Invasive species: How newcomers are causing problems for boats and harbours in the area

Anyone who takes their boat out of the water and finds a white, hard coating on the propeller and hull has probably already come across the Australian calcareous tube worm. Anyone who can no longer cool their outboard motor should check the inlets for quagga mussels. And anyone swimming in Scandinavia should keep an eye out for a tiny, transparent jellyfish whose stinging cells can cause palpitations and cramps. Invasive species have long since arrived in German and northern European waters – with tangible consequences for boats and harbours. Two marine biologists explain what’s behind this – and why we have to come to terms with it.

​A sign calls for people to be killed

Skärhamn is one of those cosy harbours on the Swedish west coast. A few boats are moored in their berths, whilst children use landing nets to catch crabs and other sea creatures. It’s a peaceful scene – were it not for a sign hanging from the jetty urging people to kill.

It features pictures of native and non-native crabs. The latter, it states, cause massive harm to native species as invasive species. One should ‘let them die humanely’: if an invasive species gets caught in a net, it should be killed quickly and painlessly – for example, by delivering a well-aimed, forceful blow with a stone on a hard surface.

Invasive species are defined as organisms which, as a result of human activity, have been introduced into an area where they did not originally occur, have become established there, and are causing adverse effects on the natural environment, the economy or human health.

They arrive from distant habitats in the ballast tanks of ocean-going vessels, attach themselves to ship hulls or escape from aquaculture – crabs, jellyfish and algae, mussels and barnacles. However small they may be, they can cause a great deal of trouble when they attach themselves to boat hulls and harbour facilities and begin their sometimes remarkably destructive work.

​Termites of the Sea

Recreational boaters and harbour operators have always had their fair share of problems with fouling, both at sea and in harbour, but have largely come to terms with old acquaintances such as barnacles, mussels and the like. Today, newer species are causing fresh concerns.

In September 2021, for example, Wismar’s West Harbour had to be closed off because of a tiny creature: the wooden piles beneath the quay edge looked sound from the outside, but inside they were a hollowed-out labyrinth of boreholes made by the so-called shipworm.

In fact, this is not a worm at all, but a mussel whose shells have evolved into highly efficient drilling tools. “Ports all along the German Baltic Sea coast as far as Rügen are affected,” says Dr Michael Zettler, a zoologist and marine biologist at the Leibniz Institute for Baltic Sea Research in Warnemünde. He has been researching marine ecosystems and invasive species for almost 40 years. “Any wooden piles placed in the water provide food for the creature.” Within a few weeks, the mussels can grow into the wood without being noticed. “The openings are so small that this only becomes apparent over the years,” explains the expert.

The shipworm is not exactly a new phenomenon. As far back as the 15th century, Christopher Columbus reported how his ships literally crumbled beneath the crew’s feet due to the actions of these ‘termites of the sea’. The shipworm is also said to have played a part in the downfall of the Spanish Armada just under 100 years later. Further north in Holland, in 1731, sea defences breached during a storm surge – the shipworm had eaten away at the wooden dyke gates.

Its exact origin remains a matter of debate to this day. It is therefore not entirely clear whether it is in fact an invasive species or rather a species that has been established for centuries, though no less harmful for that.

New underwater brake

The situation is clearer when it comes to the Australian calcareous tube worm. In the brackish water areas of the North Sea and the Baltic Sea, for the past five years or so, many a planned holiday sailing trip has begun with a visit to the shipyard, because the worm has quickly built a reef around the keel and propeller or become lodged in the through-hull fittings.

Michael Zettler and his colleagues have been recording growing populations since the end of 2020. The tube-dwelling bristle worm, just under two centimetres long, builds limestone tubes up to ten centimetres in length. Large clusters form reefs, in which new generations attach themselves to the older ones. “The growth is much thicker than with barnacles,” explains Zettler. “The layer can reach a thickness of 10 to 15 centimetres – barnacles cannot achieve that.”

Dr Christian Buschbaum, marine ecologist at the The Alfred Wegener Institute in List on Sylt, is also familiar with the problem. “The calcareous tubeworm is a specialist in calm brackish waters with fluctuating salinity,” he explains. “It avoids the open, turbulent waters of the North Sea or the Baltic Sea, as well as the low-salinity areas of the eastern Baltic Sea. But in brackish water – in harbours, marinas and behind locks – it finds ideal conditions.”

