Scientists at the University of East Anglia (UEA), U.K., are involved in a major research project investigating a possible link between global warming and falling numbers of cod.
A team from the UEA is currently analyzing sea samples containing zooplankton, the tiny marine animals that are the staple diet of the fish and other species such as haddock.
Researchers on board the science vessel Discovery collected the samples at 17 sites from the ocean surface to within a few hundred meters of the sea floor in the northern Atlantic between Greenland and Iceland.
Current flows and the water chemistry were also analyzed.
The (British) 6.5 million pound (U.S. $10 million) study by the National Environment Research Council (NERC) was prompted by concern about dwindling numbers of fish stocks.
Previous studies in the North Sea have shown a 90 percent decline of zooplankton in the past 40 years at a time when cod, haddock, herring, and mackerel levels have also fallen, fuelling speculation that overfishing may not be the sole cause of their decline.
And experts are keen to discover if a similar pattern is being repeated in the Atlantic waters.
One theory being investigated is that rising water temperatures could be affecting ocean circulation patterns, moving the shrimp-like creatures north into waters away from traditional fishing grounds.
Phil Williamson, a UEA marine biologist coordinating the research, said that early findings suggested that levels of zooplankton were also dropping in the Atlantic, which could have an impact on the rest of the food chain.
"The numbers seem to be lower than what we expect but it's very difficult to know what to expect when we haven't done a study like this before. It's early days but we have got some indications from some surveys that the numbers might have gone down a bit," he said.