Ocean acidification seen as huge threat to Atlantic cod

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Climate change to put a big hit on cod stocks.

Fish larvae seen as highly sensitive to CO2

Staff Report

New lab experiments suggest that increasing ocean acidification could take a big bite out of the economically important cod fishery in the North Atlantic. The research suggests that the buildup of CO2 in the ocean could double the mortality of newly-hatched cod larvae.

Members of the German research network BIOACID quantified the impacts, showing that recruitment could decrease to levels of one quarter to one twelfth of the recruitment of the last decades. Cod have already been under intense fishing pressure for decades, and the new study, published in the online journal PLOS ONE, identifies climate change as an emerging new threat.

When additional carbon dioxide from greenhouse gas emissions dissolves in the ocean, the water gets more acidified – with negative consequences for the behavior, growth and development of fish larvae.

Two separate experiments shows that cod larvae are very sensitive to elevated carbon dioxide concentrations. The experiments were conducted in Sweden and Norway as part of the German research network BIOACID (Biological Impacts of Ocean Acidification) and the EU BONUS project BIO-C3 („Biodiversity Changes – Causes, Consequences and Management Implications”).

For the first experiment, fertilized eggs and larvae of cod caught in the Öresund were kept in the lab of the Sven Lovén Centre at the University of Gothenburg (Sweden) for six weeks. Some were held in seawater at ambient carbon dioxide concentrations, others were exposed to CO2-conditions that are projected to occur by 2100. Temperature, light and food densities remained at natural conditions. The second experiment was conducted with cod offspring from the Barents Sea in the Centre for Marine Aquaculture Tromsø, NOFIMA (Norway).

“Even though the experiments were conducted in two consecutive years at different research stations under different conditions in relation for example to food or tank sizes and with two different stocks, their results are surprisingly similar”, said first author Martina Stiasny, PhD student in the research unit Evolutionary Ecology of Marine Fishes at GEOMAR and in the unit for Environmental, Resource and Ecological Economics at Kiel University. “The daily mortality rate for cod from the Öresund under current conditions was nine per cent compared to 20 per cent under increased CO2 concentrations. For the Barents Sea stock, we found mortality rates of seven and 13 per cent respectively.”

“The repercussions of the anthropogenic climate change need to be included into stock projections and considered in the management of fish stocks,” said Dr. Catriona Clemmesen, head of the GEOMAR working group Larval Fish Ecology. “Only this will enable us to define realistic limits for fishing pressure and to avoid overfishing and depletion of fish stocks.”

“Ocean acidification cannot be completely avoided anymore. But the bigger the stocks are and the more responsibly and sustainably fishing activities are, the bigger the recruitment will remain,” said Stiasny. “This will in the long run not only allow for larger fisheries, but also helps stocks to better adapt to climate change and other anthropogenic influences.”

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