Purse seine by-catch mitigation techniques
Tropical tuna are known to associate with objects floating at the surface of the ocean. These objects can be natural, such as branches, debris, dead animals, or artificial, coming from human pollution, or constructed and released by fishers to increase their chances of finding tuna (Dagorn et al. 2000a). These man-made floating objects are named Fish Aggregating Devices (FADs). Although tuna purse seining generates relatively low levels of by-catch and discards, the fishing mode greatly affects the composition and volume of by-catch (Romanov 2002). In free school fishing (ie. schools not associated with floating objects), estimates of tuna discards and by-catch are more than three times less than in FAD fishing (Delgado et al. 2000), where a high diversity of species is captured. FAD-fishing is thus the major source of by-catch associated with the tuna purse seine fishery and is estimated to generate significant amounts of by-catch of vulnerable species such as sharks and sea turtles as well as juvenile fish of target and non-target species (Ariz et al. 1999, Hallier et al. 1999, Fonteneau et al. 2000, Anon. 2006). Nevertheless, other fishing modalities targeting pelagic fishes have a significant effect on by-catch (Anon. 2001). The term by-catch will be used throughout this document; this term has been used in scientific and popular literature for more than half a century and has been subject to a variety of definitions (FAO, 1997). For the purpose of this document we consider by-catch to be the part of the capture made up of non-targeted sizes and species.
More than 40 different species have been caught by the FAD associated fishery (Romanov 2002). This is a subject of great concern for Regional Fisheries Management Organisations (RFMOs) which are responsible for tuna research, conservation and management (eg. CCSBT, IATTC, ICCAT, IOTC and WCPFC). Over the past twenty years, tuna purse seiners have taken advantage of the associated concentrations of fish by using FADs. In the last ten years the catch using FADs has increased considerably and now more than 70% of the total catch in the Indian Ocean is obtained using FADs (Fonteneau et al. 2000).
In general, there is a lack of studies dealing with by-catch and purse seining (there are 10 times more longlining studies). The majority of specific research into by-catch on FADs has been published only recently (Dempster and Taquet 2004). The great majority of research has been conducted on anchored FADs (AFADs), principally due to difficulties in studying large mobile fish around drifting FADs (DFADs) in the open ocean, which are transitory in both space and time. Pelagic fish may treat anchored and drifting FADs differently (Holland et al. 1990, Freon and Dagorn 2000) and the conclusions obtained on AFADs cannot reliably be extrapolated to DFADs.
Most of the recent work on DFADs is covered by two European projects: FADIO (Fish Aggregating Devices as Instrumented Observatories of pelagic ecosystems; www.fadio.ird) and MADE (Mitigating ADverse Ecological impacts of open ocean fisheries; www.made-project.eu). In both projects there has been an emphasis on collaboration and information sharing between scientists and fishermen (Calheiros et al. 2000, Moreno et al. 2007) which have been successful in addressing other issues like the one dealing with the dolphin by-catch issue.
In the present document by-catch issues relating to DFADs will be analyzed for three groups of species: 1) bigeye and small/juvenile tuna, 2) sharks and 3) marine turtles. Other species will not be explicitly considered due to the absence of robust scientific by-catch data although many of the principles discussed for these three groups will apply to other species as well. The dolphin bycatch issue will also not be discussed as dolphin mortality levels have been reduced to less than 0.1% since the early 90s (Hall 1998). The success of the AIDCP, the multilateral agreement that entered into forc