How is the North Sea changing?
Climate change is not only a global phenomenon. It is also noticeable on our doorstep. For example, in the North Sea. The water temperature has increased significantly. How this affects the animal and plant life in the North Sea is being investigated by Prof. Karen Wiltshire, Vice Director of the Alfred Wegener Institute and responsible for Marine and Coastal research.
The North Sea is a sea on our doorstep, visible from the buildings of the Alfred Wegener Institute. Why is this local sea so important for us?
For centuries, the North Sea has been a major transport route - as you can see here in Bremerhaven with its large container terminals. The North Sea is also an important source of fish. If you live here, near the coast, you have a close relationship with nature. It is natural for us to be concerned about the sea.
The global temperature has risen significantly in recent decades. How has this climate change affected the North Sea?
Our long-term data, which we have recorded off Helgoland, show that since 1962 we have experienced a temperature rise of 1,5 degrees - obviously quite substantial. This is further reflected in the fact that since the winter of 1962/63, we have had no real ice winter around Helgoland.
What implications does this temperature increase have for the animal and plant life in the North Sea?
Many warm water species have migrated into the North Sea - we estimate that there are over 40 new species. Since the winters are no longer cold, these organisms are able to overwinter. It is believed that fish like red mullet, which is a warm water species, proliferate even in the North Sea. Although the red mullet is a tasty fish it cannot replace the cod, our traditional food fish in terms of fishing effort. It is becoming too warm in our latitudes for cod, so it tends to occure further north. Other immigrants are several types of snails, jellyfish, and large algae, which are even becoming dominant in the various habitats. The Pacific oyster has already established itself near Sylt. With its robust shells, the oyster has built reefs of almost concrete-like consistency that cannot be harvested. Thus climate change has severely undermined the marine system we were used to.
We must expect continued climate warming in the future. How will this affect the fauna and flora in the North Sea?
The current system will shift even more in the direction of warm water species. Furthermore, the plankton will bloom earlier in the year and the animals that live off it will have to re-orient themselves. Ultimately, the entire food chain would be different. For the cold-water species that previously dominated the North Sea, it will become more and more difficult and they will increasingly migrate towards the north. Some have already gone or about to disappear. And we humans will have to adapt to the new species entering the North Sea.
What could these adaptive strategies look like?
If the species which humans have become used to over the centuries have disappeared, one should first consider whether fishing is still economical. For example, the fishing methods might have to be adjusted to catch the new species. Perhaps we should also consider whether, instead of fishing in the future, we should invest in fish farms and aquaculture.
Which issues are still unresolved in your field of research? What answers will you be looking for in the next few years?
One of the questions is: What happens when a system in which the top predators used to be fish, changes to one in which jellyfish are dominat? Would this essentially just be an aesthetic problem, because the jellyfish on the beaches were a nuisance or would the food web be restructured? We hope to find an answer within the next five to six years. For a long time we have asked the question: How do we obtain a better understanding of the entire ecosystem. What does it mean when, in the North Sea, one species is replaced by another?
Could it really be that one day we’ll have a North Sea full of jellyfish and the fishing industry is left empty handed?
There are examples where this has actually happened, for instance in the Caspian Sea. There a small ctenophore (comb jellyfish) brought the entire fishing industry to a standstill. This ctenophore is now also present in the North Sea. However, so far it does not appear to be a major threat. We can therefore not simply superimpose the situation in the Caspian Sea on our North Sea.