FISHERY BULLETIN: \0L. 70, NO. 3 



bility to capture existed among the various 

 school types as well as between the three species. 



MATERIALS AND METHODS 



In order to describe the schooling behavior of 

 T. thymius in the eastern North Pacific and to 

 relate this behavior to percentage of successful 

 sets and catch per successful purse-seine set, 

 California fishermen's logbooks were analyzed 

 for the period 1960-67. Abstracts of these logs 

 were made available through the courtesy of the 

 Inter-American Tropical Tuna Commission. 



A purse-seine set is defined as that operation 

 in which the net is laid out around a school of 

 fish and the bottom of the net drawn together, 

 capturing the fish, which are then transferred 

 to the vessel (Orange, Schaefer, and Larmie, 

 1957) . A complete description of the fishing op- 

 eration is given by McNeely (1961). For the 

 purposes of this paper the term "school" will 

 apply to that quantity of fish captured in a suc- 

 cessful set of the net. No assumptions as to the 

 configuration of the school nor the orientation 

 of the fish within the school will be made (see 

 Williams, 1964; Breder, 1967). 



Orange, Schaefer, and Larmie (1957) made 

 several assumptions in analyzing single set data 

 from fishermen's logbooks, and other workers 

 have followed (Broadhead and Orange, 1960; 

 Whitney, 1969). These assumptions are: 



1. A set of the net is made on a single school 

 of fish. 



2. Either the entire school is captured or each 

 set captures a constant fraction of the 

 school upon which it is made. 



3. Vessel masters can estimate accurately the 

 tonnage from individual sets of the net. 



In addition, it is assumed that the schooling be- 

 havior described in ship's logs indicated the 

 school type evident when the fish were first ob- 

 served. 



The average size of all schools may actually 

 be smaller than those cited by the fishermen be- 

 cause small schools of 2 tons or less may be 

 passed over by the fishermen in the hope of cap- 



turing a larger school later in the day. 



In regard to the second assumption, fishermen 

 generally agree that it is extremely difficult to 

 "cut" a tuna school with their nets; it appears 

 to be an all-or-nothing situation. Typically a 

 fisherman will catch ^4 ton or less when the 

 school is missed. These fish usually are en- 

 trapped in net folds during the pursing oper- 

 ation and are unable to escape with the main 

 body of the school being set upon. The constant 

 fraction, therefore, approaches zero. That the 

 second assumption does not hold in every case 

 was recognized by the original proponents 

 (Orange, Schaefer, and Larmie, 1957) . We have 

 assumed also that the fraction of fish retained 

 in the net from a school escaping capture is the 

 same for all school types, as well as for all times 

 of day; these are factors which need additional 

 study. 



Fishermen identify schools to species with con- 

 siderable skill, but the system of identification 

 is difficult to describe. Their ability to judge 

 tonnage, however, is very good after the fish 

 are inside the net and in full view of the mast- 

 man. The airborne spotters are extremely good 

 at estimating school size. 



Bluefin net sets were defined to be: 1) all 

 sets in which 90 "^r or more of the total tonnage 

 landed was T. thynnus, 2) all no-catch sets in 

 which T. thynmis was clearly identified as the 

 pursued species, and 3) all sets in which it could 

 be determined from location, water temperature, 

 date, time, and previous and later sets by the 

 same or other boats operating nearby that it 

 was T. thynnus being sought and/or captured. 

 Using the above criteria, we determined that a 

 minimum of 8,059 sets were made on T. thynnus 

 and 65,478 tons landed by the eastern North 

 Pacific high seas purse-seine fleet during the 

 period 1960-67. Of these sets, 43.9 9r were ident- 

 ified in the logbooks as to school type. 



All weight references in this paper are ex- 

 pressed in short ton units. 



SCHOOLING BEHAVIOR 



The terminology of school types used in this 

 paper is that of Scott (1969) (Table 1). T. 

 thynnus exhibited 13 diflferent types of schooling 



916 



