INGHAM ET AL: OXYCLINE AND SKIPJACK TUNA DISTRIBUTION 

 5°E 10° 15° 



5°N 



o c 



— 5°S 



— I0< 



20° 



FIGURE 9.— Depth (meters) to the 26.0 g/1 a t surface from Un- 

 daunted data, February-April 1968. 



best be demonstrated with the data collected in 

 October-November, involving 49 sightings with 

 relevant oxygen data. After grouping the oxycline 

 depth measurements into 5-m classes and plotting 

 a sighting versus depth-frequency bar graph (Fig- 

 ure 12), it appears that a smooth inverse relation- 

 ship exists for depths >10 m. By assigning the 

 central value of each depth class to each sighting 

 in the class, a least squares linear "equation" can 

 be obtained for sighting frequency as a function of 

 oxycline depth in the form: 



s = a + mz 



(1) 



where 



s 

 z 



= the number of sightings 

 = the depth of the 3.5 ml/1 surface 

 a and m = constants, in this case, equal to 

 23.15 and -0.59, respectively, 

 leading to 



5°E 



I5 e 



5°N 



5°S 



10' 



I5 C 



20° 



FIGURE 10.— Depth (meters) to the 26.0 g/1 a, surface from 

 Undaunted and Rockaway data, October-November 1968. 



s = 23.15 - 0.59z 



(2) 



as the "equation." Note that the equation is 

 defined only over the range of depths from 1 1 to 40 

 m. At depths greater than this, school sightings 

 may be difficult to make and at depths less than 

 this the fish may avoid the thin habitable layer. 



Although the relationship portrayed in the bar 

 graph appears to be nonlinear, the errors intro- 

 duced by interpolation between sampling bottle 

 depths and the arbitrary assignment of central 

 values to the frequency classes make any attempts 

 to obtain a best-fit, nonlinear "equation" unwar- 

 ranted. The linear relationship shown above is 

 about all the sophistication the data will bear, 

 particularly in view of the small number of fish 

 school sightings. 



To further pursue the role of environmental 

 conditions in influencing the distribution of skip- 

 jack tuna, we considered the concept of habit layer 



863 



