An NY Times article a couple weeks back included this snapshot of global fisheries decline, based on data compiled by the experts here at the UBC Sea Around Us Project. The image tells the story of the article, a story not repeated often enough. Without a drastic change in management, we are nearing the end of the wild harvest in fish.
The world is in the middle of phase transition from wild harvest to farming, similar to what happened with land animals. The depletion of the natural resource itself is driven in part by the rapid transition to an energy-intensive farming industry. Consider the proportion of wild fisheries required to support fish and animal feed:
Nearly one-third of the world’s wild-caught fish are reduced to fish meal and fed to farmed fish and cattle and pigs. Aquaculture alone consumes an estimated 53 percent of the world’s fish meal and 87 percent of its fish oil. (To make matters worse, as much as a quarter of the total wild catch is thrown back — dead — as “bycatch.”)
A substantial proportion of the wild harvest is used to maintain marine aquaculture of carnivorous species like salmon. It is wildly inefficient, the marine equivalent of farming wolves rather than herbivorous cattle. This is why many experts conclude that the future for pescetarians is probably the blander, lower-on-the-food-chain species like tilapia and catfish. Continued consumption of popular favourites like tuna and salmon could only happen with drastic improvements in fisheries management.
Marine aquaculture shares many of the shortcomings of industrial animal agriculture. A high input of energy (fish meal, animal feed) is required per unit of food production. Also, a large proportion of the natural resource base (wild-caught fish, agricultural land) must be used to produce the inputs. Finally, research concludes that shifting towards less energy-intensive options (lower on the food chain or "closer to the sun", effectively the same statement) would help sustain the natural resource.
Sure, fish did not evolve to eat grain. Then again, neither did cattle.