Jellyfish
With warming waters in the North Atlantic, jellyfish have become an increasing problem for fish farmers. While most common jellyfish are relatively harmless, two types present a more serious threat.
Hydrozoans, tiny marine predators, while technically a different order (Cnidaria) from true jellyfish, usually go through a free-floating “jellyfish” or medusa stage before transforming into polyp colonies. In their jellyfish stage they can enter the gills of fish and, as they are usually armed with poisonous stings, they can do great damage. Their attacks can also leave fish vulnerable to bacterial infection.
“String jellyfish”, known in Norway as Perlesnormanets (“pearl Norman”) have also posed a serious threat in recent years. This species, Apolemia uvaria, look like a long thread, but they are actually a colony of many individuals. They do not occur naturally in Norwegian waters, but tend to drift in with the ocean currents. When a colony hits a fish farm net it can break up, causing stinging threads to enter the pen. So far there are no truly effective measures against this pest.
Meanwhile, it’s a little known fact that jellyfish themselves can be farmed. The Jellyfish Farm in Germany produces a range of species including moon jellyfish, Japanese sea nettle, Australian spotted jellyfish and more.
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