However as shocking as the facts about the dolphin friendly method of purse seine fishing is it isn’t as shocking as the conclusion drawn by whysharksmatter, the author of the article.
He quotes the following bycatch rates from here (a broken link to what looks like a Powerpoint presentation).
Ten thousand sets of purse seine nets around immature tuna swimming under logs and other debris will cause the deaths of 25 dolphins; 130 million small tunas; 513,870 mahi mahi; 139,580 sharks; 118,660 wahoo; 30,050 rainbow runners; 12,680 other small fish; 6540 billfish; 2980 yellowtail; 200 other large fish; 1020 sea turtles; and 50 triggerfish.
Ten thousand sets of purse seine nets around mature yellowfin swimming in association with dolphins, will cause the deaths of 4000 dolphins (0.04 percent of a population that replenishes itself at the rate of two to six percent per year); 70,000 small tunas; 100 mahi mahi; 3 other small fish; 520 billfish; 30 other large fish; and 100 sea turtles. No sharks, no wahoo, no rainbow runners, no yellowtail, and no triggerfish and dramatic reductions in all other species but dolphins.
Then whysharksmatter writes this
If you work out the math on this, you find that 1 dolphin saved costs 382 mahi-mahi, 188 wahoo, 82 yellowtail and other large fish, 27 sharks, and almost 1,200 small fish.
By trying to help dolphins, groups like Greenpeace caused one of the worst marine ecological disasters of all time. Few other fisheries are as bad for groups like sharks and sea turtles as the purse seine fishery, and none are as large in scale.
This is entirely disingenuous because it suggests that there are only two methods of catching tuna both of which use purse seine nets and that the dolphin friendly usage of purse seine nets is the ecologically worse of the two.
Greenpeace in fact advocate ditching the use of purse seine nets entirely and catching tuna (plus other commercial species of fish) with pole and line methods and many major British retailers are in line with this thinking.
There is a brilliant new documentary called The End of the Line that is currently on limited release in the UK that explains these issues in far greater detail than I can here or pick up the book by Charles Clover that the film was based on (available at Amazon US or UK).