Overfishing

From giant blue marlin to mighty bluefin tuna, and from tropical groupers to Atlantic cod, industrial fishing has scoured the global ocean. There is no blue frontier left. Since 1950, with the onset of industrialised fisheries, we have rapidly reduced the resource base to less than 10% - not just in some areas, not just for some stocks, but for entire communities of these large fish species from the tropics to the poles
Ransom Myers, former fisheries biologist

Oceanic life starts with the smallest creatures. Microscopic plankton convert sunlight and atmospheric carbon dioxide into oxygen — one species of blue-green algae, Prochlorococcus, is present in such abundance that it produces one out of every five breaths we take. These are scooped up by grazers such as krill, which are then eaten by small baitfish — anchovies, herrings and their ilk — and much larger filter-feeders like baleen whales. Further up the food chain, baitfish provide food for larger fish, eventually reaching the top predators: tuna, marlin, sailfish, sharks as well as dolphins, seals, otters and other mammals.

Since the Second World War fishing by humans has rapidly disassembled this complex web of sea life from the top down. A 2003 study in Nature found that tuna and other large predators had declined by on average 90% since the middle of last century. Much of this is to satisfy consumers in Asia, Europe and North America – bluefin tuna is so valued in Japanese sushi that they can sell for as much as £250,000 - but many other species for which no market exists are simply thrown overboard as ‘bycatch’.

Our global demand for seafood, and our inability to govern the oceans, has had dramatic repercussions. Firstly, removal of top predators can cause knock-on effects known as ‘trophic cascades’, which ripple down the food chain with unpredictable consequences. Overfishing of large sharks off the US eastern seaboard allowed their prey, cownose rays, to boom to 20 times their former numbers. As these middle predators became more common, their scallop prey crashed, and a 100-year-old North Carolina scallop fishery collapsed.

Secondly, as technology has progressed and subsidies to fishing fleets have grown, boats have been able to go farther out to sea and fish deeper than ever before. Because many coastal fisheries have collapsed, much fishing has shifted to the high seas — a vast, unregulated global commons where destructive fishing techniques catch millions of fish, birds and mammals as bycatch and trawl the sea floor in search of the last remaining fish stocks. Bottom trawling has been compared to forest clearcutting in its impacts on biodiversity and ecosystems, and covers an area roughly 150 times that deforested each year.