Population of cod has come into increasing pressure
The increased acidification of the ocean may lead to twice as many newly-hatched cod larvae to perish, placing this economically valuable fish stock under increasing pressure. This finding was reached after two experiments lasting several weeks conducted by members of the German research network BIOACID.
The population of cod has come into increasing pressure for the past decades, and this is a cause for concern as it is one of the most important commercial fishes in the North Atlantic. Its population has been drastically reduced and faces collapse (as we have reported here: taucher.net/diveinside- overfishing_causes_collapse_of_cod_fisheries_in_baltic_sea-kaz).
Indeed, overfishing has caused cod stocks to collapse several times in the past. Now, a recent article in the online journal PloS ONE cites another major stressor faced by the cod population: ocean acidification. This is because as additional atmospheric carbon dioxide dissolves in the ocean, it causes the seawater to become more acidified, leading to negative consequences for the development, behaviour and growth of fish larvae.
The results of two separate experiments conducted in Sweden and Norway showed that the mortality of cod larvae during the critical period between the hatching and the development of the gills was twice as high at elevated carbon dioxide concentrations as at current conditions.
In the first experiment, fertilised eggs and cod larvae caught in the Öresund were kept in the laboratory at the Sven Lovén Centre at the University of Gothenburg in Sweden for six weeks. All were exposed to the same temperature, light conditions and food densities compatible with natural conditions. The difference was that some were held in seawater at ambient carbon dioxide concentrations, while others were exposed to carbon dioxide concentrations projected for 2100.
As for the second experiment, it was conducted at the Centre for Marine Aquaculture Tromso, NOFIMA (Norway) with cod offspring from the Barents Sea.
“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 lead author Martina Stiasny in German. She is a PhD student at GEOMAR Helmholtz Centre for Ocean Research Kiel.
“The daily mortality rate for cod from the Öresund under current conditions was nine per cent compared to 20 percent under increased carbon dioxide concentrations. For the Barents Sea stock, we found mortality rates of seven and 13 percent respectively.”
Taking into consideration the increased mortality rates into their model calculations (which were based on inventory data of the International Council of the Exploration of the Sea (ICES), the researchers then determined the number of cod offspring that would enter the population. Based on their scenarios, ocean acidification may cause a drop of a quarter to one-twelfth of the current numbers.
“Our results show for the first time how ocean acidification can add up to the fishing pressure on stocks of a commercially important fish species,” said Dr Catriona Clemmesen, head of the GEOMAR working group Larval Fish Ecology. “The repercussions of the anthropogenic climate change need to be included into stock projections and considered in the management of fish stocks. Only this will enable us to define realistic limits for fishing pressure and to avoid overfishing and depletion of fish stocks.”
According to Stiasny, fisheries would need to adjust to climate change to retain their fish stocks, as ocean acidification could not be avoided anymore. However, she added that “the bigger the stocks are and the more responsibility and sustainably the fishing activities are, the larger would be the larvae repopulation. 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.“
Link to the study: journals.plos.org/...journal.pone.0155448