BLACKBURN and WILLIAMS: DISTRIBUTION AND ECOLOGY OF SKIPJACK TUNA 



nylon parachute cord lines. The Cromwell could 

 fish only 4 lines from the stern, compared with 11 

 fished by the Jordan, 8 from outriggers and 3 from 

 the stern. On the Jordan the 4 lines on each out- 

 rigger were connected to a set of hydraulic power 

 gurdies for rapid hauling. 



Charts and sections were contoured by hand ex- 

 cept those of dissolved oxygen content which were 

 prepared by computer and Calcomp plotter. Gear 

 and methods used for neuston and midwater trawl 

 samples are not discussed because the results are 

 not utilized in this report. The same applies to 

 observ^ations on birds, fish schools, and marine 

 mammals. 



RESULTS OF THE 

 EXPERIMENTAL FISHING 



Based on the position of the surface current 

 boundaries and distributions of temperature and 

 skipjack forage derived from Part I operations, 

 the latitudinal zones investigated in Part II 

 fishing operations were similar on the 1970 and 

 1971 cruises: 12°-14°N, 9°-ll°N, 6°-8°N, 3°-5°N, 

 1°-3°N, and 2°-4°S. Figures 3 and 4 show the 

 cruise tracks during Part II fishing operations in 

 the above-mentioned zones. They also show 

 approximate positions of surface current bound- 



aries, which were obtained from data on ther- 

 mocline topography. 



The total fishing effort (number of line-hours) in 

 or immediately adjacent to the study area was 

 very much higher on the November-December 



1970 cruise than in March-April 1971, because of 

 the curtailment of the latter cruise (Table 1). The 

 catch by species on each cruise is given in Table 2, 

 which shows the number boarded and kept, tagged 

 and released, and lost but identified, and the 

 overall size range of each species. Skipjack was 

 ob\iously the dominant species on each cruise. 



DISTRIBUTION AND RELATIVE 



ABUNDANCE OF SKIPJACK 



AND OTHER TUNA 



This section deals with results from the 1970 and 



1971 cruises. Relative abundance of skipjack was 

 calculated in terms of catch per line-hour on track. 

 Catch equals number boarded, tagged, and lost but 

 identified. Fish taken when the vessel circled 

 following an initial strike, or when chumming 

 with live ancho\y, are not included. Troll catches 

 of skipjack made on track and separated, arbi- 

 trarily, by > 10 min are considered to have come 

 from separate schools or aggregations of fish, and 

 an index of schools encountered per hour of 

 trolling has been derived. There are highly sig- 

 nificant positive correlations between catch /line- 

 hour and schools/hour for each cruise (r = +0.901 

 for data of Table 3 and -1-0.716 for Table 4, both 

 significant at the I'^c level). Schools/hour is a more 

 consenative indicator of relative abundance of 

 skipjack than catch/line-hour because each 



Table 2.-Sumniar>- of fish catch by species: Cruises Jordan ol-Cromirell 51, November-December 1970, and Jordan 60, March-April 



1971. 



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