Research at Dalhousie University in Halifax, Canada, believe that the amount of phytoplankton - the plants at the base of the oceanic food chain - has declined by 40% in the last 40%. (Nature, vol 466, p591) They see this decline, which is based on over half a million observations, in eight out of ten oceanic regions.
Not every oceanographer agrees. As New Scientist reported on 7 April others have performed data analyses which show increases, at least in some oceans, and though some climate models forecast the decline others don't.
But there's another thing. There are more levels in the food chain in the tropics than in temperate seas. Since 90% of the captured solar energy is lost as you rise one level tropical seas host less fish than temperate ones. (That's why whales go north each year.) And the boundary between the two kinds of sea in the North Atlantic has moved 1,000 km north in recent decades.
However you look at it, this is not good.