710 



Fishery Bulletin 97(3), 1999 



25 



20 



15 



•S 10 



o 



5 - 



W=l,5Wn 



W=Wo 



Sep Oct Nov Dec Jan 



Month 



Figure 6 



The effect of weighting fact, W, on the estimation of catchability for the 1995-96 season. 



Feb 



vided. The results of our study show that the catch- 

 ability of the Kuwait shrimp fishery is not constant; 

 there were high values at the beginning of the fish- 

 ing season, lowest values at the end. The two-peak 

 pattern of catchability may be associated with school- 

 ing behavior at the time of recruitment to the fish- 

 ery and the spawning migration of the species. There 

 is also great variation in catchability on an 

 interannual basis because schooling is associated 

 with high recruitment (Mathews et al., 1994) and 

 certain environmental conditions (Penn, 1984). 



The general declining trend in catchability may 

 be attributed to schooling behavior. As fishing pro- 

 gressed, shrimp schools were depleted or dispersed, 

 and catchability declined (Drobisheva and Aseev, 

 1976; Garcia and Reste, 1981; van Zalinge, 1984; 

 Mathews et al., 1994). Reduced activity caused by 

 low temperature is another identified cause for the 

 decline of catchability of Spencer Gulf prawns 

 iPenaeus latisiilcatus) in Australia during the win- 

 ter months (June to August) (Sluczanowski, 1984) 

 and P. semisulcatus in Kuwait (Mathews and Al- 

 Hossaini, 1982). Both schooling and effects of decreas- 

 ing temperature likely contributed to the variation 

 of catchability in the Kuwait shrimp fishery. 



The major spawning season for P. semisulcatus is 

 from December to April (Drobisheva and Aseev, 1976; 



Siddeek et al., 1989). Many penaeids undertake a 

 short shoreward migration before spawning (Garcia, 

 1988; Ye, 1984). Penaeus seryiisulcatus in the Gulf of 

 Carpentaria migrate from offshore into shallow wa- 

 ters in the spring before spawning (Dall et al., 1990). 

 Drobisheva and Aseev (1976) reported that P. 

 semisulcatus in the Arabian Gulf forms prespawning 

 schools or aggregations in the spawning area. Al- 

 though the migration pattern of P. semisulcatus in 

 watters off Kuwait is still not clear, the pattern of 

 catchability strongly indicates that this species may 

 return to Kuwait waters before spawning season and 

 concentrate in certain areas. Such shoreward move- 

 ment by spawning schools resulted in the highest 

 catchability, and a peak in December or January (Fig. 

 4), which rapidly decreased as fishing progressed. 



There is also the possibility that shrimp of non- 

 Kuwait origin migrate to Kuwait waters. El-Musa 

 ( 1982) analyzed mark-recapture data and concluded 

 that the P. semisulcatus released in February 1979 

 at Dohat Al-Zaur, close to the border with Saudi 

 Arabia, exhibited a strong northward movement. The 

 timing of such a migration could be affected signifi- 

 cantly by environmental conditions associated with 

 a spawning migration, resulting in an apparent in- 

 crease in catchability earlier in some years than in 

 others. 