Many boat owners respond by using stronger antifouling paint. “But in the end, that just increases the amount of toxins entering the water,” says Buschbaum, himself an experienced sailor and boat owner. “It makes more sense to take action early and clean the boat mechanically.”

However, boat owners sometimes only notice the growth once it has become extensive. “This can happen relatively quickly; thick mats can grow within a few weeks if the boat is not moved. The main settlement period for the heat-loving, planktonic larvae is August and September – that’s when the larvae seek out new substrate,” explains the marine ecologist.

​The only thing that helps is prevention. Once a species has become established, it is no longer possible to intervene in the sea without causing more harm than good.

​Here to stay

There is little cause for hope that the worm will one day disappear from the scene again. “Once a species is there and has become established, there is absolutely no chance of intervening in the sea without causing even more damage. All you can do then is watch and monitor,” says Zettler, speaking from decades of experience, adding: “The only thing you can do is prevention: stop it from happening in the first place.”

The good news is that nature can regulate itself. “First, there’s this explosive proliferation. Populations skyrocket because there’s no one there to eat them. You reach a peak with very high densities,” reports Zettler. But then the environment often adapts to the new organisms. Predators or parasites keep their numbers in check. “As a result, the densities fall steadily until they return to normal levels.”

​Boom, peak and stabilisation

According to Zettler, the quagga mussel is a good example of mass reproduction. Originally native to the Black Sea–Caspian region, it has spread across Europe over the decades – initially along the major river systems, and now also in numerous lakes. Lake Constance, many lakes on the edge of the Alps and parts of the Mecklenburg Lake District are affected.

The mussels attach themselves to hulls in vast numbers, colonise buoys and even block pipes in waterworks. At the same time, they filter vast quantities of plankton. In doing so, they ensure that lakes remain clear, but they also deprive other species of their means of survival.

To curb further spread, boats in Switzerland must already be professionally cleaned with water heated to 70 degrees whenever they move between lakes. There are plans to extend this approach to Lake Constance as well.

“The quagga mussel is making headlines on Lake Constance – yet it has been found here in Mecklenburg since around 2010,” reports Zettler, drawing a historical comparison that may help to put concerns about its spread into perspective: “If you read old newspaper articles from the 1920s, when the zebra mussel first arrived – the headlines are exactly the same: blocked water pipes, swimming bans, the end of the world.”

His prediction is that the mussel population will stabilise, just as the zebra mussel did. However, it could take years for that to happen. Zettler therefore considers the Swiss boat-washing scheme to be a sensible preventive measure. “It won’t completely prevent the spread, though,” he adds, because “fishermen drag the larvae in their nets, and ducks carry them in their plumage – the routes of introduction are unfathomable. But delaying the process is better than doing nothing.”

​A distinction between positive and negative is a human construct. It does not exist in nature. In nature, there is only this: it works or it doesn’t. But there is no need to conjure up worst-case scenarios.

​Different rivers, different wildlife

In 2022, Christian Buschbaum, together with other researchers, published an article in “Non-native species on the German North Sea and Baltic Sea coasts. Introduced species in German coastal waters” More than 150 non-native species have been recorded in German coastal waters. Around two species a year are introduced anthropogenically – that is, as a result of human activity. They have amusing names such as ‘moss animals’ or ‘candelabra algae’. Some, however, have the potential to disrupt entire ecosystems.

According to Zettler’s observations, inland waters have changed particularly drastically over the last few decades. “If you survey the fauna in the Havel, Elde or Müritz today and compare it with data from 20 to 30 years ago, you’d think you were in a different river. Certain species of mussels, snails and crayfish have disappeared completely. Instead, other, new ones have appeared.”

​Good and evil?

Change is not always a bad thing. The Wadden Sea, for example, has for centuries been a destination for new arrivals such as the American razor clam, which has been establishing itself there since the late 1970s and is now regarded as an important source of food for coastal and seabirds.

The Pacific oyster is another example of just how complex developments can be. Buschbaum has been researching them for many years. Originally imported into the Netherlands and Sylt as a delicacy farmed in aquaculture, they have since run amok. What’s particularly interesting is that, as he explains, “A great many other species have arrived along with the oysters.” “An oyster shell is like a ship’s hull – all sorts of things tend to attach themselves to it.”

