Researchers have known for some years that the Atlantic cod beats the retreat in the direction of the Arctic when the waters in its traditional habitat become too warm. In summer, shoals from the Atlantic Ocean, for example, are now moving up as far as Spitsbergen into the waters the Arctic cod calls its own. In the next two and a half years, biologists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, together with scientists from Kiel, Bremen, Düsseldorf and Münster, will be seeking to discover the consequences of this climate-related migration on the stocks of these two commercial fish species, how the fish are responding to the water becoming warmer and more acidic and at which stages of life the changes are most dangerous to them. The first investigations are already in progress as part of the joint project BIOACID with focus placed on the early life stages.
Until recently, Flemming Dahlke, fishery biologist at the Alfred Wegener Institute (AWI), could not have imagined that he would one day have to resort to the fishing rod to pursue his doctorate. But after several fruitless attempts at getting hold of an Atlantic cod ready to spawn, a fishing trip proved to be the most profitable method. Flemming Dahlke stripped off the eggs from his catch, fertilised these with Atlantic cod sperm and was finally able to start on his actual research work.
At the Swedish research station of Kristineberg, the AWI fishery biologist observes, documents and measures how the cod eggs develop at different water temperatures. He would like to know whether as many larvae hatch from eggs which have matured in sea water at a temperature of 12 degrees as from eggs which have been kept in water at a temperature of six degrees, and how the quantity of dissolved carbon dioxide in water impairs the survival chance of the fish spawn.
These two questions are among the focal research topics of the BIOACID fish consortium headed by AWI biologist Dr. Felix Mark. In this research project marine biologists from the Alfred Wegener Institute, the GEOMAR, and the Universities of Bremen, Düsseldorf and Münster together with partners from Norway and Sweden investigate how sensitive the two commercial fish species, Atlantic cod and Arctic cod, are to the increasing warming and acidification of the sea water. In nine closely linked sub-projects they study all life stages of the fish and their genetic patterns: from spawn and the development of the larvae, through the juvenile fish and their favourite food, the copepod, to the mature parent fish.
“As for all other organisms, the Atlantic cod and Arctic cod feel the most comfortable in a specific temperature range. During the spawning season, for example, the Atlantic cod prefers temperatures of between three and seven degrees Celsius. By contrast, the Arctic cod breeds at temperatures of between zero to four degrees Celsius. If the temperature of the sea now increases due to climate change, the animals become stressed, a condition which is greatly exacerbated by the increasing ocean acidification. We suspect that these new environmental conditions will lead to the comfort ranges of both species becoming smaller and that the habitat of the fish will increasingly overlap. This means that the Atlantic cod can be expected to seriously compete with the Arctic cod “, says Dr. Daniela Storch, biologist at the Alfred Wegener Institute.
Just which fish species has the best chances of survival will be investigated by the project members in complex behavioural experiments and during a four-week expedition. “From mid August to mid September this year we will be fishing the fjords of the north, west and south coasts of Spitsbergen on the research vessel Heincke. We firstly wish to document where we find which species at this time of year. Secondly, we are interested in catching a great deal of fish which we will bring back alive to Bremerhaven and can then study in the over 100 new basins of our aquarium facility”, explains Felix Mark.
The planned experiments include performance analyses in the ultra-modern flow channel and the two MRI scanners of the Alfred Wegener Institute in Bremerhaven. “Using this apparatus we can not only look into the brain of the fish but even into its individual cells. We are able to recognise, for example, how its metabolism alters within the cells, how heart and blood circulation of the fish react to the rise in water temperature, at which pH value of the water the fish reaches its performance limits or in which way temperature and degree of acidification affect its senses”, explains Felix Mark.
The researchers know from investigations of tropical fish, for example, that their offspring have a reduced sense of smell as ocean acidification rises. The consequence: the young fish find it more difficult to return home and are more likely to fall prey to others. So are heat-shy Atlantic cod and its Arctic cousin facing a similar fate in view of climate change? Flemming Dahlke’s first results are at least interesting: “There are many signs that the water temperature plays an important role in the breeding of the cod. There was as virtually no life in those eggs fertilised in sea water at a temperature of twelve degrees”, reports the phd student.
Atlantic cod (Gadus morhua) is one of the most important commercial fish species in the North Atlantic. Stocks of Atlantic cod are divided over both sides of the North Atlantic but the fish also live in the coastal waters off the southern tip of Greenland and around the island as well as in the North and Baltic Sea. Whilst the young fish prefer the shallow waters with sea grass beds or rock crannies, older fish tend to be found in shoals during the day at a depth of 150 to 200 metres. At night each animal searches for food alone, hunting for invertebrates, smaller fish and also juvenile fish of their own species under some circumstances. Adult fish reach a body length of around one metre. However, outstanding specimens two metres in length have also been caught. The Atlantic code tolerates water temperatures of up to 15 degrees Celsius and is brownish to greenish or grey dorsally and pale ventrally. The stocks of Atlantic cod along the American east coast and in European waters have dropped steeply due to intensive fishing in the eighties. Nowadays, primarily Norwegian, Icelandic and Russian fishermen find Atlantic cod in their nets.
Industrial production of Atlantic cod has been attempted primarily in the USA, Canada, Iceland, Norway and the United Kingdom. However, the financial market crisis in 2010 and the as yet unsolved problems in breeding itself have so far prevented any further growth in this economic sector.
More information on the Atlantic cod and its significance as a commercial fish may be found in the web portal of the Food and Agriculture Organization of the United Nations
Artic cod (Boreogadus saida) lives in the central Arctic Ocean and in the northern Atlantic and Pacific Oceans. The fish occur both beneath the sea ice and in the ice-free and also near-shore waters. Arctic cod are usually around 25 centimetres long and feed on copepods, amphipods and smaller fish. They are brownish along the back with many fine dark points. Sides and belly are silvery. The animals are currently caught primarily by Russian and Norwegian fishermen for the production of fish meal and oil.
More information of the Arctic cod and its significance as a commercial fish may be found in the web portal of the Food and Agriculture Organization of the United Nations