LENARZ and ZWEIFEL: INTERACTION BETWEEN LONGLINE AND SURFACE FISHERIES 



cruitment, recruits are divided equally among the 

 five cells 51, 52, 69, 83, and 84, which resemble the 

 recruitment areas proposed by Fink and Bayliff 

 (1970). For the other alternative, denoted as uni- 

 form recruitment, recruits are divided equally 

 among all cells except those on the boundaries or 

 on land. Total annual recruitment is 100 fish. We 

 assumed 1 ) that fish are 1 yr old when recruited, 2 ) 

 growth proceeds according to the von Bertalanffy 

 curve of LeGuen and Sakagawa (1973), and 3) the 

 coefficient of instantaneous natural mortality is 

 0.8 on annual basis and is independent of time and 

 location. Fish >6 yr old (175 cm) were removed 

 from the system. Consequently, under constant 

 conditions the fishery reaches equilibrium in 5 yr. 

 The system was always run for 5 yr before an 

 experiment was begun. 



We first examined the effects of sampling loca- 

 tion, dispersal, and location of recruitment on age 

 distribution and the resulting apparent rate of 

 natural mortality obtained from unbiased sam- 

 ples from an unfished population. Mortality was 

 estimated with the standard linear regression 

 model {\n Nt = In Nq - Mt) from the age distribu- 

 tion offish in each cell. It is assumed that mortal- 



ity is constant after full recruitment, and that the 

 modal age represents first age of full recruitment. 

 The results reveal that M is usually overestimated 

 as would be expected when fish emmigrate from a 

 sampled area (Figure 15). Estimates of M tend to 

 be relatively high near areas of spawning with 

 inshore recruitment. In the case of uniform re- 

 cruitment, estimates of M tend to be highest on the 

 western boundary where fish are lost to the sys- 

 tem. Modal age tends to increase in a westerly 

 direction for inshore recruitment and stay rela- 

 tively constant for uniform recruitment (Figure 

 15). The modal size of actual catches of surface- 

 caught yellowfin tuna in the eastern Pacific in- 

 creases in a westerly direction (Figure 16). Al- 

 though the surface fishery probably does not take 

 an unbiased sample of the size distribution of the 

 population, the data are suggestive of reduced re- 

 cruitment in the western areas. 



We simulated a 20-yr hypothetical yellowfin 

 tuna fishery to examine interactions among a 

 longline fishery, inshore surface fishery, ocean- 

 wide surface fishery, and ocean-wide surface 

 fishery that does not heavily exploit young fish as 

 follows: 



0--0- -0--0-- 



. MODAL AGE 



ROW 



ROW 



COLUMN 



COLUMN 



Figure 15. — Estimates of coefficient of instantaneous natural mortality on annual basis ( M) and modal age 

 of yellowfin tuna by row and column: (A) inshore recruitment, and (B) uniform recruitment. 



819 



