Ye and Mohammed: Analysis of variation in catchability of Penaeus semtsukatus in waters off Kuwait 



71 1 



The schooling behavior of P. semisulcatiis was as- 

 sociated with years of higher catches (van Zahnge, 

 1984; Mathews et al., 1994). When recruitment to 

 the fishery was good, larger schools, and thus higher 

 catchability, could be expected. The catches at 

 the beginning of the 1995-96 season were much 

 higher than those of the same period in the 1994-95 

 season ( Fig. 2 ); catchability in 1995-96 was also much 

 higher (Fig. 4). With intensive fishing effort, the 

 shrimp stock was reduced to a very low level by the 

 end of the fishing season, despite its initial abun- 

 dance. Therefore, catchability is associated with stock 

 abundance. 



Although the variation in total mortality and catch 

 cannot be explained by variation in natural mortal- 

 ity, violation of the assumption that natural mortal- 

 ity remains constant within a season may still be 

 the source for biases in the estimation of monthly 

 catchability. The relatively larger discrepancy be- 

 tween predicted and observed catches, the predicted 

 being always lower than the observed, in the first 

 month (Fig. 2) may result from a departure from the 

 natural mortality rate that is generally used. An in- 

 flated natural mortality for the first month will give 

 lower predicted catches because the first month's 

 catchability was related to the initial population 

 number, which in turn was connected to all the other 

 estimates of catchability in the fitting scheme used 

 in this study. The fit cannot be improved by adjust- 

 ing catchability for the first month. Although it may 

 be true that natural mortality is constant during a 

 fishing season, a possible second recruitment in early 

 September, which was not considered in our study 

 and is probably rather weak, may compensate natu- 

 ral death to some extent and lead to a lower net natu- 

 ral mortality than the value we used. 



This study reports apparent changes in catch- 

 ability of Kuwait's P. semisulcatiis fishery. It should 

 be noted that M, q, CPUE, and schooling must be 

 interrelated to a certain extent. Schooling may bias 

 CPUE with respect to years in which it does not oc- 

 cur, and M estimates from any method applied in 

 Kuwait may also be influenced if schooling occurs 

 only in certain seasons. Changes in catchability to a 

 fishery may be associated with both fishing power 

 and population characteristics. A better understand- 

 ing of the mechanisms underlying these results will 

 improve stock assessment. The variations in 

 catchability suggest that changes in catch-rate indi- 

 ces may not actually reflect variations in abundance. 

 Although other results support the conclusion of this 

 study, functional relationships among catchability 

 and population characteristics, fleet characteristics, 

 and environmental conditions should be studied 

 further. 



Acknowledgments 



This study was part of the Shrimp Fisheries Man- 

 agement Project supported by the Kuwait Institute 

 for Scientific Research. We would like to thank James 

 Bishop for many constructive discussions and com- 

 ments on the manuscript, and two anonymous refer- 

 ees for extensive reviews. The contributions of the 

 project staff are highly appreciated. 



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