Seafood could be going off a lot of menus as the world warms. More than half of a group of fish crucial for the marine food web might die if, as predicted, global warming reduces the amount of oxygen dissolved in some critical areas of the ocean – including some of our richest fisheries.
The prediction is based on a unique set of records that goes back to 1951. California has regularly surveyed its marine plankton and baby fish to support the sardine fishery. “There is almost no other dataset going back so far that includes every kind of fish,” says Tony Koslow of the Scripps Institution of Oceanography in La Jolla, California, who heads the survey. The survey records also include information on water temperature, salinity and the dissolved oxygen content.
Koslow’s team studied records of 86 fish species found consistently in the samples and discovered that the abundance of 27 of them correlated strongly with the amount of oxygen 200 to 400 metres down: a 20 per cent drop in oxygen meant a 63 per cent drop in the fish. There have been several episodes of low oxygen during the period in question, mainly in the 1950s and since 1984.
Global climate models predict that 20 to 40 per cent of the oxygen at these depths will disappear over the next century due to warming, says Koslow – mainly because these waters get oxygen by mixing with surface waters. Warmer, lighter surface waters are less likely to mix with the colder, denser waters beneath.
Of the 27 species most affected by low oxygen, says Koslow, 24 were “mesopelagic”: fish that spend the daytime in deep, dark waters below 200 metres to avoid predators such as squid that hunt by sight. There are 10 billion tonnes of mesopelagic fish globally – 10 times the annual global commercial catch – and they are a vital food for other fish and marine birds and mammals.
Out of the depths
In large segments of the Indian, eastern Pacific and Atlantic Oceans called oxygen minimum zones (OMZs), patterns of ocean currents already permit little downward mixing of surface water, so the dark depths where mesopelagics hide have barely enough oxygen for survival. Worldwide, OMZs are expanding both in area and vertically, pushing “hypoxic” water – water with too little oxygen for survival – to ever-shallower levels. Last year, Japanese researchers reported that this has nearly halved the depths inhabited by Pacific cod.
The California coast is an OMZ. When oxygen levels are even lower than usual, the hypoxic zone starts up to 90 metres closer to the surface. This means fish must stay in shallower, more brightly lit water, says Koslow, at greater risk from predators – which, he suspects, is what kills them. In the California data, predatory rockfish in fact boomed during periods of low oxygen.
“This is important work,” says William Gilly of Stanford University’s marine lab in Pacific Grove, California. He studies Humboldt squid, an OMZ predator whose recent movements seem consistent with Koslow’s idea.
“These findings are an example of the kinds of changes we will see more broadly throughout our oceans in coming decades, especially in OMZs,” says Frank Whitney of the Institute of Ocean Sciences in Sidney, British Columbia, Canada. Unfortunately, he notes, water and nutrient movements within OMZs make them among our richest fishing grounds.
Journal reference: Marine Ecology Progress Series, DOI: 10.3354/meps09270