Increasing ocean acidification can double the mortality rate of newly hatched cod larvae. Now German scientists, in collaboration with Nofima, are sounding the alarm.

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Anne-May Johansen  

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‘- Various scenarios show that recruitment of cod stocks in the Barents Sea and Baltic Sea in the year 2100 may decline to between one quarter and one-twelfth of what has been the level in recent decades, due to increased concentrations of CO2. There should be a strong call for action by the fisheries authorities, the German research network BIOACID confirms.

The increase in CO2 emissions impacts the oceans in two ways: Higher temperatures and a lower pH, in other words more ocean acidification. To some extent, fish can avoid the temperature problem by moving closer to the poles, but ocean acidification is not linked to geography and the fish cannot avoid it by swimming away.

Survival is significantly reduced

Through the EU project AquaExcel, Nofima has played host to three German research groups that having been studying the effect of ocean acidification on cod reproduction. This work, which Nofima’s scientists have been actively engaged in, was carried out at the Nofima Centre for Marine Aquaculture at Kraknes, in Tromsø in 2014 and 2015.

The results of one of the research groups, Geomar in Kiel, are now clear. They show that the survival rate for cod larvae is significantly reduced by decreasing pH, and that this may have far-reaching consequences for the development of cod stocks.

– This work is the first to have dealt with the effects of ocean acidification on a commercially important fish species, and we expect that the findings will receive a great of attention and make an important contribution to the debate on the effects of increased CO2 emissions on marine life, our ability to harvest food from the sea and not least how we should manage marine resources to lessen the negative effects as much as possible, says Senior Scientist at Nofima, Atle Mortensen. He heads Nofima Centre for Marine Aquaculture, known locally as The Cod Breeding Station.

Double mortality

It is the accumulation of CO2 from the atmosphere that dissolves in the ocean and creates acidification. In two independent tests, conducted in Sweden and at Nofima’s facilities in Tromsø, scientists have documented that, compared to the current level, the mortality rate among cod larvae in the first critical phase after hatching, was twice as high when carbon dioxide levels in the water were at the level one expects in 2100.

– Although the experiments were performed in two consecutive years by different research stations under different conditions, and with two different cod stocks, the results are surprisingly similar, explains PhD student Martina Stiasny of the research unit Evolutionary Ecology of Marine Fishes with Geomar and the University of Kiel.

– Our results show, for the first time, how ocean acidification can put pressure on the stock of a commercially important species, Geomar’s Dr Catriona Clemmesen points out.

Avoid overfishing and depletion

She is of the opinion that the consequences of anthropogenic climate change must be taken into consideration in connection with the future management of fish stocks.

– Only in this way will we able to define a realistic framework for quota allocation and avoid overfishing and depletion of Fish stocks.

– To sustain the stocks, the fisheries should adapt to climate change, Martina Stiasny recommends.

– Ocean acidification cannot be completely ignored any longer, but the larger the stocks are and the more responsible and sustainable manner in which fishing is carried out, the greater the recruitment will be, she states.

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