Gaertner et al Bycatch of biilfishes by the European purse seme fishery 



687 



Introducing some elements of uncertainty in the inputs 

 highlighted the large variability of the bycatch estimates 

 (see the values obtained for the lower and upper CI [confi- 

 dence intervals); Table 4. ). However, even including uncer- 

 tainty in the inputs, these values remained very low com- 

 pared with bycatches reported from other fisheries. Based 

 on these results, the total bycatch of biilfishes taken by the 

 entire purse-seine fleet operating in the eastern Atlantic 

 was tentatively estimated. This calculation is supported 

 by the facts that 1) the European fleet is the main com- 

 ponent of the purse-seine fishery operating in this part 

 of the ocean and 2) it is reasonable to assume that other 

 fleets of purse seiners adopted the same fishing strate- 

 gies as the European fleet. With this approach, the ratio 

 of the billfish bycatch per tons of tunas (obtained from 

 the European fleet; Table 3) was raised to the total tuna 

 catch taken by the entire purse-seine fishery (Table 4). 

 To account for the change in fishing strategies caused by 

 the ban on FAD fishing operations, we performed new 

 billfish ratio estimates for the years 1997, 1998, and 1999. 

 These results give an indication of the bycatch of biilfishes 

 in the eastern Atlantic purse-seine fishery (Table 4). 



Discussion 



The ecosystem approach to assessing and managing large 

 coastal marine ecosystems has been developing since the 

 early nineties (Sherman and Duda, 1999). To date, with the 

 exception of the central Pacific Ocean (Kitchell et al., 1999), 

 this approach generally has not been used for monitoring 

 large pelagic fisheries in offshore waters. Our study, in 

 presenting data on bycatch of biilfishes taken by the tuna 

 purse-seine fishery, helps to extend this approach to the 

 monitoring of the eastern Atlantic epipelagic ecosystem. 



IVIarllns 



200 



150 



100 



50 







150 

 120 



90 



60 



30 







-A 



U- 



au 



175 225 275 325 375 425 475 



Sailfish 



 Moratorium 



n Without moratorium 



:jliii 



4=L 



5 15 25 35 45 55 65 



Bycatch (t) 



Figure 5 



Histograms of Monte-Carlo-generated total billfish bycatch 

 ( marlin group in the upper histogram and sailfish group in the 

 lower histogram) taken by the European Union tuna purse- 

 seine fishery, taking into account whether a moratorium on 

 FAD fishing was applied or not, in the eastern Atlantic Ocean. 



A purse-seine fishery cannot be assessed purely in terms 

 of the tuna catch. In the eastern Atlantic Ocean, it can be 

 assumed that the large catches of tunas taken by the purse 

 seiners (around 200,000 t per year in the last decade, Table 

 4) affect the abundance of biilfishes 1) directly, by generat- 

 ing bycatch and 2) indirectly, by increasing or decreasing 



