In the western Pacific, tuna are to be 

 found along fronts, localized in cool or warm 

 eddies, and in zones of upwelling. Variations 

 in concentration and location of these tuna are 

 associated with the growth, decay, and change 

 in position of the eddies. Such eddies are found 

 to be associated with the Polar Front, Subtropi- 

 cal Convergence, and near the Equator . Individ- 

 ual eddies are probably more or less transient, 

 but may form and disappear in a regular pro- 

 gression. Certain areas may be classed as 

 "eddy -prone . " These areas vary with latitude 

 and with position of the North Pacific High. In 

 the eastern Pacific, there are very interesting 

 fronts, such as the one off Cape San Lucas at 

 the tip of Baja California, in which the tempera- 

 ture change is much more rapid than in the large 

 frontal zones described for the western Pacific. 

 While the Cape San Lucas Front is semiperma- 

 nent, many other such sharp temperature dis- 

 continuities are very transient . One explanation 

 for the aggregation of tunas in frontal zones is 

 that the tuna have converged to feed on forage 

 organisms associated with these surface 

 features . 



Skipjack and yellowfin of the eastern 

 Pacific usually occur in waters warmer than 

 21° C. Skipjack may also be excluded from 

 areas in which the temperature is greater than 

 28° C. Annual changes in the position of the 

 21° C. isotherm produce corresponding changes 

 in the extent of the fishery in the northern 

 (California) and southern (Chile) extremes . The 

 central area of the fishery (roughly latitudes 

 5° S. to 22° N.) is characterized by a shoal 

 thermocline, and skipjack and yellowfin are 

 present, with some seasonal variation, through- 

 out the year. Island groups located at some 

 distance from the mainland form other areas of 

 good tuna fishing with some degree of seasonal 

 variation. 



The reasons for the aggregation of tuna 

 near islands are not known. There is some evi- 

 dence that at Clarion Island tuna may forage on 

 detritus -feeders such as galatheid crabs, and on 

 trunk fish (Lactoria). Also near Clarion Island 

 in close inshore waters there is no significant 

 correlation between phytoplankton and zoo- 

 plankton such as is found in other areas of 

 the eastern Pacific. Near the Marquesas 

 and certain eastern Pacific islands, 

 zooplankton abundance increases with proximity 

 to the island. This may be associated with the 

 lack of a surrounding reef, which would tend to 

 filter the runoff and remove a significant por- 

 tion of the nutrients. Correlation among PO4, 

 carbon-14, zooplankton, and climax predators 

 may not be apparent unless the total standing 



crop of climax predators is included, rather 

 than that of a single species or group of species. 



When using commercial catch records 

 to evaluate productivity in a fishery it is neces- 

 sary, as was previously pointed out, to consider 

 change in fishing methods. As an example, cer- 

 tain islands and areas in the eastern Pacific are 

 not suitable purse seine grounds because of the 

 presence of sharks, strong currents, or other 

 reasons, but they are suitable for live -bait 

 fishing. 



During the summer, albacore off Oregon, 

 Washington, and British Columbia (latitudes 

 42° N. to 50° N.) are normally found in waters 

 with temperatures between 54° and 63° F. (12° 

 and 17° C), with the greatest concentration be- 

 tween 58° and 61° F. (14° and 16° C), and in 

 areas where the top of the thermocline is 50 to 

 75 feet (15 to 23 m.) below the surface. Within 

 these general limits, the distribution of albacore 

 may be associated with feed or other biological 

 determinants. Control of distribution within 

 rather broad temperature limits by factors other 

 than temperature applies not only to albacore in 

 temperate waters but also to tunas in tropical 

 waters. To the south, off California, albacore 

 are found in waters with temperatures between 

 57° and 70° F. (14° and 21° C), with albacore 

 less than 20 pounds in weight most abundant in 

 57° to 65° F. (14° to 18° C.) water, and those 

 above 20 pounds most abundant in 65° to 70° F. 

 (18° to 21° C.) water. Thus the upper distribu- 

 tional limit of albacore in 62° F. (17° C.) water 

 in the northern area merely indicates a lackof 

 warmer waters. However, reduced commercial 

 catches during years of favorable water tempera- 

 tures, 58° F. (14° C.) or above, in the northern 

 waters should not always be construed to indi- 

 cate a scarcityof albacore. Economic or mar- 

 ket conditions may act to prevent or discourage 

 the formation of a fishery when albacore may in 

 fact be abundant. A particular problemconcern- 

 ing the use of commercial catch statistics is that 

 such records often indicate only the port of land- 

 ing, not the actual catch area, which may be 

 hundreds of miles away, e.g., albacore caught 

 off California may be landed in Oregon or 

 Washington. 



The shallow thermocline mentioned above 

 may act as a physical barrier limiting the verti- 

 cal migration of albacore. It is likely that gill 

 nets and purse seines are most successful under 

 such conditions. In certain areas, as in the 

 eastern tropical Pacific, temperature may not 

 be the only barrier to vertical migration. Re- 

 duced oxygen content of thermocline waters 

 may also have an effect in inhibiting the descent 



