FISHERY BULLETIN: VOL. 79. NO. 4 



Figure 4. — Annual 1972-79 catch and catch per unit effort data 

 for three species of thread herring captured in the Golfo Dulce 

 area of Costa Rica ( zone 3 ) . Catch and effort were compiled from 

 captains reports and verified from sales receipts. 



tion is based on the relation between the number 

 of gill rakers and standard length (Berry and Bar- 

 rett 1963). Over 3,000 specimens of thread herring 

 collected from commercial catches by the Costa 

 Rican Ministry of Agriculture during the period 

 April 1975 to December 1977 were separated into 

 two groups based on external coloration and mor- 

 phological features. One group ("blue sardines") 

 consisted offish which more easily lost their scales 

 and which were characterized by a blue dorsal 

 coloration and more shallow, elongated bodies. 

 These fish were predominantly O. hulleri. The sec- 

 ond group ("green sardines") consisted primarily 

 of O. medirastre and O. libertate. Fish in this group 

 retained their scales, had deeper bodies, and a 

 greener dorsal coloration. The second group was 

 more abundant than the first, accounting for more 

 than 907^ of the fish sampled from commercial 

 landings except during February -July 1977 when 

 blue sardines made up 25-63% of the samples. 

 Since the sampling procedures used were not 

 strictly random and exact species identifications 

 were not attempted, it can only be concluded that 

 O. medirastre and O. libertate appeared to be the 

 predominant species in the catch during most of 

 the sampling period. 



In the absence of species-specific catch and effort 

 information all three species were, for the pur- 

 poses of analysis, assumed to compose a single unit 

 stock confined to Costa Rican coastal waters. The 

 degree to which the different species shared com- 

 mon growth or natural mortality rates was not 

 knowm, nor was it certain whether they school 

 together and were thus exploited with the same 

 intensity in the same areas. Although commercial 

 fishermen in Costa Rica have observed that O. 

 libertate and O. medirastre intermingle in the 



same schools, O. bulleri are rarely captured to- 

 gether with the other two species and apparently 

 do not school with them. Berry and Barrett (1963) 

 reported that the size ranges of all three coastal 

 Pacific species collected in Central and South 

 America were nearly equal, an observation that 

 was generally confirmed in samples taken from 

 commercial catches in Costa Rica and which indi- 

 cated that all three species were equally suscepti- 

 ble to capture over the same size range. All three 

 species harvested in Costa Rica during 1975-77 

 reached a maximum size of about 24 cm SL. 



The assumption of a single geographic stock in 

 Costa Rican waters was largely an arbitrary one 

 since there was no information available concern- 

 ing the migratory behavior of the Pacific species. 

 Klima (1971) reported that Atlantic thread her- 

 ring, O. oglinum, in the northeastern Gulf of 

 Mexico seldom are found in depths >100 m and are 

 most common in depths <40 m. Similar observa- 

 tions have been reported for Pacific thread herring 

 in Central America (Magnusson 1971). Kinnear 

 and Fuss (1971) reported seasonal north-south mi- 

 grations of O. oglinum on the west coast of Florida 

 in response to changes in surface water tempera- 

 tures. A southerly fall migration of 6-7 mi/d has 

 been observed for O. oglinum between North 

 Carolina and Florida (Pristas and Cheek 1973). 

 Since seawater temperatures along the Pacific 

 coast of Central America presumably are more 

 constant, such directed migration may not occur. 



Despite the fact that the three coastal Pacific 

 species of Opisthonema are distributed continu- 

 ously from the Gulf of California to Peru ( Berry 

 and Barrett 1963), there is some justification for 

 treating the Costa Rican thread herring resource 

 as a single geographic unit stock since there are no 

 fisheries for thread herring in any of the other 

 Central American countries except Panama. In 

 Panama, Opisthonema spp. are harvested only in 

 the Gulf of Panama and not in the intervening 

 coastal waters of western Panama between the 

 Gulf and the Costa Rican border. 



METHODS 



Estimation of Catch and Effort Statistics 



Total annual catch and effort data were com- 

 piled from sales receipts obtained from the can- 

 ning companies in Puntarenas and Golfito (Table 

 2). Catch data were reliable since the entire catch 

 in Costa Rican waters was harvested by Costa 



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