than in (1). Results from the area most exten- 

 sively studied (Gulf of Tehuantepec), if extra- 

 polated to the other areas, suggest association 

 between depth ofthermocline, biological 

 productivity of the surface waters, and tuna 

 abundance. In areas of shoal thermoclines, 

 destratificationby the winds leads to enrichment 

 of the surface waters. 



(3) Offshore islands. Tuna, yellowfin 

 and/or skipjack, occur at each island or island 

 group in greater abundance than in the imme- 

 diately adjacent ocean waters. It is not clear 

 why they do so, but little oceanogr aphic work 

 has been done except at Clarion Island. Sea - 

 sonal variations in tuna abundance are compa- 

 rable with those in (2). 



Brown, Robert P. , and Kenneth Sherman 



Oceanographic observations and skip- 

 jack distribution in the North Central 

 Pacific. /Conference Paper V - \lj 



Recent investigations by the staff of the 

 Bureau of Commercial Fisheries Biological 

 Laboratory, Honolulu, have indicated that sea- 

 sonal fluctuations in availability of the com- 

 mercially important skipjack tuna, Katsuwonus 

 pelamis (Linnaeus), in Hawaiian waters are 

 related to seasonal movements of surface water 

 types within the Hawaiian region. Five cruises 

 were undertaken from January to October 1959 

 by the staff of the Laboratory within the general 

 area of the North Pacific bounded by latitude 

 15° N. and 26° N. , and longitude 150° W. and 

 170" W. , with two primary objectives: (1) To 

 delineate, by surface temperature and salinity 

 values, the boundaries between the North Pacific 

 Central, intermediate, and North Pacific Equa- 

 torial water types, and (2) to monitor the sea- 

 sonal movements of these water types and their 

 associated biota, particularly skipjack tuna. 



Background material and sources of in- 

 formation concerning oceanographic conditions 

 in the Hawaiian Islands region are given. The 

 results of the five 1959 cruises were discussed, 

 and it was shown that a reliable description of 

 the seasonal movements of the surface water 

 types within the survey region could be obtained 

 by monitoring the surface salinity of these 

 waters. Tonguelike features noted in the sur- 

 face salinity distribution were discussed, and 

 possible causal mechanisms were discussed. 

 Salinity gradients associated with the boundary 

 zones between the water types were found to 

 move seasonally with the water types and to 

 undergo a widening as the year progressed. The 

 possible cause for this was discussed. 



Available scouting data indicated that 

 skipjack sightings during January-February and 

 March- April prior to the summer appearance of 

 large fish, which comprise the bulk of seasonal 

 skipjack landings, were concentrated in the 

 boundary between the intermediate and adjacent 

 water types. However, additional observations 

 of skipjack distribution and boundary conditions 

 are needed before any definite conclusions re- 

 garding a relationship can be reached. 



The greater frequency of occurrence of 

 skipjack schools west of longitude 155° W. sug- 

 gested movement of summer "season" skipjack 

 to the island region from the west. 



Survey results showed no evident rela- 

 tion between water type and seasonal distribu- 

 tion of zooplankton. It was suggested, however, 

 that a relation may exist between the observed 

 increase in zooplankton abundance and skipjack 

 larvae during the summer months attributable 

 to a spawning periodicity of adult skipjack. 



Distribution of large predators caught 

 during longline and trolling operations and 

 aquatic mammal sightings did not appear re- 

 lated to water types. Bird flocks were usually 

 associated with fish schools. 



Clemens, Harold B. 



The distribution of California bluefin tuna 

 in the eastern North Pacific. /Con- 

 ference Paper I - \] 



California bluefin are caught by purse 

 seine from Cape San Lucas to Pt. Conception 

 primarily from May to November. The species 

 is taken mainly in coastal waters and favors 

 water with temperature -salinity characteristics 

 intermediate between the cold low-saline water 

 in which albacore occur and the warm high- 

 saline water in which yellowfin occur. In warm 

 years, the bluefin ranges considerably farther 

 north than normal. There has been a marked 

 change in size of the fish landed during the past 

 50 years, and there also are large annual fluc- 

 tuations in the catch. 



Clemens, Harold B. 



The distribution of albacore in the North 

 Pacific, /Conference Paper 1-57 



Pacific albacore are creatures of the 

 open sea. They are most abundant in the tem- 

 perate regions, where they make far-ranging 

 seasonal migrations. 



The most productive fishing grounds in 

 the eastern North Pacific are located between 

 central Baja California and the Columbia River. 



