Last Monday, I wrote about how climate change can facilitate the spread of non-native and invasive species. Today, we look at a species that whilst problematic now, could spread further throughout Norwegian waters as temperatures rise.

The last time we looked at an ocean-dweller in this series, we saw that while some species may not be great for ecosystems, they can provide an obvious benefit to other aspects of the region, in this case the fishing industry. The Pacific oyster (Crassostrea gigas) was also introduced intentionally for cultivation and is now on the verge of becoming a major problem in Norwegian waters.

What are they?

Because of its tolerance of most environments, the Pacific oyster has become the most widely cultivated oyster in the world, and thus one of the most widely distributed alien species in the world. Originating from the North-West Pacific, around Japan, it’s sometimes referred to as the Japanese oyster. There is some confusion regarding its taxonomy, with it also sometimes referred to as the Portuguese oyster, though it’s possible the two are separate species. They are large, jagged oysters, and occur in marine coastal waters.

How did they get here?

The oysters were imported into waters throughout Scandinavia and most of Northern Europe to replace dwindling stocks of native oysters at various points through the 20th century. Naturally, they eventually established wild populations as well, and are now abundant along Norway’s southern coast. Whilst they have taken over coastlines through much of Europe, their dislike of colder waters means that for now, their local populations are largely constrained to the south of Norway. But increases in temperature, which will occur at an accelerated rate in the Arctic and sub-Arctic, mean that the oyster could spread further north in the coming decades.

What do they do?

Much like the Red King Crab, they transform the local ecosystem into a homogenous mass. They can transform substrate from soft bottomed and muddy to filled with rocks and other oysters and mussels, also paving the way for other alien species, and lowering regional biodiversity by outcompeting and displacing local species. Interestingly though, presence of oysters can often improve water quality in the surrounding regions and heighten ecosystem productivity, though the position of the oyster in novel food webs is not particularly well understood. They also have negative effects for local human populations, making certain areas impossible to use for recreation, as they’re extremely sharp.

How do we stop them?

In other countries, attempts to eradicate wold populations by harvesting them have proved futile, and a 2005 study showed the oyster eradication would also cause substantial harm to the local ecosystem. Warming seas will mean the expansion of the oyster’s range, however this is likely to happen very slowly, so by focusing on the ranges edges it may be possible in the future to limit expansion.

For more information on the oyster, we recommend that you read the following articles:

Invasive Alien Species Fact Sheet – Crassostrea gigas by the Online Database of the North European and Baltic Network on Invasive Alien Species

Crassostrea gigas – Cultured Aquatic Species Information Program by the Fisheries and Aquaculture Department of the United Nations