Figure 3. — Comparison between average Hawallsin 

 skipjack landings (1950-1952) and POFI surface 

 sightings of skipjack schools in the Hawaiian 

 Islands area. 



and Kingman Reef; the Phoenix 

 Islands are Hull, Canton, 

 Sidney, Phoenix, Blrnle, 

 Enderbury, Gardner, and 

 McKean. 



For the Hawaiian Islands 

 it Is possible to examine 

 the reliability of sightings 

 as a measure of relative 

 abundance by comparing our 

 sightings of skipjack with 

 the commercial landings of 

 skipjack in Hawaii. The 

 same seasonal trends are 

 evident in both the sightings 

 and landings (fig. 3). Dis- 

 crepsincles are minor and oc- 

 cur only in the details. 

 For Instance, commercial 

 landings (expressed in 

 pounds) treble in passing 

 from the March-Kay period to 

 the June -August period, while 

 the number of schools sighted 

 only doubles. Because the 

 fishing fleet is usually 

 larger in summer and the size 

 of skipjack is also larger, 

 the greater increase in land- 

 ings than in sightings is not surprising. It is clearly evident that the relative- 

 ly sparse surface sightings from POFI vessels furnished a description of seasonal 

 variation In skipjack occurrence that is very similar to the description one gets 

 from the catching activity of a much larger number of commercial fishing vessels. 



The graphic summary of the observations made at the three principal island 

 groups (fig. 4) reveals a considerable difference in the species composition of the 

 tuna schools between the Hawaiian Islands on the one hand and the Line and Phoenix 

 islands on the other. The skipjack is by far the most cocmonly sighted tuna in the 

 Hawaiian Islands. This species also is by far the dominant one in the catch of the 

 commercial fishery (June 1951). In the Line and Phoenix islands skipjack form a 

 substantial part of the surface sightings, but they are considerably outnumbered by 

 the yellowfin (Ikehara 1953) . This difference between the Line and Hawaiian islands 

 is also found In the populations of subsurface tunas detected by longline fishing. 

 In the Line Islands, where surface yellowfin are abtandant, subsurface yellowfin are 

 also abundant (Murphy and Shomura 1953a). In the Hawaiian Islands, where surface 

 yellowfin are scarce in relation to skipjack, the subsurface yellowfin are less than 

 one-fourth as abvuidant as in the Line Islands. These differences may well be caused 

 by differences in basic productivity or by combinations of ecological factors that 

 tend to favor one or the other species of tuna in each of the localities. 



In contrast to the differences in species composition among the three island 

 areas, the general patterns of seasonal abundance are surprisingly similar (fig. 

 4). Each of them shows a peak in abundance of skipjack, yellowfin, and all species 

 combined during the period June to November. Minor differences among island groups 

 are to be expected in view of the small number of observations during some of the 

 periods in some areas. This similarity of seasonal pattern suggests that common 

 factors are operating in the three areas. It is difficult to suggest common fac- 

 tors that might affect fish abundance In all three island areas simultaneously, 

 since they are widely separated in space and are in different current systems. 

 Possibly these seasonal cycles are merely apparent changes in abundance brought 

 about by fluctuations in the bird populations, for, as has been shown, most fish 

 schools are located by birds. 



