Ye and Mohammed: Analysis of variation in catchability of Penaeus semisu/catus in waters off Kuwait 



705 



Month 



Figure 1 



The catchability estimates of the UFK fleet in 1985 (■) and 1986 (Al. The survey catch rates in November 1985 and 

 October 1986 were missing because the whole data set of five year-round surveys was destroyed during the Iraqi occupa- 

 tion in 1991. 



rate (kg per haul of 45 minutes) of a survey vessel in 

 1985 and 1986. There are no survey catch rates in 

 number available, but using catch rate in weight 

 would not introduce any great bias in the investiga- 

 tion of catchability here. The catchability, q, is rela- 

 tively stable within the period of the investigation, 

 although a great difference can be seen between 1985 

 and 1986. Therefore, the UFK CPUE was used as an 

 index of stock abundance of P. semisulcatiis within a 

 fishing season in our study. 



Results 



Natural mortality is supposed to be known and in- 

 dependent of time within a fishing season, which may 

 violate reality, but this is a more conservative as- 

 sumption than assuming that natural mortality is 

 constant over periods of years (Hannah, 1995). The 

 natural mortality rate of Kuwait's P. serriisulcatus 

 has been investigated by a number of authors 

 (Mathews et al., 1987; Siddeek et al., 1989; Xu et al., 

 1995; Mohammed et al., 1996; FAO^ Al-Hossaini'^); 

 these results range from 1.8 to 4.0/yr In this study, 

 we used the minimum, median, and maximum esti- 



mates of natural mortality rates for our estimation 

 of catchability. 



The fits of the above developed depletion model to 

 catch data are shown in Figure 2, in which we used 

 a median natural mortality rate (M=2.9/yr) and pro- 

 duced 95% confidence intervals by the parametric 

 bootstrap (BO method (Efron andTibshirani, 1986). 

 The predicted catches for the 1994-95 and 1995-96 

 seasons are very close to the observed values in gen- 

 eral. There is relatively greater discrepancy at the 

 beginning of the seasons, however. The predicted 

 catches are lower than the observed ones in the first 

 month for both seasons (Fig. 2). This is caused by 

 the assumption of Equation 4. 



The calculated stock abundance indices also fol- 

 lowed the pattern of observed population indices (Fig. 

 3), showing an exponential depletion of a single co- 



FAO (Food and Agriculture Organization of the United Na- 

 tions). 1982. Assessment of the shrimp stocks of the west 

 coast of the Gulf between Iran and the Arabian Peninsula. Food 

 and Agriculture Organization. FAO Rep. FI:DP/RbyAB/80/015, 

 Rome, Italy, 162 p. 



Al-Hossaini, M. 1993. Finfish fisheries and shrimp fisher- 

 ies management. Tech. Rep., Kuwait Institute for Scientific 

 Research. Food-4 KISR 4367, 53 p. 



