LAURS and LYNN: SEASONAL MIGRATION OF THUNNUS ALALUNGA 



face layer develops and warms to preferred temp- 

 eratures, initially in the Transition Zone and then 

 in more nearshore waters. It is near the end of May 

 and through June that the preferred temperature 

 range occurs in the Transition Zone and is gener- 

 ally restricted to depths <70 m. The depth limita- 

 tion of preferred waters greatly improves the vul- 

 nerability of albacore to surface trolling gear. 



BIOLOGICAL PRODUCTIVITY.— Tagging 

 data show that migration of albacore from the west- 

 ern to the eastern North Pacific is active with an 

 average migration speed of 48 km/day for 78- and 

 80-cm fish (Japanese Fisheries Agency 1975). This 

 suggests that an albacore requires considerable 

 energy to complete the transpacific migration. 

 Sharp and Dotson (1977) calculated that the 

 caloric expenditure per hour for a swimming alba- 

 core 63 cm in fork length is 5.02 kcal/h. They also 

 speculated that fat stores may be an important 

 energy source utilized by albacore for migration. 

 Studies of the food habits of albacore caught dur- 

 ing the surveys 11 show that albacore feed actively 

 in offshore waters during their shoreward migra- 

 tion. The composition of the food found in the 

 stomachs is different from that of fish caught in 

 inshore waters (Pinkas et al. 1971, Laurs and 

 Nishimoto MS 12 ), but average volumes of food in 

 stomachs from the two regions are similar. There- 

 fore, availability of forage is likely to be an impor- 

 tant factor influencing the route of albacore mi- 

 gration. 



There are three major oceanic habitats in the 

 North Pacific which are separated by pronounced 

 latitudinal faunal boundaries and steep latitudi- 

 nal gradients in standing stocks of phytoplankton 

 and zooplankton (McGowan and Williams 1973). 

 These species and biomass boundaries are coinci- 

 dent with the boundaries of the Pacific Subarctic, 

 Transition Zone, and Pacific Central waters 

 (Johnson and Brinton 1963). A northward increas- 

 ing step-cline occurs among the North Pacific 

 habitats in standing stocks of phytoplankton 

 (Venrick et al. 1973; McGowan and Williams 

 1973 ), zooplankton ( Reid 1962; McGowan and Wil- 

 liams 1973), and micronekton (Aron 1962), and in 



"Laurs, R. M., and R. N. Nishimoto. 1973. Food habits of 

 albacore caught in offshore area. In Report of joint National 

 Marine Fisheries Service-American Fishermen's Research 

 Foundation albacore studies conducted during 1973, p. 36-40. 

 (Unpubl. rep.) 



12 Laurs, R. M., and R. N. Nishimoto. Food habits of albacore in 

 the eastern North Pacific. (Unpubl. manuscr.) 



primary production ( Koblents-Mishke 1965). 

 Zooplankton and micronekton standing stock es- 

 timates made during the offshore albacore surveys 

 show similar results with values generally being 

 highest in Subarctic waters, intermediate in 

 Transition Zone waters, and lowest in Central 

 waters. 



Since biological productivity is higher in Sub- 

 arctic waters than in Transition Zone or Central 

 waters, it would be most advantageous from the 

 standpoint of food availability for albacore to 

 confine their migration path to Subarctic waters. 

 However, during spring months the temperature 

 of the Subarctic waters is much lower than the 

 habitat preference for albacore. We conclude, 

 then, that the northern limit of the albacore mi- 

 gration route during spring is determined by 

 ocean temperature and that the limiting tempera- 

 ture is found near the northern boundary of the 

 Transition Zone. The temperature of the upper 

 layer of the Central waters is higher than the 

 habitat temperature preference for albacore, but 

 there are temperatures below the upper layer 

 which lie within the habitat temperature prefer- 

 ence for albacore. Thus, temperature could restrict 

 the distribution of albacore from the upper layer 

 but not at some depth interval below the upper 

 layer. We propose that while temperature may 

 play a role in determining the southern limit of the 

 albacore distribution and migration route, the 

 major factor is the abundance and availability of 

 forage organisms which drop off sharply near the 

 southern boundary of the Transition Zone. 



OCEAN THERMAL GRADIENTS AND 

 THERMOREGULATION OF ALBACORE.— 

 Thermoregulation processes by albacore may be 

 an important factor in determining their associa- 

 tion with the Transition Zone and its frontal 

 boundaries. Thermoregulation is characteristic of 

 tunas and certain other fishes (Carey et al. 1971). 

 According to Neill ( 1976) for fishes as a group, the 

 only effective means of regulating body tempera- 

 ture is by behavioral regulation of the immediate 

 environmental temperature through locomotory 

 movements. 



Computer simulation models developed by Neill 

 (1976) indicate that where environmental condi- 

 tions are characterized by large expanses of 

 isothermal or nearly isothermal water separated 

 by relatively narrow thermal discontinuities (e.g., 

 oceanic frontal systems), fishes will be relatively 

 concentrated near the discontinuities. 



819 



