You go out for some tuna on your big ol’ boat. Your fish finder spots a large school of fish. You lower the net, you scopp them up, and they splat onto the deck and voila, 1000 lbs of tuna!
But…how did they know it was going to be tuna? And there are no other random fish in there? Or a sea turtle, shark, or dolphin or something? And lobster traps/crab pots…how does a random crab not get into a lobster trap, or visa versa?
Tuna schools may be spotted on the surface while feeding, and can then be identified.
However, fishermen often do take a large number of non-target species, known as “by-catch.” Some that are commercially valuable may be kept. More often, the “trash” fish are simply thrown back, frequently dying due to trauma suffered in the nets or traps. Fishing can be a very wasteful process.
By-catch mortality of dolphins with tuna, or sea turtles with shrimp, has been a major conservation issue.
As mentioned, they don’t. Anyone fishing just follows the best technique to capture what they are looking for. Everything else needs to be thrown back ethically if not legally. Lots of fish are thrown back, even the legal ones when it comes to sport fishing. Bass fisherman for example usually throw fish back because they just aren’t that good to eat. Good fisherman tend to be pretty conscience of conservation.
IIRC, tuna is fished for with long lines, not seine nets. Different techniques for different fish help target the fish you are looking for. But there are always “by-catch” as mentioned.
Actually, tuna are frequently caught using purse seines. In the eastern tropical Pacific, these are deliberately set around schools containing both dolphin and tuna. This article discusses the tuna-dolphin issue in some detail.
Sea turtle bycatch with shrimp can be avoided by the use of turtle excluder devices (TEDs), but the use of these has been resisted in some areas.
One major problem that has recently become apparent with long-lining is the incidental mortality of albatrosses. If the line floats too close to the surface while being released, albatrosses take the bait and are dragged under and drowned. This is presently a serious concern with regard to the decline of several albatross populations.
Seine nets have been outlawed in the U.S. but they still use them in Mexico, IIRC.
Thanks for the correction.
The fish that is on the other end of my line is always the right one - for that moment.