CHARLES-DOMINIQUE: CATCH EFFICIENCIES OF PURSE AND BEACH SEINES 



E = U V W (beach seine) 



(3) 



where capital letters refer to the beach seine and 

 small letters to the purse seine (Table 1). 



Table 1.— Summary of the methods used In this paper to 

 estimate the parameters of the catchabillty, as defined in the 

 text and on figure 1 , Small letters refer to the purse seine and 

 capital letters to the beach seine. 



Parameters 



Purse 



Beach sine 



Retention rate 

 after escape- 

 ment through 

 mesh 



Retention rate 

 after other 

 forms of es- 

 capement 



Catchabillty 



Efficiency 



Avoidance 



V = for fishes greater 

 than Lioo. 100% 

 mesh retention 

 size 



IV = 15 independent 

 mark-recapture es- 

 timates 



g = 5 measures us- 

 ing two methods 

 (from captures and 

 from recaptures) 



e = q  a//4 



a = area covered by 



the purse seine 



(0.72 ha) 

 A = area covered by 



the beach seine 



(9.4 ha) 



u = elw 



V= 1 (idem) 



W = 2 meas- 

 ures ( H/i , 

 W2). 5 replica 



W 



supposed negli- 

 gible 



Two independent experiments were design- 

 ed 1) to estimate the purse seine retention rate 

 and 2) to estimate both the beach seine retention 

 rate and the purse seine efficiency. 



To estimate the purse seine retention rate, 15 

 mark-recapture experiments ((' = 1 . . . 15) were 

 conducted in the open lagoon.^ 



Marking was done without anaesthetic by clip- 

 ping either the superior lobe of the caudal fin, 

 the right or left pelvic fin, or the adipose fin. 

 Fish were stored in 1 m^ floating cages with up 

 to 100 fish per cage. The minimum size of the 

 marked fish (mm) was gi-eater than the Lioo-" 



At each experiment i, rw, fish were marked 

 and released within the "closed" purse seine 

 (i.e., when the two ends of the seine are joined 

 together before pursing). At the end of the fish- 

 ing operation, the species and sizes of the recap- 

 tured fish ((■,) were noted, and the retention rate 

 {lOi = rj/m,) was calculated for each set. The 

 weighted average (w) and variance (i'(m')) were 

 then calculated (Table 2). 



w = 



In. 



'The experience was conducted in the Aby lagoon, one of 

 the largest lagoons of the Ivory Coast (424 km-), situated on 

 the southwest of the country. 



^Fish were larger than 9 cm {Chrysichthys spp.) or 10 cm 

 (Tilapia guineensis). See footnote 3: Cantrelle et. al. 

 (1983). 



Table 2. — Estimation of the purse seine retention rate (w) from releasing m fish within the closed purse seine and 

 recapturing them (r). C.V. is the coefficient of variation in percentage. 



913 



