INGHAM ET AL: OXYCLINE AND SKIPJACK TUNA DISTRIBUTION 



developed by Barkley et al. (see footnote 4). They 

 defined the habitat of adult skipjack to be bounded 

 above by the sea surface or 22°-26°C (for 9- to 4-kg 

 fish) and below by 18°C or 3.5 ml/1 oxygen concen- 

 tration, whichever is shallower. We plotted the 

 skipjack tuna school sightings on a horizontal 

 chart of habitat layer thickness ( using 24 °C ) for the 

 October-November cruise period (Figure 13). The 

 distribution of school sightings at various habitat 

 layer thicknesses (Figure 14) is considerably dif- 

 ferent from that at various oxycline depths. Many 

 points in the school sightings versus oxycline 

 depth plot (Figure 12) have shifted to shallower 

 classes in the school sighting versus habitat 

 thickness plot, including seven observations in 

 habitat thicknesses of 5 m or less. This shift is the 

 consequence of regarding the 18 °C isothermal 

 surface as the floor of the habitat when it is shal- 

 lower than the oxycline and assuming that it has a 

 constraining effect equal to that of the 3.5 ml/1 

 oxygen surface. The validity of this assumption is 

 unknown, but comparison of the two distributions 

 (Figures 11, 13) suggest that the 3.5 ml/1 oxygen 

 surface has a stronger effect on the skipjack tuna 

 than the 18°C isothermal surface. 



The question of whether it is school distribution 

 or availability (to a fishing method) which has 

 been related to oxycline depth cannot be resolved 

 without an independent assessment of tuna school 

 distribution by a different method. The means 

 used to locate tuna schools is essentially that 

 employed by crews of purse seiners and live-bait 

 boats; a watch is maintained for bird activity 

 above feeding or "breezing" schools. This 

 technique reveals only those schools which are 

 available to seines or pole-and-line fishing 

 methods, hence it would be more accurate to con- 

 sider the factor portrayed in Equation (2) as avail- 

 ability rather than distribution. Those fish not 

 closely approaching the surface would not be de- 

 tected and would not be available to these harvest- 

 ing methods. 



The pragmatic significance of the relationship 

 between skipjack tuna school availability and 

 oxycline depth lies in its use by fishermen and 

 fishery scientists, the former for more efficient 

 harvest strategy and the latter for more accurate 

 resource assessment. The coincidence of the oxy- 

 cline and thermocline should provide a very strong 

 lower barrier to downward excursions of tropical 

 tunas, perhaps even strong enough to prevent an 

 encircled school from escaping by sounding before 



the seine is pursed. If this were true, the efficiency 

 of capture by purse seine would be greater in wa- 

 ters containing a shallow oxycline. 



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