Today, the Pacific oyster can be found throughout the Wadden Sea and has now also reached Kiel on the Baltic Sea. “At first, we thought that the introduced oysters would overgrow the native mussels, as the oyster larvae settled on the mussels’ shells, causing the mussels to die off,” explains Buschbaum.

“And that was indeed the case at first – the mussel population declined.” Until the winter of 2009/10, which was colder than usual. For reasons that scientists have not yet been able to fully explain, the system changed: From then on, the oysters mainly attached themselves to other oysters, rather than to the mussels. Buschbaum describes what emerged as “a new system”: mixed reefs comprising oysters and mussels, which are more stable than pure mussel beds.

The mussels actually actively migrate down into the reefs – to escape predators such as birds and crabs. “They don’t grow quite as large there, because they don’t get much food. But they survive.”

The sharp-edged oyster shells can leave painful marks on both the keels and the hulls of beached boats. From an ecological point of view, however, the situation has stabilised.

They have even created entirely new structures that hold back the water at low tide. Much like tidal pools on rocky coasts, this has resulted in areas with permanent water cover, where new species communities have been able to establish themselves.

What’s more, the Senckenberg Institute by the Sea in Wilhelmshaven classifies them as part of the ‘group of marine ecosystem engineers’, as their massive reef structures act as natural breakwaters and protect the coasts from erosion and flooding.

“Classifying things as positive or negative is purely a human construct,” Buschbaum clarifies. “That doesn’t exist in nature. In nature, it’s simply a case of: it works or it doesn’t. And so far, it seems to be working very well.” In the Wadden Sea, no native species has yet been displaced by an introduced one. “I don’t think it’s a good thing that so many species are being introduced either. But there’s no need for doomsday scenarios.”

​A hidden danger in harbours and marinas

A look at Buschbaum’s study shows, however, that there is still untapped potential in some introduced species that have hitherto been virtually unknown amongst water sports enthusiasts. Japanese kelp, for example – a brown alga that can grow to a length of up to several metres – has become established on Heligoland since 1988 and later in the Sylt mudflats. “Due to their size, strands of seaweed can become entangled around the propellers of recreational boats, thereby rendering them unable to manoeuvre,” the study states.

The same applies to a sea squirt from the north-western Pacific, which was first recorded near Hörnum on Sylt in 2016 and is considered highly invasive. In 2018, it appeared in the harbour at Langeoog, and in 2020 in the recreational boat harbour at Wyk on Föhr. Its yellowish-white mats cover everything – hulls, ropes and posts in harbours and marinas.

Another species of the same genus, conspicuously red to orange in colour and forming colonies, has been recorded on Heligoland and at floating jetties in Hörnum for a good 15 years. Both examples illustrate how quickly new species can become established in marina – once introduced by international shipping, they are now unwittingly carried further afield on the hulls, ropes or fenders of pleasure craft.

​Take care when swimming

But it is not just boats and harbour facilities that are affected. Anyone who takes a dip in the water may also encounter uninvited guests. For example, the Swedish cruising yacht club Svenska Kryssarklubben recently issued a warning about the Kläng jellyfish in Askeröfjord on the west coast of Sweden.

This tiny cnidarian is native to the north-western Pacific. Contact with its stinging cells is extremely painful and can cause severe reactions such as chills, a racing heart and cramps. What makes them particularly treacherous is that they are transparent, only two to three centimetres in size and almost impossible to spot.

​If you can’t beat them, eat them

Invasive species have many facets – some cause problems for boats, some threaten ecosystems, and some cause a stinging sensation when you go swimming. Still others end up on our plates. The Chinese mitten crab, for example, which tears German anglers’ nets to shreds, is a highly sought-after delicacy for Chinese gourmets.

Anyone who fills their plate with bright red crustaceans at the traditional Swedish crayfish feast, Kräftskiva, these days is usually eating the North American signal crayfish – because the native noble crayfish is threatened with extinction following a devastating crayfish plague. A well-intentioned attempt to save the species turned into a disaster: when Sweden introduced the supposedly resistant signal crayfish from 1960 onwards, it ended up spreading the pathogen further. “This ultimately dealt the noble crayfish the final blow,” explains a Senckenberg study.

