Catch Efficiencies of Purse and Beach Seines in Ivory 



Coast Lagoons 



Emmanuel Charles-Dominique 



ABSTRACT: Catch efficiencies of two commonly 

 used fishing gears in Ivory Coast lagoons, purse 

 seine and beach seine, were studied. Only fish 

 larger than the 100% mesh retention size (Lion) 

 were considered. Escapement was estimated from 

 the retention rates of marked fish released within 

 the closed seines in shallow waters. Simple and 

 reliable upper estimates of the catch efficiency 

 were provided by these retention rates, which 

 ranged from 10% to 79% for the purse seine and 

 from 35% to .53% for the beach seine depending on 

 the species. The purse seine efficiency was esti- 

 mated by performing 25 sets (one set covering 0.72 

 ha) inside the closed beach seine (covering 9.4 ha), 

 on both marked and unmarked populations of the 

 bagrid Chrysichthys spp. and the cichlid Tilapia 

 guineensis. The efficiency was close to 15% for the 

 species considered, but this estimate may be sensi- 

 tive to experimental bias (marking and "enclosure" 

 effects). Avoidance, calculated for the purse seine 

 from escapement and efficiency, appears to be con- 

 siderable. Comparison of the catch rates by the two 

 gears showed interspecies selectivity ("species se- 

 lectivity") and intraspecies selectivity ("size-selec- 

 tivity", regardless of mesh size). An understanding 

 of both types of selectivity appears to be essential 

 for an interpretation of the catch rates. 



Artisanal fisheries are well developed in Ivory 

 Coast lagoons, yielding from 10,000 to 20,000 

 tons of commercially valuable fish per year. Var- 

 ious stock assessment progi'ams have been ini- 

 tiated at the Abidjan C.R.O. (Centre de 

 Recherches Oceanographiques) for fisheries 

 management purposes during the last few years, 

 but more direct methods of estimating fish abun- 

 dance from catch rates are needed. The catch 

 rates, which can be considered as relative abun- 

 dance indices, can be converted into absolute 

 abundance measurements, if the efficiency of 

 fishing gear is known or can be estimated 

 (Beverton and Holt 1956). Unfortunately, this is 



Emmanuel Charles-Dominique, Institut Francais de 

 Recherche pour le Developpement en Cooperation 

 (ORSTOM), 2051 Avenue du Val de Montferrand, BP 5045, 

 34032 Montpelher Cedex 1. France. 



always a difficult process because efficiency de- 

 pends upon various factors such as the behavior 

 of fish, the environmental conditions (depth, na- 

 ture of the bottom, etc.), and the physical char- 

 acteristics of each particular gear. Previous 

 studies have mostly dealt with towed gears such 

 as trawls and plankton nets (Barkley 1972; Kjel- 

 son and Colby 1977; Merdinyan et al. 1979). The 

 efficiency of these gears appeared to be a func- 

 tion of the active avoidance rate by fish, and 

 models can be designed to explain, at least par- 

 tially, the process (Barkley 1964). Measures of 

 gear efficiency have been based on the "swept- 

 area method" (Beverton and Holt 1956). Mark- 

 recapture experiments have been made in well- 

 defined areas, and the recapture rate has been 

 compared with the ratio between the area swept 

 by the gear and the area where the marked fish 

 have been distributed (Kuipers 1975; Loesch et 

 al. 1976; Watson 1976; Kjelson and Johnson 

 1978). Unlike trawl nets, surrounding nets and 

 beach seines have rarely been studied in terms of 

 efficiency, except the Danish seine (Hemmings 

 1973), which can be compared with a beach 

 seine. For the surrounding-type gears, there 

 are two different phases: 1) the shooting of the 

 net, during which active avoidance takes place, 

 and 2) the hauling of the net on board once the 

 circle is closed, during which escapement can 

 occur. 



In this report, an experiment, based on an 

 estimation of catchability according to the areas 

 swept by the gears, in conjunction with a mark- 

 recapture procedure, is described. This study 

 was designed to better understanding multi- 

 species catch-rates and to provide estimations of 

 catchability for stock assessment. 



MATERIAL AND METHODS 



The two main fishing methods used in the 

 Ivory Coast lagoons are the beach seine without 

 a bag for shallow waters (about 1,200 m long) 

 and the purse seine for depths of 2 m or more 

 (ranging from 300 to 500 m long). Both gears 

 reach the bottom and catch pelagic species as 



Manuscript accepted June 1989. 

 Fishery Bulletin, U.S. 87: 911-921. 



911 



