FISHERY BULLETIN: VOL. 73, NO. 2 



T.'VBLE 3.-Relative abundance of troll-caught skipjack and other 

 tuna. Cruises Jordan ol-Cromwell 51, November-December 1970. 



'Data on this line are from fishing on passage, <5Q line-h. 



Table 4.— Relative abundance of troll-caught skipjack and other 

 tuna, Cruise Jordan 60, March-April 1971. Data in square 

 brackets are from fishing on passage and are included in totals 

 to the left. 



aggregation is given equal weighting irrespective 

 of the number of fish caught. The index does not 

 reflect the size of the aggregations, which on both 

 cruises were considered to be relatively small. No 

 large surface schools of skipjack or other tuna 

 were seen in the study area. 



Tables 3 and 4 show the relative abundance of 

 skipjack, yellowfin tuna and total tuna as 

 catch /line-hour and skipjack as schools/hour for 1° 

 latitudinal zones fished in Part II operations. 

 Approximate boundaries of surface current sys- 

 tems are also indicated: NEC, NECC, and SEC 

 mean North Equatorial Current, North Equatorial 

 Countercurrent, and South Equatorial Current. In 

 November-December 1970 the highest level of 

 relative abundance of skipjack was between lat. 

 0°40' and 2°N, with a secondary maximum at lat 3° 

 to 5°N and other high levels north of the NECC 

 and south of the Equator. Within the NECC at lat. 

 7° to 10°N, relative abundance in terms of catch 

 rate was variable, but generally low in terms of 



schools encountered. With the added contribution 

 of yellowfin and other tuna, there was a marked 

 maximum north of the NECC. In March-April 

 1971, overall relative abundance was much lower, 

 and the principal maximum was situated north of 

 the NECC, between lat. 9° and 11°N. Secondary 

 maxima occurred at lat. 1° to 3°N and south of the 

 Equator. The relative abundance of skipjack in the 

 NECC was again low. 



In Figures 5, 6, 7, and 8, daily values of relative 

 abundance (catch rates and schools) are plotted 

 and contoured. The results are more difficult to 

 interpret in this form, but there are some general 

 agreements with the zonally averaged data. 



Off- track Catches of Skipjack 



Off-track troll catches of skipjack were made 

 with the use of anchovy, Engraulis mordax, live 

 bait on Jordan 57. On five occasions schools of 

 skipjack were chummed with live bait, following 

 initial jig strikes (4) or fish sighting (1). On two of 

 these occasions the chumming and circling of the 

 vessel produced a substantial additional catch. Use 

 of live bait on cruises of this type is advantageous 

 in order to increase sample size of fish. 



Distribution of Skipjack by Time of Day 



Percentages of the total numbers of skipjack 

 schools encountered on track in each 1-h period 

 have been calculated and are given in Table 5. 

 Schools are defined as above. Some 60-min periods 

 included station time (i.e., not fishing), and the 

 numbers of schools per unit time have been ad- 

 justed. Variability in occurrence is considerable 

 between cruises for 1-h periods. However, when 

 presented by 2-h periods, the temporal occurrence 

 of skipjack shows remarkable similarity on the 

 two cruises. Surprisingly, fewest aggregations 

 were encountered before 1000 h. Aggregations 

 were encountered most frequently between 1200 

 and 1500 h, and again, as expected, in the predusk 

 period, 1700 to 1759 h. 



Biological Characteristics of 

 Skipjack and Other Tuna 



Size of Skipjack 



Measurements of fish length (tip of snout to tip 

 of median caudal fin rays) were made to the 

 nearest millimeter on all fish. Table 6 shows the 



388 



