FISHERY BULLETIN VOL 77. NO. 1 



CYRA. Although they had small numbers of sets, 

 the percentage of successful flotsam-associated 

 sets from 1962 to 1966 was 67.6'*. Changes in 

 percentage of successful sets can not adequately 

 explain the increased catch per set. 



No satisfactory explanation for the increase in 

 catch per flotsam-associated set has been found. 

 Overall increases in abundance or increased skill 

 of the fishermen can not explain the increase. The 

 above factors may account for some of the increase. 

 Technological advances may account for the in- 

 creased catch rate. It is also reasonable to believe 

 that fishermen have learned to catch flotsam- 

 associated tuna more efficiently and the residence 

 time of tuna with flotsam has increased since 

 1967. 



Changes in catch per set on flotsam-associated 

 sets may have been due to technological advances 

 such as bigger nets. If technological advances can 

 explain the increased catch per set on flotsam, 

 then either the catch per set on unassociated 

 schoolfish should have also increased or sets as- 

 sociated with flotsam prior to the technological 

 advances must have caught a low proportion of 

 potential catch. Nets have increased in size, 

 perhaps increasing the probability of catching yel- 

 lowfin and skipjack tuna which may aggregate 

 around flotsam. It is possible that bigger nets 

 could account for increased catch/set of flotsam- 

 associated sets without likewise affecting catch/ 

 set on unassociated schoolfish sets. 



Fishermen often will drift with logs for consid- 

 erable time, waiting for tuna aggregations to 

 reach an optimal size before setting the net. The 

 spread of such behavior throughout the fleet could 

 cause the overall catch per set of flotsam- 

 associated tuna to increase. Adequate data for 

 testing this "increased knowledge" hypothesis 

 was unavailable. 



The marked changes occurring in flotsam- 

 associated tuna catch in 1963-75 coincided with a 

 large increase of effort and technology in the 

 porpoise-associated fishery (Green et al. 1971). It 

 is hypothesized that the increased effort and 

 technology in the porpoise-associated fishery may 

 have been related to changes in the catch rate of 

 tuna schools associated with flotsam. 



When purse seiners set on porpoise, there is 

 often an incidental kill of the marine mammals. 

 Due to recent technological advances, the porpoise 

 kill has been reduced, but in earlier years of the 

 porpoise-associated fishery (the mid-1960's) por- 

 poise mortality was higher (Southwest Fisheries 



Center^'). This incidental kill may have reduced 

 the porpoise population. The porpoise-associated 

 fishery first developed near shore and thus the 

 nearshore porpoise stocks have been affected for a 

 longer time than offshore stocks. One may reason- 

 ably speculate that, on a species basis, nearshore 

 porpoise stocks have been affected more by inci- 

 dental kills than offshore porpoise stocks. 



The bond between tuna and porpoise is not un- 

 derstood. It is possible that the mechanisms in- 

 volved in the association of tuna with porpoise is 

 similar to those responsible for their association 

 with flotsam. Tuna associated with flotsam are, on 

 the average, smaller than tuna associated with 

 porpoise (Calkins 1965, tables 2 and 9; Sharp''). 

 Knudsen (1977) gave some evidence that tuna 

 caught in areas where porpoise fishing predomi- 

 nates were generally older and larger than in tra- 

 ditional schoolfish areas. Size overlap, however, 

 did occur (Calkins 1965). Assuming that the 

 number of porpoise schools have declined, the 

 probability of tuna encountering porpoise schools 

 has decreased. The probability of tuna aggregated 

 near flotsam encountering porpoise schools has 

 also decreased. Thus, as a result of decreased en- 

 counter rates with porpoise (slower transition 

 from flotsam to porpoise), the size of the aggrega- 

 tions of tuna near flotsam have increased. 



In conclusion, the most likely sources of flotsam 

 are the large rivers of Central America. Indirect 

 evidence indicates that tuna caught in unas- 

 sociated schoolfish sets are from the same popula- 

 tion as tuna caught associated with flotsam. It 

 appears that the increase of flotsam-associated 

 sets from 1963 to 1975 was due to an increased 

 interest by fishermen and hence an increased 

 fishing effort on floating objects. The observed in- 

 crease in catch per set may have been a biological 

 change rather than a change in fishing technology 

 or skill. 



ACKNOWLEDGMENTS 



I would like to express my thanks to John 

 Hunter who provided guidance in several phases 

 of this study. His comments were extremely help- 



"Southwest Fisheries Center. 1976. Report of the Work- 

 shop on Stock Assessment of Porpoises Involved in the Eastern 

 Tropical Pacific Yellowfin Tuna Fishery. SWFC Adm Rep, 

 LJ-76-29, 54 p. Southwest Fisheries Center. La Jolla, CA 

 92038. 



"G. Sharp, Inter-American Tropical Tuna Commission. 

 Southwest Fisheries Center. La Jolla, CA 92038, pers. commun 

 April 1977. 



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