SECKEL: SKIPJACK AND ENVIRONMENT 



AS = VM. 



The velocity of the fish school, V, consists of two 

 parts: the velocity of the water relative to 

 fixed coordinates, Vw, and the velocity of the 

 school relative to the water, Vf. The displace- 

 ment equation is 



AS = (Fw + Vf) M. 



The velocities are functions of location and of 

 time and the net displacement of a school after 

 a time T is 



S = ^ l(Vw + VF)M]i 



or 



S ^ Sw + Sf ^ -^ iVwM)i + ^ (VFM)i 



i=l i=l 



with T — nAt if the time increments are all equal. 

 Although this displacement equation is true 

 for any time and space scale, current or swim- 

 ming velocities averaged over time intervals. At, 

 of a week or a month are of interest here. Ran- 

 dom motions of fish schools and eddying currents 

 therefore make no contribution to the net dis- 

 placement in the scales under consideration. It 

 is evident that in migrations of thousands of 

 kilometers taking a time of 1 to 2 years, ocean 

 currents cannot be ignored unless 



Vw << Vf 



and the displacement of the school due to the 

 current, Sw, is therefore small compared to that 

 relative to the water. 



If the environmental conditions are known, 

 then a numerical integration can be performed 

 to determine the displacement of a fish school 



o 



3 

 I- 



I 

 \- 



o 



1.0 



.5 

 DYNES CM 



-2 



Figure 8. — Schematic presentation of average zonal 

 wind stress, long 120° to 160°W, in latitude bands 10°- 

 15°N, 15°-20°N, 20°-25°N, and associated meridional 

 component of wind-driven current in miles per month 

 (thick arrows). 



caused by currents alone. An example of a drift 

 model that is applicable to skipjack originating 

 in the eastern North Pacific will be given in a 

 subsequent section. 



Table 5. — Skipjack tagged in the eastern Pacific and recaptured in the 



central Pacific. 



Data source: Dr. William H. Bayliff, Inter-American Tropical Tuna Commission. 



773 



