Tuna longline and sustainability

Recently, “sustainable” and “eco-friendly” seem to have become most fashionable words in the world. From politicians to high school students, these terms are being used as “papal indulgence”. Any action or object becomes holy and justified the moment these words are attached to them: sustainable eco-friendly source of energy; eco-friendly fridge and so on.

In the fisheries science, the word, “sustainable” can be found in text books as early as over 50 years ago. Therefore, it is nothing new for us. None-the-less, recently, this type of argument has become very active. Some say that tuna longline fishing is not sustainable as they target spawners, while others even argue that longline fishing is not eco-friendly as they have by-catches of non-target animals and consume more combustion.

However, these arguments are all relative (i.e. less sustainable or more sustainable etc.) since human beings themselves are destructive forces to the earth’s eco-system. Any human action – including industry, agriculture, and fisheries – changes the natural environment and eco-system, and any substance in the world is non-sustainable in the long-run.

Taking these facts into consideration, the question we must then ask is whether tuna longline fishing is indeed more destructive than other fishing methods? Longline fishing is a very passive method, just waiting for the fish to bite the baits. It only randomly extracts a part of the fish stock and hardly causes any excessive catches. Particularly, the longline catch rate (hooking rate) is correlated very closely with the stock size. Therefore, if stock size shows any reduction, it is immediately reflected by the proportional reduction in the catch rate of longline fishing, which is not the case in other tuna fisheries. For example, in surface fisheries, when stock size is reduced, the area of fish distribution or number of fish schools might be reduced, but the school size tends to stay stable. Therefore, stock reduction is not directly reflected in the catch rate.

As longliners are fishing at the margins of economic break point, the declined catch rate – which means reduced profit – would discourage fishing. Hence, the quantities of efforts of this fishery are controlled by economics responding to the stock size. When longline used to be the only offshore tuna fishing method in the world (i.e. 1950-1970), tuna stocks had never experienced serious depletions by excessive fishing. All the problems of excessive fishing started only after the development of large-scale surface fisheries, such as the cases with west Atlantic bluefin, Pacific bluefin, southern bluefin, and bigeye tunas all over the world.

Ironically, the more efficient the fishing gears are, the more destructive they are for the fish resources. If the choice is between more efficient gears and sustainable use of fish resources, then scientists would choose sustainable use of fish resources. The only problem with this choice、if extreme, is that the fish stocks would be sustainable but fishery might not be.