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



v{w) = 



S(if/w,) 



Xnii J 



In the second experiment, the beach seine was 

 set in a shallow area of a lagoon where the aver- 

 age depth was 3 m. Fish were marked with a 

 type-1 mark (Mj fish) at the beginning of the 

 experiment and released into the closed beach 

 seine. A 30 min delay allowed the fish to disperse 

 in the enclosure. Five successive purse seine 

 sets were made randomly within the beach 

 seine. Each set lasted about 22 minutes (the net 

 was closed after about 3 minutes; the rings were 

 placed on-board after about 9 minutes). 



Species and size of captures and recaptures 

 were recorded for each purse seine set; the un- 

 marked fish were marked with a type-2 mark 

 and stored in the floating cages. After the fifth 

 purse seine set, the marked fish in good con- 

 dition were released (M-? fish). The retrieval of 

 the beach seine took 6 hours. Species and size of 

 the captures and recaptures (both Mi and M2 

 types) were recorded (Table 3). The experiment 

 was repeated five times. 



In this experiment, one should note that the 

 storing period differed for marked fish: the 

 marked- 1 fish, captured the day before, were 

 held captive approximately 15 hours, while the 

 marked-2 fish on average were retained only 1 

 hour in the cages. Based on an aerial observa- 

 tion, the shapes of the seines were almost cir- 

 cular, although the beach seine was in some 



cases distorted by tidal currents; however, it 

 was assumed that the nets were perfect circles 

 (purse seine area 0.72 ha, beach seine area 9.4 

 ha). 



Two retention rates by the beach seine (Wi 

 and W2) were calculated for the whole experi- 

 ment: 



Wi = Ri(Mi - (rii + ry, + n^ + r^ + ^5)) 



and for the second period of the experiment (be- 

 ginning after the fifth purse seine set): 



Wo = Ro/Mo. 



W2 and Wo correspond to fish that have stayed 

 an average of SVa hours and 6 hours in the en- 

 closure respectively; thus, Wi was expected to 

 be lower than or equal to Wo. The difference 

 between Wi and W2 was tested with a x^ test at 

 the 0.05 level (Dagnelie 1975, p. 88). If W^ and 

 Wo did not differ significantly, escapement was 

 assumed not to have occurred during the purse 

 seine fishing period. The mean retention rate 

 (W) was calculated: 



W ={Ri+ Ro)l(Mi - (r„ + ri2 

 + ris) + M'2). 



'13 



+ ri4 



Assuming that the fish were equally available 

 inside the enclosure, the overall accessibility 

 (Pa) is 1 and the expectancy of the local accessi- 

 bility (pa) is equal to the ratio of the areas cov- 



Table 3. — Experimental scheme used to measure the purse seine efficiency and the beach 

 seine retention rate: a large beach seine reaching the bottom is set and closed over a shallow 

 area, making a circular enclosure. /W, fishes are marked and released within it at time 0, and 

 a 30 min delay allows them to disperse. Five purse seine sets are then successively made 

 within the enclosure. At every set /, the catches c, and recaptures r,, are recorded. The 

 standing stock within the enclosure before set /is noted N,. The unmarked fishes caught with 

 the purse seine are marked with a type-2 mark (mz) and stored, then are released together 

 after the fifth purse seine set (M2). The beach seine is then retrieved and catches (C) and 

 recaptures of both type-1 (M,) and type-2 (M2) are recorded. 



Operations 



Beach seine 

 closed 



Five purse seine sets 

 (in hours elapsed) 



Beach seine Beach seine 

 retrieved catches 



A/6 



/W, 



/T721 fT?22 (TI23 ^2i '^2b 



Mo 



c 



a 



914 



