Among the worst marine litter are ghost nets. These are nets that have been lost from fishing trawlers. While nets will float around the sea they will continue catching and killing fish and they can also kill dolphins, whales, seals and sea turtles. Some of these nets get caught up in mangroves where fish can still get caught and killed.
The best guess is there are 5.25 trillion pieces of plastic floating in the sea. Plastics take hundreds of years to break down. These plastics break down into smaller pieces called micro plastics. Some of the bacteria that live on these accumulate chemical pollutants. Shellfish and small fish eat the micro plastic along with the pollutants. As these small fish are eaten, many of the polluting chemicals are concentrated as it moves up the foodchain. Some other bacteria on micro plastics can cause disease in fish.