As they're slower breeders than skipjack, many yellowfin are caught before they're old enough to breed. This makes them more vulnerable to over-fishing. Tuna fishing has grown into an eight-billion dollar industry. And over four million tons of tuna are caught worldwide each year, a four-fold increase in as many decades. Almost two-thirds of the catch now comes from the Pacific. In the Atlantic, yellowfin catches have been shrinking since 1990. Now a similar decline has begun in the Pacific. Tuna need to swim constantly to keep water flowing over their gills, otherwise they can't breathe. The fishermen want to get them out of the water as quickly as possible. When starved of oxygen, a build-up of lactic acid in their muscles causes the quality of their meat to deteriorate. The fish are scooped up from the water, a tons or two at a time. Every last fish from this school of 7,000 yellowfin and skipjack tuna is plucked from the water. With fishing techniques now so efficient, and with ever more vessels plying the Pacific, there is real concern among biologists that even the resilient skipjack may begin to decline. This vessel is not one of the newcomers. It's a Papua-New-Guinea-flagged ship, fishing in their territorial waters. So it's subject to catch limits and regulations that are amongst the strictest in the Pacific, designed to ensure that tuna fishing remains sustainable. But New Guinea's fishermen are concerned about the increasing numbers of foreign vessels now fishing for Pacific tuna. They were the first nation to propose that the high-seas pockets beyond their national waters be declared marine reserves, as now advocated by Greenpeace.