BLACKBURN and WILLIAMS: DISTRIBUTION AND ECOLOGY OF SKIPJACK TUNA 



126° W I20*W 



IS-N. 



II5*V» 



ZOOPLANKTQN 

 ( ml/ 1000 m3 ) 



10^. 



<I00 



100-200 



>200 



9*N 



aru 



.urn 



8*N 



• JORDAN 

 O CROMWII.L 



s-s 



125* W 



I20*W 



.9*S 



Figure 21.-Night standing stock of zooplankton by 0.5-m net 

 (ml/1,000 m"), cruises Jordan 5T-Cromwell 51, November- 

 December 1970. 



centration and related measurements with skip- 

 jack availability for all data from the study area. 

 Table 11 gives the results of the tests using con- 

 centrations of day forage, day zooplankton, and 

 their arithmetic product. Table 12 gives similar 

 results using night concentrations. All variables 

 were transformed to logarithms in order to bring 

 distributions closer to normal, before correlation 

 coefficients were calculated. A distinction is made 

 between all skipjack and large skipjack; the latter 

 excludes skipjack < 45 cm, which seem to be a 

 separate age-group and exhibited some segrega- 

 tion from the other skipjack in space and time. 



I5*N 



I20"W 



II5«W 

 -1 — 1 1 ] i_ 



ZOOPLANKTON 

 ( ml/ 1000 m3 ) 



lO'N- 



<25 



25-50 



50-100 



(00-200 



>200 



5'N 



5'S- 



I5*N 



lO'N 



5*N 



120'* 



5*S 



II5*W 



Figure 22.-Night standing stock of zooplankton by 0.5-m net 

 (ml/ 1,000 m'), cruise Jordan 60, March-April 1971. 



Zooplankton data are from hauls of the 1-m net 

 only, as explained above. 



None of the 72 correlations in Table 11, involv- 

 ing day concentrations of forage and zooplankton 

 with skipjack, are significant. On the other hand 4 

 of the 48 correlations in Table 12, involving night 

 concentrations of forage and zooplankton with 

 skipjack, are significant by the usual criteria and 

 positive: two coefficients are above the 5% level of 

 probability and two are above the 1% level. They 

 refer only to availability of large November- 

 December skipjack measured as catch per line- 

 hour or schools per hour, in relation to night forage 

 and to the product of night forage and night 

 zooplankton, with both variables averaged over 



399 



