But, according to Takayanagi, the important piece missing from this picture is what the fish eat. Marine ecosystems ultimately depend on phytoplankton, the production of which, in turn, depends on nutrients such as nitrogen and phosphorous. These are brought to the surface by upwelling or vertical circulation (see diagram, ‘Environment System’). In some places upwelling will increase, in others it will decrease, and the result directly affects fish distribution. But we don’t know enough to make firm predictions yet, he says.
The picture is muddied further by the problem of over-exploitation of high-value, commercially important stocks. “Catches are changing, but we don’t yet know if the main cause is global warming or over-fishing.” And there is another issue that will slowly come into play over the next century — the increase in acidity as the ocean absorbs more and more carbon dioxide.
An increase in acidity will make it much more difficult for organisms to extract calcium from sea water to form shells, and will put coral reefs and shellfish, including oysters, scallops and mussels, at risk. More importantly, it will severely affect a huge variety of plankton, the ocean’s basic food source.
Whatever happens offshore, Takayanagi says we will see effects much earlier in coastal waters. And that is already beginning to happen, as warm-water species invade southern Japan. “In the Kyushu area, divers are seeing unknown warm-water species of seaweeds. Some don’t even have Japanese names yet, but they affect the distribution of small fish that spawn and breed in seaweed.”