One of the mainstays of commercial fishing of pelagic fish species (think tuna, marlin, mahi-mahi, etc.) is the use of Fish Aggregating Devices (FADs).
Basically what this involves is throwing overboard something that floats with a GPS transmitter and then leaving it in the ocean for a while before coming back and catching everything that’s hanging out by the device. Pretty much anything can act as a FAD, from natural things like floating sargassum mats to discarded fishing nets, to off-shore aquaculture nets. Some of the FADs used in commercial fishing are moored to the bottom while others are allowed to float around on the ocean currents.
For reasons that vary by species up to 300 different species gather around these FADs. Generally speaking they provide some protection for juvenile fish species by providing structure in the open ocean, these juveniles in turn attract predators (like tuna) which then attract bigger predators, and so on.
There are three main results of the increased use of FADs in commercial fishing. First, there has been an increase in the number of fish caught by pelagic fishing vessels, second the overall size of the fish caught has gone down, and finally the amount of bycatch has gone up significantly. All of these results prove problematic to say the least in terms of ocean conservation.
Looking at the first two results together indicates that smaller fish, possibly juveniles which have not had a chance to reproduce are being caught in record numbers. This leads to a decline in adults, which in turn produce fewer juveniles, which in turn are caught in ever increasing numbers. Obviously this is problematic when looking at a fish like tuna, of which most species are already over-exploited and have dwindling numbers.
Increasing bycatch is another issue altogether. Basically bycatch is when the wrong animal comes up in a fisherman’s net/line/trawl/trap etc. All different kinds of fishing gear have different rates of bycatch, from spearfishing where it is virtually nonexistent to deep-ocean trawling and FAD fishing where it is very high. This means that using FADs to catch tuna leads to catching far more non-target species than using a hook and line does. This video really drives home what can and does get caught using FAD fishing techniques.
So when you’re next off to the market to buy fish it really is important to not only pay attention to what kind of fish you’re eating but also how it was caught because some methods are much worse than others!