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



Table 5. — Comparison of the retention rates of purse seine ( W) and 

 beach seine (W, and Wz), calculated by size group: one-way 

 analysis of variance by ranks (Kruskall-Wallis test). Critical value of 

 X^o.95 is 7.81. M = mean, C.V. = coefficient of variation. 



Catchability and Efficiency of the Purse 

 Seine 



The catchability equivalent to a purse seine 

 set was estimated using both methods from cap- 

 tui-es (Table 6) and recaptures (Table 7). The 

 mean and the standard deviation were then cal- 

 culated. 



For Chrysichthys spp., using the method from 

 captures, q^ equaled 2.35% (SD = 5.38), and 

 using the method from recaptures, q,- equaled 

 0.97% (SD = 2.03). Most of the variability of the 

 q^ values comes from one set (#3, Date 13), 

 where one quarter of the fish were caught. We 

 tested the two q measures obtained for this set 

 (24.2, 6.85) as outliers in their respective series 

 (DagneHe 1975, p. 34). With this procedure, the 

 value to be tested was initially removed from the 

 data, and a new mean and standard deviation 

 were calculated (q/ = 1.20, SD = 1.63). A 

 Student's t statistic was then calculated (15.3 

 and 4.4 respectively) and compared with the 5% 

 critical value <o.o5 = 3.6. This allowed us to dis- 

 card the data from set #3. After removal of the 

 outHer set, the difference between the two esti- 

 mations of q, tested using the Wilcoxon's signed 

 rank test, was not significant at the 0.05 level. 



Thus, the 39 measures of q were pooled for the 

 calculation of the mean {q = 0.93) and SD = 1.58. 

 The efficiency was then calculated using Equa- 

 tion 1 to be 12% {e = 0.93 • 9.4/0.72). 



For T. guineensis, the two catchabihty esti- 

 mation methods yielded the following results: 

 using the method from captures (Table 6), the 

 mean, noted q,. equaled 3.54 (SD = 1.70) and 

 from recaptures (Table 7), the mean, noted q,., 

 equaled 1.39 (SD = 1.44). 



We compared the two samples (Wilcoxon's 

 rank test) and found a highly significant differ- 

 ence {P = 0.01). The q,. value was considered to 

 be more reasonable and the reasons will be dis- 

 cussed later. The efficiency follows was then cal- 

 culated to be 18% (1.39 • 9.4/0.72). 



Avoidance of the Purse Seine 



The avoidance rate («) was estimated using 

 Equation (2), e = u  v  w (pooled mean from 

 Table 2), knowing e, w, and with i.' being equal to 

 1 in our experimental conditions. For Chry- 

 sichthys spp., e = 0.12 and iv = 0.73, thus, u = 

 e/iv = 0.16. Thus active avoidance rate appears 

 to be the main factor in the efficiency. For T. 

 guineensis, the q,. estimation from recaptures 



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