxed, the fishing arena found that the sticklebacks along the coasts were running out. Violent depletion had gone too far.

Since the early 20th century stickleback frenzy, the fish have recovered. Mildly put. And knowledge of the role of spigots in the ecosystem has been lost over the same period, as has interest in stickleback fishing.

But this is about to change.

– Vi vet att det finns ett stort intresse att börja fiska spigg från det storskaliga trålfisket i utsjön. Då tänker man sig i första hand att man ska göra fiskmjöl av den och mata andra arter som lax till exempel, där en stor del av fodret består av fiskmjöl.

Since fatty fish contain a lot of Omega 3, another possible use is in food supplements. Other experiments have been carried out to digest stickleback to produce biogas. And diesel. Buses running on fish diesel already exist in Åland.

Ulf Bergström hopes that stickleback can once again be seen as a resource. A large-scale fishery could stop the rampant development of spiny swarms grazing cleanly in coastal areas. But stickleback fishing has its own challenges to deal with. By-catch of other species risks putting additional stress on the highly stressed sea.

Another way to stop the big stickleback ravages is to favour its natural enemies. Abbore and pike inside the coastal areas. Cod and large herring out at sea.

Marine ecologist Ulf Bergström is seriously concerned about the spigot explosion. It will require a major effort to overcome the problems it causes. Getting pike and perch stocks back to a healthy level is at the top of the scientists’ agenda.

– There are big swarms of sticklebacks coming in and a small army of predatory fish to counter it. They have a tough job. I hope that we can support and strengthen the reproduction and survival of predatory fish, so that we can continue to have the fish stocks in the archipelago that we are used to.

Text: Martin Widman
Header image: Magnus Eriksson/TT

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