Warmer summers, changing salinity levels and increased shipping traffic are creating new opportunities for species to spread and become permanently established. Not everything can be prevented, and hardly anything can be reversed. And who knows: perhaps one or two species will evolve from being a nuisance to becoming an integral part of the ecosystem – just like the Pacific oyster in the Wadden Sea.

Identifying and reporting invasive species​

• The Leibniz Institute for Baltic Sea Research in Warnemünde (IOW) is compiling a record of invasive species in the Baltic Sea and is asking boat owners to photograph and report any unusual growth on their boats. io-warnemuende.de
• The Alfred Wegener Institute (AWI) on Sylt is conducting research into invasive alien species in the North Sea and the Wadden Sea – using its own invasive alien species monitoring platform. AWI.de
• In Denmark Among other things, the Environment Agency provides information on invasive species in marine areas. mst.dk
• The official species portal, Arter, documents the occurrence and distribution of species and collects reports on invasive species. Arter.Dk
• In Sweden provides the Marine and Water Authority with information on invasive species in the sea. Havochvatten.se
• Sightings of non-native and invasive species can be reported via the Swedish Species Portal. Invasivaarter.nu
• Senckenberg am Meer in Wilhelmshaven also carries out research into invasive species on the North Sea coast. Senckenberg.de

​Fact sheets: Five species that recreational boaters should be aware of

​Chalk tube worm (Ficopomatus enigmaticus)

• Origin: South-West Pacific, probably the Indian Ocean
• Features: White, cement-like limestone tubes form thick mats
• A problem for pleasure boats: Clogs through-hull fittings, can jam the rudder and propeller, forms layers up to 15 cm thick, places an enormous weight on the hull
• Distribution: Brackish water in the North Sea (Hooksiel, Jade) and the Baltic Sea (Trave, Schlei, Warnow as far as Rügen). Avoid open, rough seas and low-salinity areas of the Baltic Sea
• Special feature: Known in Germany since 1975, it has spread rapidly since 2020

​Quagga mussel (Dreissena rostriformis bugensis)

• Origin: Black Sea/Caspian Sea
• Features: A small striped mussel, similar to the zebra mussel, but with a rounded belly
• A problem for pleasure boats: Massive fouling on hulls, clogs cooling water pipes, increases drag. In inland waters, up to 30,000 individuals per square metre
• Distribution: Lake Constance, the lakes on the edge of the Alps, Lake Müritz, Lake Müggelsee, and many German rivers and lakes
• Special feature: Larvae survive in the bilge or cooling system, whilst adults attach themselves to the hull – and are thus carried along

​Pacific oyster (Magallana gigas)

• Origin: Pacific
  (originally imported for aquaculture)
• Features: Large, sharp-edged shells; they grow faster and larger than native oysters
• A problem for pleasure boats: Sharp shells can cause injuries and damage to the hull when the boat runs aground
• Distribution: The entire Wadden Sea, and now also the western Baltic Sea (Kiel)
• Special feature: Forms stable reefs with mussels and can therefore help to protect the coast. Known as ‘Sylter Royal’, it is a delicacy.

​Shipworm (Teredo navalis)

• Origin: Unclear (cryptogenic species); recorded in north-western Europe since the early 18th century
• Features: Tiny drill holes in the wood that are barely visible
• A problem for pleasure boats: Destroys wooden piles and harbour structures, and poses a threat to untreated wooden boats. From the outside, the wood looks healthy; inside, it is a labyrinth of tunnels.
• Distribution: The Baltic Sea as far east as Rügen, the North Sea
• Special feature: It has been permanently in the Baltic Sea since 1993 and grows within a few weeks

​Barnacle (Amphibalanus improvisus)

• Origin: Cryptogen – origin disputed, probably North American; established in German coastal waters since the 19th century
• Features: White, cone-shaped calcareous shells, found either singly or in groups. When alive, they can be recognised by the violet-white edge of their mantle.
• A problem for pleasure boats: Increases flow resistance; classic hard fouling
• Distribution: Commonly found in the Baltic Sea; also in North Sea ports and river estuaries