Along with overfishing, they are having an increasingly damaging impact on the word's oceans, the report by the U.N. Environment Program says.
Fish catches are predicted to decrease in nearly all areas by 2050, and fisheries worldwide will be heavily dominated by smaller species lower down on the food chain, it says.
A continuing decline in marine biodiversity will leave marine and coastal ecosystems vulnerable to climate change, the report concluded.
"Decoupling growth from rising levels of pollution is the No. 1 challenge facing this generation," program Executive Director Achim Steiner said.
"This is nowhere more starkly spotlighted than in the current and future health of the world's sea and oceans.
"Multitrillion-dollar services, including fisheries, climate-control and ones underpinning industries such as tourism are at risk if impacts on the marine environment continue unchecked and unabated."