ASSOCIATIONS OF TUNA WITH FLOTSAM IN 

 THE EASTERN TROPICAL PACIFIC 



Paul R. Greenblatt' 



ABSTRACT 



The fishing record for flotsam-associated tuna in the eastern tropical Pacific was examined. The rivers 

 of Central America are probably the major source of flotsam. Correlation analysis ofthe number of sets 

 occurring in an area indicates that unassociated tuna and flotsam-associated tuna are related. The 

 number of sets made on floating objects has increased dramatically since 1971. The percentage of 

 flotsam-associated sets has increased, indicating that flotsam-associated sets are more important to the 

 tuna fishery than in 1963. The catch per set of tuna associated with flotsam has also increased markedly 

 since 1967. Analysis of length-frequency data indicate that, on a single set basis, tuna fork length is 

 more variable in sets associated with flotsam than with unassociated schoolfish sets. Results of the 

 length-frequency analysis support the idea that flotsam aggregates tuna. 



The catch of the eastern tropical Pacific tuna 

 fishery consists of mostly yellowfin tuna, Thunnus 

 albacares, and skipjack tuna, Katsuwonus 

 pelamis. The catch is frequently categorized by the 

 conditions under which the purse seine set is 

 made. Scott 1 1969) made a major distinction be- 

 tween associated schools and unassociated 

 schools. Associated schools are caught either in 

 "porpoise sets" (sets associated with porpoise) or 

 "floating object sets" (sets associated with logs or 

 other flotsam). Unassociated schools are caught in 

 "night sets" (sets made at night with the aid of 

 bioluminescence) and "schoolfish sets" (schools 

 seen and set upon during the day). Night sets 

 compose a very small proportion of total sets and 

 will not be discussed in this paper. Porpoise sets 

 catch mostly yellowfin tuna. Floating object sets 

 and schoolfish sets catch yellowfin and skipjack 

 tuna, either as pure or mixed species. 



Little is known about the attraction of tuna to 

 flotsam. Gooding and Magnuson (1967) and 

 Hunter and Mitchell (1968) observed fish gather- 

 ing around flotsam. These authors attracted some 

 tuna to their flotsam, but never large schools. 

 Tuna were a minor portion of the observed fish 

 assemblages. Hunter and Mitchell ( 1968) postula- 

 ted a connection between schooling behavior and 

 the attraction of fish to flotsam. They concluded 

 that flotsam had the function of providing (p. 27) 

 "... a visual stimulus in an optical void." Gooding 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service. NOAA, La Jolla. Calif; present ad- 

 dress: Code A-008, Scripps Institution of Oceanographv. Univer- 

 sity of California, La Jolla, CA 92093. 



Manuscript accepted August 1978 



FISHERY BULLETIN VOL. 77, NO, 1. 1979. 



and Magnuson ( 1 967) concluded that fish gathered 

 around floating objects at sea primarily because 

 the objects provided shelter from predation. It may 

 be possible that the same factors attracting small- 

 er fish also attract large tuna schools. 



This paper examines historical tuna fishery 

 data on the catches of yellowfin and skipjack tuna 

 associated with floating objects in the eastern 

 tropical Pacific from 1963 to 1975. The objectives 

 ofthe paper are to 1 ) establish the main sources of 

 flotsam, 2) determine if there is a connection be- 

 tween various set types, 3) see if flotsam- 

 associated sets have become more important to the 

 tuna fishery, 4) determine if the catch rate on 

 flotsam-associated sets has changed, and 5) assess 

 whether flotsam does aggregate tuna schools. 



METHODS 



Since the catch of tuna associated with flotsam 

 depends on the presence of tuna, flotsam, fisher- 

 men, strength of attraction, and suitable fishing 

 conditions, I examined each factor in light ofthe 

 published literature and available fishery data 

 from the eastern tropical Pacific tuna fishery. 



The Inter- American Tropical Tuna Commission 

 ( lATTC) collects information from tuna fishermen 

 operating in the eastern tropical Pacific. Informa- 

 tion collected in logbooks includes date and loca- 

 tion of sets, catch of various species, type of set, 

 and environmental conditions. Although these 

 logbooks remain confidential, it is possible to ob- 

 tain summaries of the information for certain 

 time-area strata. During the beginning portion of 



147 



