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



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Another interesting point is that there was a rise 

 in catchabihty in December for the 1994-95 season, 

 in January for the 1995-96 season. The second 

 catchability rate peak was even sHghtly higher than 

 the first one in both seasons. All these results show 

 that catchability in the P. semisulcatus fishery var- 

 ies within a fishing season and from year to year, 

 but that the variation patterns in different seasons 

 are similar, with two peaks (Fig. 4). 



There are many biases in the estimation of natu- 

 ral morality ( Vetter, 1988 ). The natural mortality rate 

 used in this analysis was the median value of esti- 

 mates from various sources. Different values of natu- 

 ral mortality will result in different estimates of 

 catchability. Figure 5 shows comparison of the re- 

 sults derived from the seasonal 1995-96 data when 

 the minimum (M=1.8/yr), median (M=2.9/yr), and 

 maximum (M=4.0/yr) estimates of natural mortality 

 were used. The variation of the pattern of catchability 

 was similar within this range of natural mortality. A 

 lower natural morality results in higher catcha- 

 bilities and a relatively large variation in catchability 

 within a fishing season, demonstrating the correla- 

 tion between natural mortality and catchability. 

 Therefore, we suggest that the variation pattern 

 derived by this method is reliable, although absolute 

 values may be biased by the natural mortality used. 



The weight, W in Equation 5, can be assigned a 

 high or low value depending on the reliability of the 

 auxiliary information compared with the catch data 

 (Hilborn and Walters, 1992). Here, we set Wto6 sim- 

 ply because there were six points of catch and effort 

 data each month, but the auxiliary index had only a 

 single datum point each month. Actually, the auxil- 

 iary information was used to constrain the predicted 

 stock's declining track, and thus control total mor- 

 tality within a certain level. A sensitivity analysis 

 showed that reducing W by 50% resulted in an aver- 

 age decrease of 3.5% in catchability, ranging from 

 2.1% to 4.8%; increasing W by 50% leads to an in- 

 crease in catchability of 7.4% on average, with a 

 range of 5.3% to 9.6% (Fig. 6). The primary concern 

 here is the pattern of catchability variation; it is not 

 sensitive to the weighting factor in the case of Kuwait's 

 P. semisulcatus fishery (Fig. 6). One method to deter- 

 mine a suitable value for W is by comparing the geo- 

 metric fits of both the catch and abundance index in 

 the case of unknown reliability over either set of data. 



Discussion 



An analytical framework to estimate changes in 

 catchability of single cohort fisheries has been pro- 



