Net positive

FISH are slippery characters, with little regard for international agreements or borders. The speediest, such as crescent-tailed bluefin tuna, can slice through the ocean at 70 kilometres per hour. Their routes take them beyond areas that come under the jurisdiction of individual coastal states, and into the high seas. These wildernesses were once a haven for migratory species. No longer.Under international law the high seas, which span 64% of the surface of the ocean, are defined as “the common heritage of mankind”. This definition might have provided enough protection if the high seas were still beyond mankind’s reach. But the arrival of better trawlers and whizzier mapping capabilities over the past six decades has ushered in a fishing free-for-all. Hauls from the high seas are worth $16 billion annually. Deprived of a chance to replenish themselves, stocks everywhere pay the price: almost 90% are fished either to sustainable limits or beyond. And high-seas fishing greatly disturbs the sea bed: the nets of bottom trawlers can shift boulders weighing as much as 25 tonnes.