FISHERY BULLETIN: VOL. 70, NO. 3 



more birds had been available. Such pelagic 

 populations as these must represent a vast un- 

 tapped resource, judging from the 5,000 tons or 

 so taken annually in a limited area around the 

 Hawaiian Islands (June, 1951). 



The distribution of oceanic schools is at var- 

 iance with the distribution of deep-swimming 

 yellowfin tuna (Figure 38B), the most striking 

 differences being the high density of schools in 

 the Countercurrent (lat 5°-10°N) and south of 

 the equator. In fact, the distribution of surface 

 schools is almost the inverse of the distribution 

 of deep-swimming tunas. This apparent inverse 

 relation poses a serious problem, for a priori we 

 would expect surface schools to respond to the 

 equatorial enrichment in the same manner as 

 the deep-swimming tunas. 



The problem can be approached by considering 

 the environmental factors that make it possible 

 for an area to support large numbers of surface 

 schools. The obvious one that comes to mind 

 is the standing crop of forage, as indicated by 

 zooplankton. We have already indicated, how- 

 ever, that zooplankton tends to peak at the equat- 

 or (Figure 25), and thus food supply as indi- 

 cated by the standing crop of zooplankton does 

 not account for the north-south distribution of 

 surface fish. In a possibly parallel situation. 

 King and Hida (1954) found little or no more 

 zooplankton around the Hawaiian Islands than 

 in the North Pacific Current and were unable 

 to account for the population of skipjack tuna 

 around Hawaii on the basis of the standing crop 

 of zooplankton. 



A suggestion of an important environmental 

 factor controlling the abundance of skipjack tuna 

 was given by Robins (1952), who found a posi- 

 tive association between skipjack tuna troll 

 catches and temperature discontinuities or 

 "fronts." The circulation associated with fronts 

 involves horizontal convergence and sinking, ac- 

 cording to Cromwell ( 1956) . This type of move- 

 ment could act to concentrate organisms, partic- 

 ularly those that float or can actively resist 

 sinking. Fronts might act as modifiers of the 

 average standing crop by altering its distribu- 

 tion in space so that fishes can more effectively 

 forage. 



The effect of fronts on the distribution of for- 



age might account for the abundance of schools 

 near islands. An island mass in a moving ocean 

 current must set up patterns of "frontlike" circ- 

 ulation cells and eddies, as has been shown for 

 the Hawaiian Islands (McGary, 1955). These 

 features, although perhaps not increasing the 

 basic supply of food, should concentrate some 

 of it so that fish can feed more efficiently. 



In an effort to determine whether this same 

 mechanism could be offered as an explanation 

 of the irregular distribution of surface schools 

 in the equatorial region (Figure 38), we deter- 

 mined the frequency of temperature irregular- 

 ities at the sea surface, confining ourselves to the 

 best-sampled zones, i.e., lat 5°S to 10°N, long 

 140° to 170 °W. The frequency of occurrence of 

 temperature irregularities or "fronts" was esti- 

 mated from thermograph traces made while the 

 ships were crossing the equatorial system in a 

 north-south direction. An irregularity was 

 simply defined as a temperature change that was 

 completed during 15 min or less when the ships 

 traveled at a speed of about 8 knots, i.e., long, 

 even temperature clines were not considered. If 

 the temperature rose (or fell) quickly to a new 

 level, this was counted as one "front." If it both 

 rose and fell during any interval, it was counted 

 as two "fronts," providing the rise and the fall 

 each took no more than 15 min. 



An inquiry can logically be made into the re- 

 lation of wind velocity to the number of such 

 fronts, for on calm, hot days there may be many 

 irregularities caused by convection currents set 

 up by diurnal heating. A plot of wind force 

 against the number of fronts in unit time, how- 

 ever (Figure 39), suggests that we are dealing 

 with phenomena that are independent of the im- 

 mediate wind stress and heating. 



When we compare the rate of crossing fronts 

 with the rate of sighting surface schools, we find 

 a remarkable correspondence (Figure 40), en- 

 abling us to advance the same explanation for 

 differences in abundance of surface schools 

 among different areas of the open ocean and be- 

 tween the vicinity of islands and the open ocean. 

 The responsible feature of the environment, the 

 "front," meets the test of biological and physical 

 logic, for temperature fronts are symptomatic of 

 horizontal movement of water toward the dis- 



906 



