674 



Fishery Bulletin 88(4). 1990 



tions may result from fish behavior (Thorne 1977b). 

 However, most of these surveys are performed on a 

 single pelagic or semipelagic species, and the correla- 

 tion between acoustic surveys and CPUE by trawling 

 for multispecies tropical stocks is currently poorly 

 documented. Soetre and Paula e Silva (1979) found 

 from comparison between demersal trawl catch and 

 echointegration on a muitispecific stock off Mozam- 

 bique that the catchability of the otter trawl was 0.3. 

 These authors state that this value is "suspiciously low 

 for demersal species." Guillory et al. (1982) concluded 

 from replicate sampling with otter trawls that this type 

 of gear should be used strictly for qualitative purposes 

 in a multispecies situation. This is in part supported 

 by the present work, which indicates a need for alter- 

 nate methods of stock assessment. Longlines show 

 similar problems to trawling. Thus, correlation between 

 longline CPUE and observed densities from a submer- 

 sible was weak at the multispecies level (Richards and 

 Schnute 1986, Ralston et al. 1986) but satisfactory for 

 some selected species. Similar work involving longline 

 CPUE and observed densities by visual transects in- 

 dicated a good correlation for all species (r = 0.88) but 

 considerable variations depending on species (Kulbicki 

 1988). Visual census either by diving or from submer- 

 sible might be an alternative, but these methods also 

 have numerous limitations. In particular, they do not 

 allow a quick coverage of large areas and are not usable 

 in turbid waters. If reasonably accurate density esti- 

 mates are required in a multispecies tropical trawl 

 fishery, it is likely that this can be achieved only by a 

 combination of several independent methods. 



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1984 Prawn resource management in Ihe west coast of penin 

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