FISHERY BULLETIN: VOL. 87, NO. 4. 1989 



well as demersal species (Durand et al. 1978; 

 Charles-Dominique 1983). 



In this experiment, the purse seine was 305 m 

 long on the lead-line and 14 m deep; the net was 

 in excellent condition. The beach seine was 1,100 

 m long and 8 m deep; although the net was care- 

 fully checked, a few holes may have been over- 

 looked owing to its very large size. The mesh 

 size for both seines was 14 mm (bar measure). 



Catchability (q) is the probability of capturing 

 one fish from the standing stock by one unit of 

 effort iq = C/N, where C is the catch in number 

 per unit of effort and A^ the total standing stock). 

 It may be divided into three elements (Laurec 

 and Le Guen 1981): 1) overall accessibility (pa), 

 the probability of the presence of one fish in the 

 fishing area A; 2) local accessibility (p„), the 

 probability of the presence of one fish in the area 

 a that has been swept by the gear in one fishing 

 operation; and 3) efficiency (e), the ratio of the 



number of fish caught to the number of vulner- 

 able fish that were present in the area swept in 

 one set (see Figure 1 ). Thus, 



Pa  Pa- e 



(1) 



In shallow waters, where the net reaches the 

 bottom, the efficiency can be broken down into a 

 product of three retention rates, corresponding 

 to three successive phases: 1) avoidance (u), 

 beginning with the net-shooting and ending with 

 its closing; 2) escapement through the mesh 

 (v), occurring if the fish size is less than Ljoo (the 

 size at which 100% of fish are retained by the 

 mesh); and 3) other forms of escapement (»'), 

 i.e., jumping over the net, burrowing or slipping 

 through holes in the net or under the lead-line. 



This catch efficiency can be written as follows: 



e = u  V  w (purse seine) 



(2) 



Standing slock : N 



Accessible stock 



Vulnerable stock : N 



Remaining after avoidance : N. 



Remaining after mesh selection : N 



Catches (remaining after other forms of 

 escapement) C 



Catchability : q = C / N 



captured vulnerable accessible 



q = 



vulnerable accessible standing 



■- Ph 



p. supposed = 1 



area covered by the seine a 



fishing area 



erficiency e = — = — x — x — 



N N N. N 



a a b c 



w : retention rate after others 

 forms of escapement 

 '- V : retention rate after mesh selection 

 '- u : retention rate after avoidance 



Figure 1. — Schematic representation of the catchability parameters for a seine in shallow 

 waters and the catchability equation that comprises these parameters. 



