702 



Abstract.— A method for assessing 

 variation in catchability of single cohort 

 fisheries based on catch and effort sta- 

 tistics was developed. The results of its 

 application to the Kuwait shrimp, 

 Penaeus semisulcatus, fishery showed 

 that catchability declined gradually 

 with progress of fishing in general, but 

 there was a rise in December or Janu- 

 ary. The pattern of variation was asso- 

 ciated with possible schooling behavior 

 af the beginning of a fishing season. 

 Initially, catchability was high, but, as 

 intensive fishing dispersed the schools, 

 or reduced population density, catcha- 

 bility declined. The midwinter increase 

 in catchability was probably due to an 

 inshore spawning migration. There 

 were also substantial interseasonal 

 variations in catchability because school- 

 ing was associated with strong recruit- 

 ment and environmental conditions. 



An analysis of variation in catchability of 

 green tiger prawn, Penaeus semisulcatus, 

 in waters off Kuwait 



Yimin Ye 



Hussain M. A. Mohammed 



Mariculture and Fisheries Department 

 Kuwait Institute for Scientific Research 

 P O Box 1638, 22017 Salmiya, Kuwait 

 E-mail address (for Y Ye)) yye q.'safat l<isredu l^w 



Manuscript accepted 2,5 June 1998. 

 Fish. Bull. 97:702-712 ( 1999). 



Most fishery management regula- 

 tions are based on estimation of re- 

 source abundance and fishing mor- 

 tahty, of which catchabiUty is an 

 essential element. The existing lit- 

 erature on population dynamics 

 rarely deals directly with catchability 

 per se. For most production models 

 and cohort models that use abun- 

 dance indices, catchability is usu- 

 ally taken as constant and, subse- 

 quently, catch per unit of effort 

 (CPUE) from commercial catch and 

 effort statistics is used as an index 

 of stock abundance. 



The assumption of constant catch- 

 ability is rarely, if ever, valid within 

 a fishing season (Garcia, 1988; 

 Atran and Loesch, 1995; Hannah, 

 1995;AiTeguin-Sanchez, 1996), par- 

 ticularly for short-lived species such 

 as certain shrimp in Kuwait's wa- 

 ters. Catchability depends on many 

 factors: sensory capabilities, behav- 

 ioral response of the target species 

 (Penn, 1984; Wassenberg and Hill, 

 1990), environmental factors in- 

 cluding temperature (Chittelbor- 

 ough, 1970; Morgan, 1974; Hill, 

 1985) and wind (Ehrich and Gro- 

 ger'), stock area, the relative dis- 

 tribution of fish and fishing (Win- 

 ters and Wheeler, 1985; Hannah, 

 1995), and stock abundance (Mac- 

 Call, 1976, 1990; Murphy, 1977; 

 Csirke, 1989). Ricker (1975) stated 

 that variation in catchability is 

 likely to be the greatest source of 

 error in models based on CPUE 

 with constant catchability. 



Shrimp fisheries are often man- 

 aged by a minimum spawning stock 

 strategy to prevent overfishing of 

 stocks. Under such a strategy, 

 CPUE during a spawning season is 

 usually used as an index of spawn- 

 ing stock abundance, and fishing is 

 closed when CPUE falls below a cer- 

 tain level (Penn, 1984; Morgan, 

 1989). If catchability is highly vari- 

 able, indices of spawning stocks and 

 the underlying stock recruitment 

 relationship could be obscured and 

 perhaps lead to erroneous manage- 

 ment decisions (Hannah, 1995). 



Studies on catchability are essen- 

 tial for relating stock abundance to 

 observed CPUE. By investigating 

 within-season changes in catcha- 

 bility, properties of a stock that are 

 not apparent when only annual 

 data are examined, can be under- 

 stood better. The objective of this 

 paper is to develop a method to es- 

 timate changes in catchability from 

 commercial catch and effort data. 



Materials and methods 



The model 



The Penaeus semisulcatus fishery in 

 waters off Kuwait has a single ma- 

 jor recruitment in June and July, 



Ehrich, S., and D. Groger. 1989. Diurnal 

 variation in catchability of several fish spe- 

 cies in the North Sea. ICES Council Meet- 

 ing (CM) 1989/B:35, 10 p. 



