AbStfclCt. Controversy concern- 

 ing the validity and accuracy of recent 

 assessment results for giant bluefin 

 tuna, Thunnus thynnus, led us to ex- 

 amine alternative methods of estimat- 

 ing their abundance. In collaboration 

 with a New England giant bluefin tuna 

 industry group, we tested the feasibil- 

 ity of using commercial spotter pilots 

 and aerial photography as a means of 

 obtaining tuna counts in the Gulf of 

 Maine and adjacent New England wa- 

 ters. Nine commercial spotter pilots 

 photographed a total of 126 schools of 

 bluefin tuna during the summer of 

 1993, representing 13,973 fish, with a 

 maximum one-day count of 4,894. 

 Three spotter pilots contributed nearly 

 70% of the total photographic effort. 

 Differences in photographic ability and 

 commercial involvement in the fishery 

 appeared to influence spotter pilot par- 

 ticipation. Aerial photographic surveys 

 may provide a means of obtaining area- 

 specific minimum abundance and dis- 

 tribution data for giant and large-me- 

 dium bluefin tuna. 



The feasibility of direct photographic 

 assessment of giant bluefin tuna, 

 Thunnus thynnus, 

 in New England waters 



Molly Lutcavage 

 Scott Kraus 



Edgerton Research Laboratory, New England Aquarium 

 195 State Street. Boston, Massachusetts 02109 



Manuscript accepted 31 January 1995. 

 Fishery Bulletin 93:495-503 (1995). 



Stock assessments of the highly 

 migratory northern Atlantic bluefin 

 tuna, Thunnus thynnus, are prima- 

 rily based on age-structured and 

 lumped biomass models derived 

 from landings data and various 

 abundance indices (Scott et al., 

 1993). These production or CPUE 

 (catch per unit of effort) models pro- 

 vide the framework for interna- 

 tional management of the commer- 

 cially valuable bluefin tuna. A prob- 

 lem with CPUE-derived estimates 

 of stock biomass, however, is that 

 they are affected by changes in fish- 

 ing effort, technology, fish density, 

 and the marketplace (Lo et al., 

 1992). How accurately recent as- 

 sessment models portray seasonal 

 bluefin abundance in the west At- 

 lantic, a fishery with a 1993 catch 

 of 1,047.2 metric tons of giant tuna 

 (>77 inches straight fork length 

 [SFL]) and of large-medium cat- 

 egory tuna (between 70 and 77 

 inches SFL), remains controversial 

 (Clay, 1991; Suzuki and Ishizuka, 

 1991; Safina, 1993). 



Aerial surveys have been used to 

 obtain relative indices of abundance 

 in fisheries worldwide, including 

 northern anchovy (Engraulis mor- 

 dax), jack mackerel (Trachurus 

 symmetricus), menhaden (Breuoor- 

 tia spp.), mullet (Mugil spp.), and 

 other pelagic fishes (Squire, 1961, 

 1972, 1993; Williams, 1981; Scott et 



al., 1989; Lo et al., 1992). Estimates 

 of fish biomass from these surveys 

 are based on appraisals of school 

 size in tonnage per unit of area, or 

 by size of remotely detected signals 

 such as bioluminescence or turbid- 

 ity fields. Visual biomass estimates 

 from aerial survey data are rela- 

 tively easy to construct but are dif- 

 ficult to interpret without ground 

 truth or information on surfacing 

 behavior (Lo et al., 1993). 



In contrast, direct enumeration of 

 pelagic fish is notoriously difficult. 

 In the 1950's, U.S. fisheries scien- 

 tists attempted to count giant blue- 

 fin tuna migrating along the Bahama 

 Banks (Rivas, 1978) and later un- 

 dertook direct assessment with pho- 

 tographic and video techniques 

 (NMFS 1 ). Despite dedicated search 

 time, very few fish were detected 

 during the survey. More recently, 

 other countries have explored aerial 

 and remote sensing methods as a 

 means of calibrating catch-related 

 indices for tuna species. Since 1990, 

 transect surveys have provided es- 

 timates of regional abundance and 

 recruitment indices for southern 

 bluefin tuna, Thunnus maccoyii, in 



1 Anonymous. 1975. A study of the applica- 

 tion of remote sensing techniques for de- 

 tection and enumeration of giant bluefin 

 tuna. Rep of Southeast Fish. Sci. Cent., 

 Natl. Mar. Fish. Serv., NOAA. Contrib. 

 No. 437. 



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