3,000 



2300 



2f00 



2,400 



2^00 



ZpOO 



1,800 



1,600 



1,400 



1,200 



1,000 



800 



600 



400 



200 







1961 

 1962- 

 1963- 

 1964 

 9-Yeor Meon- 



SEASONAL 

 MEANS 



JULY AUG 



SEPT, 



OCT 



Fir.tnF. 16. — Monthly averages of catch per unit of cfTort 

 (pounds of albacore per boat-day of fishing) for the 

 area north of hit. 40° N., cast of long. 130° W. Data 

 from Aycrs and .Mci'lian (1963) and James Meeh.an 

 (personal communicatioti) for effort exceeding 5 days 

 per 1° .square in the month. 



catch rates for the commercial fishery in the 

 phime jjrovince, althotio;li imperfect, indicates that 

 the catch rates for exploratory fishing are adetjiiate 

 for use as ratios in the following discussion. 



Still another factor is salinity. Nowhere over 

 their luiown distribution do migrating albacore 

 encounter a greater range of salinity than in the 

 region off Oregon and Washington. Albacore thus 

 may respond not only to temperature variation, 

 as described above, but also to variation of salinity 

 in the plume — for example through its effect tipon 

 balance of osmotic pressure. This balance, the 

 difference between the organism's internal and 

 environiuental osmotic pressure (both referred to 

 pure water at 0° C), was studied by Sakamoto 

 (1962) as a mechanism for determining movements 

 of pelagic fishes and was applied to the Japanese 

 fishery for yellowtail (Seriola quinqueradiata, 

 Temminck and Schlegel). 



Albacore, like other teleosts, must maintain 

 their internal osmotic pressure at a nearly con- 

 stant level so that pressure balance is a function 

 only of environmental osmotic pressure. Rewritten 

 from equations of Thompson (1932) and of Lyman 

 and Fleming (1940) in terms of salinity and tem- 

 perature, the imified expression for osmotic pres- 

 sure of sea water, w. is (in millibars) 



7r(S'7oo, 9) = (130.067 S%o 



+ 5.051) (0.018 e + 5.051). 



Salinity variation clearly dominates variation of 



524 



environmental osmotic pressure in the surface 

 layer, and hence the osmotic pressure balance 

 experienced by albacore. 



Tf albacore move so as to maintain a constant 

 balance of osmotic pressure, the response of alba- 

 core to more favorable temperatures in the plume 

 would be diminished or even canceled by their 

 negative response to low salinity. Support for a 

 salinity effect is gained by comparing ratios of 

 average catch rates within the plume to rates 

 outside the plume (table 3) with concurrent sa- 

 hnities (fig. 4) and salinity gradients (table 2) in 

 tlic plume. Salinity and intensity of the plume 

 are inversely ordered \vith catch-rate ratios (see 

 rank columns in tables 2 and 3). 



In summary, 1 suggest that higher tempera- 

 tures within the plume, produced by relative con- 

 straint of heat, give rise to greater concentration 

 of albacore within the plume province than be- 

 yond it once the fish move into the area off Oregon- 

 Washington; plume salinity, to which temperature 

 change is inversely related, qualifies the degree to 

 wliich temperature difference can be effective (or 

 even negates it during times of extremely low 

 salinity, such as July 1964). Although the now 

 unknown year-to-year differences in total abun- 

 dance of albacore should be considered in this 

 discussion, the degree to which the proposed 

 mechanisms for differential distribution of alba- 

 core are supported by fishery data indicates that 

 variation of albacore distribution over the study 

 area nuiy be as important as variation of total 

 abundance in determining the yield of that fishery. 



Results of this study indicate effects of variation 

 of temperature and sahnity on albacore distribu- 

 tion. At present experimental information is 

 lacking on physiological and behavioral responses 

 of albacore to such variations. Other factors may 

 affect albacore and may themselves be related to 

 variation of temperature and salinity: the role of 

 forage in the plume province as an attractant to 

 albacore may be worth investigating. 



The commercial fishery for albacore off Oregon 

 aiul Washington has extended infrequently to, 

 and seldom beyond, the offshore hmit of the 

 plume province, in part because the range of 

 Oregon-Washington fishing boats is generally 

 short. Consequently, I believe that only a small 

 fraction of the total albacore in the area con- 

 sidered has been exposed to fishing. Results of 



U.S. FISH AND WILDLIFE SERVICE 



