Posted by Victor Restrepo and Laurent Dagorn
Tuesday, 30 August 2011

Some words have different meanings to different people. Or even different meanings to the same person, depending on how they are used. “Bycatch” is one of them.

In a general sense, Bycatch is the catch of something that is not the main objective of a fishing fleet. Vessels that go fishing for certain tuna of certain size end up catching other fish. This happens in pretty much all fisheries to some extent.

But the world doesn’t always agree on the precise definition of Bycatch. Even among scientists, the term has caused confusion for some time . These are the three main uses of the term:

• Anything that is caught and discarded at sea (including fish that were targeted by the fishing trip, but are discarded because they are unmarketable due to being damaged or too small);

• Anything that is caught and taken back to port, but that was not the target of the fishing trip (these are known as “non-target” species);

• A combination of the above.

So, there are two main concepts in these uses of the term Bycatch. The first one is whether the fish are retained onboard or discarded at sea. The second is whether they were caught intentionally (targeted) or not (incidental). No wonder there is confusion when a single word, Bycatch, is used to describe various combinations of these.

The confusion grows when we associate numbers with these concepts. If someone says “The bycatch in fishery X is 10%” and someone else says “The bycatch in fishery Y is 5%”, you might be tempted to say that fishery Y is better in some sense. But the reality is that one cannot realistically compare the two statements without knowing what each of the two persons had in mind as Bycatch. They could be apples and oranges.

One of the most widely used studies of bycatch in fisheries is a report published in 2005 by the United Nations Food & Agriculture Organization . It compares discards in many types of fisheries across the globe and indicates that tuna purse seine fisheries discard about 5% of what they catch. Another study published by an Australian government scientific organization states that bycatch in purse seine fisheries that use FADs (Fish Aggregating Devices) is about 10%. In a technical report that the two of us wrote this year , we reported that the catch of non-target species in FAD fisheries varies between 1.7% and 8.9%, depending on the Ocean region.

What do the above percentages (5%, 10% and 1.7% – 8.9%) have in common? Well, for one thing, they all refer to Bycatch in tuna purse seine fisheries. But, they refer to different types of bycatch. Let’s compare the apples and oranges:

• The 5% in the FAO report by Kelleher is an estimate of the average amount of discards in purse seine fisheries, around the world, that target tunas. It includes non-tuna species that are caught unintentionally, as well as tunas that were caught and the skippers discarded for whatever reason (too small, damaged, etc.). And, it is for all sets done by purse seiners in general, not only those on FADs.

• The 10% in the Australian report by Bromhead and colleagues is for non-target species as well as for tunas that they considered to be too small. It includes both discarded and retained fish. And this number is for purse seining with FADs only.

• The 1.7%-8.9% range in the ISSF report by us is only for non-target species (everything but tunas). It includes both discarded and retained fish. And this number is for purse seining with FADs only.

See how comparisons can be difficult? All three examples above share some similarities but also have some major differences in what they refer to (discarded or not, target or not). Bycatch sounds like a simple term, but it isn’t. It’s too general, too imprecise, when it’s not mentioned together with a definition of what it’s meant to represent.

Another point that is important to keep in mind is that different species of marine life have different biology. Some are more easily overfished than others. For this reason, the percentages of bycatch that are mentioned in different studies imply different impacts. A 10% bycatch of an abundant and resilient species may be less of a problem than a 1% bycatch of a depleted and vulnerable species. All kinds of species end up being lumped together in these statistics.

Aside from the confusion created by the lack of a unique definition of Bycatch, one thing is clear: We need to work hard to tackle all bycatch issues. The potential solutions require research and testing of methods of fishing that reduce the amount of bycatch. And, for some species, they require developing new markets so that they can be utilized in order to reduce waste.

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_{[1] Alverson D.L., M.K. Freeberg, S.A. Murawski, and J.G. Pope. (1994) A global assessment of fisheries bycatch and discards. FAO Fisheries Technical Paper No 339 Rome, FAO 1994
[2] Kelleher, K. (2005). Discards in the World’s Marine Fisheries: An Update. FAO Fisheries Technical Paper No 470. Rome, FAO 2005
[3] Bromhead, D., J. Foster, R. Attard, J. Findlay, and J. Kalish. (2003) A Review of the impact of fish aggregating devices (FADs) on tuna fisheries. Australian Bureau of Rural Sciences, Canberra, Australia.
[4] Dagorn, L., and V.R. Restrepo. (2011). Questions and answers about FADs and bycatch. ISSF Technical Report 2011-03. International Seafood Sustainability Foundation, McLean, Virginia, USA.}