Fish and other types of seafood are an important source of protein for over half of the world's population. According to the Food and Agriculture Organisation of the United Nations the number of fishermen worldwide stands at 54 million, with 660 to 820 million people directly or indirectly economically dependent upon fisheries. Global marine fisheries landings are estimated officially at between 80 and 85 million per year since 1990, with corresponding mean annual gross revenues of around USD 100 billion. But global warming is beginning to disrupt the habitat that underpins this major source of food. Since 1880, the global air temperature has risen by 0.8 degrees Celsius, leading to an increase in ocean temperature. Ocean warming is driving changes in ocean circulation and stratification (density layers of ocean in the vertical) due to losses in oxygen, acidification due to dissolution of more carbon dioxide and shifts in primary productivity. As a result, marine fish populations are experiencing large-scale redistributions, increased physiological stress, and altered food availability.
According to the results published in the journal Science, the study looked at historical abundance data for 124 species in 38 regions, which represents roughly one-third of the reported global catch. The researchers compared this data to records of ocean temperature and found that 85 of populations were significantly negatively impacted by warming, while 4% had positive impacts. Overall, the losses outweigh the gains. It found that fish that could be caught each year (maximum sustainable yield) dropped by 4.1% during the period 1930-2010, as a result of climate change.
Depending on the future climate change scenario, tropical fish currently living in waters with temperature between 27 and 29 degrees Celsius will be faced with water that is one to 2 degrees higher. However, many species will disappear unable to adapt to these conditions, also because warm water contains less oxygen. As a result, the fish have to be forced to migrate to cooler waters in search for a new habitat to survive.
Warmer oceans speed up fish metabolisms; fish, squid and other water-breathing creatures will need to draw more oxygen from the ocean. At the same time, warming seas are already reducing the availability of oxygen in many regions. Scientists from the University of British Columbia pointed out that since the bodies of fish grow faster than their gills, they eventually will reach a point where they can't get enough oxygen to sustain normal growth. They also reported that the body size of fish decreases by 20 to 30% for every degree increase in water temperature.
These changes will have a profound impact on many marine food webs, upending predator-prey relationships in ways that are hard to predict. In the tropical belt, fish populations may be hard to save from excessive warming. There, already warm water temperatures will continue to increase in coming decades due to climate change. If it continues, reduced fish catch may impact the global fisheries industry leading to economic losses for African and Southeast Asian countries.