LENARZ and ZWEIFEL: INTERACTION BETWEEN LONGLINE AND SURFACE FISHERIES 



Table 2. — Composite catch in numbers of yellowfin tuna by gear, sex, and size. Length composition by gear is based on 

 data from Lenarz et al. ( 1974) on the Atlantic fishery. Sex composition is based on data from the Pacific (Murphy and 

 Shomura 1972). 



span and obtained results similar to Lenarz et al. 

 (1974). 



The computer program MGEAR, written by W. 

 H. Lenarz, was used to obtain estimates of yield 

 per recruit using the Ricker ( 1958) yield equation. 

 A description and listing of MGEAR is available 

 from its author. The program was slightly mod- 

 ified to calculate indices of egg production using 

 the following equation 



Ef,.,, = 0.5 (t2 - t,)N,^ {FI>^ + FI,fi -'^'. ^ '^'i'"^-''') 



where E,^,,^ = index of egg production between 

 age ^1 and t.2, 

 FI,^ = index of fecundity for age ^. , 

 A^,_ - number of females in population 



of age ^, 

 F,^ = coefficient of instantaneous fish- 

 ing mortality between age t^ and 

 age t2, and 

 Mi^ = coefficient of instantaneous 

 natural mortality between age ^i 

 and age ^2- 



For this equation it is assumed that the estimates 

 of FI are proportional to egg production per 



female, which is assumed to be continuous, and 

 that the rate of egg production is linear over the 

 interval (^i, ^2*- 



A computer program MIGR was written by J. R. 

 Zweifel to perform the calculations used for the 

 third section of this paper. Since new methodology 

 is developed, a description of the calculations will 

 be given in that section. 



AVAILABILITY OF THE STOCK(S) 



OF ATLANTIC YELLOWFIN TUNA 



TO SURFACE AND LONGLINE GEAR 



In previous works on yield per recruit, yellowfin 

 tuna of all ages in either the entire Atlantic (e.g., 

 Hayasi and Kikawa 1970; Wise 1972; Hayasi et al. 

 1972; Lenarz et al. 1974), or in the eastern Atlan- 

 tic (e.g., Fonteneau and Lenarz 1974) were as- 

 sumed to be equally available to both longline and 

 surface gear. However, since the surface fishery 

 for yellowfin tuna occurs very close to the west 

 African coast (Fox and Lenarz 1973) while the 

 longline fishery for yellowfin tuna is distributed 

 throughout the tropical Atlantic, it seems possible 

 that the longline fishery is exploiting some fish 

 that are not available to the surface fishery. It is 



809 



