“There’s a big competition out there with foreign vessels, especially from China,” said Moshwood Kuku, a fishmonger at Afikala Afrikane, a stall that specializes in African fish at Billingsgate. “Locals can only fish the coast.” Chinese National Fisheries, which first sent boats to the Atlantic in 1985, now has offices up and down the coast of West Africa, accounting for more than half its international offices. It also has a huge compound in Las Palmas. While small local fishermen in West Africa tend to fish sustainably, large seagoing boats use practices that are dangerous to the environment, particularly their habit of trawling the seabed with vast nets. The nets destroy coral, and unsettle eggs and fish breeding grounds. They gulp up fish that cannot be sold because they are too small. Their competition decimates local fishing industries. By the time huge mechanized vessels have thrown the unsellable juveniles back into the sea, they are often dead, bringing stocks another step closer to extinction. Of the estimated 90 million tons of fish caught each year, about 30 million tons are discarded, Vesper of the World Wide Fund for Nature said.