Concentrations of sockeye salmon between the two op- 

 posing current systems south of the Aleutian Islands were 

 examined to determine if their abundance could be attributed 

 to differences in the availability of food organisms (fig. 13). 

 Differences in relative abundance of phytoplankton and 

 zooplankton in the Ridge Area and in the Alaskan Stream 

 may have come about from different grazing rates or vary- 

 ing physical-chemical conditions. Large zooplankters are 

 consistently abundant in the Ridge Area which corresponds 

 to the area of known high concentrations of immature salmon 

 in autumn. 



Biological Productivity 



How does the availability of food organisms afl'ect the 

 movement of salmon ? Why is the production of organisms 

 low in some parts of the ocean in spite of adequate vital 

 nutrients? Research on biological productivity may provide 

 the answers to these and other questions. 



The most basic food organisms are the phytoplankton 

 which are tiny single-celled plants. Produced continuously 

 wherever light enei'gy from the sun and nutrients are in the 

 right combinations, phytoplankton may live only a few days 



or even a few hours but reproduce at fantastic rates. As 

 they multiply, they exhaust one or more of the nutrients 

 in the water. Phytoplankton also constitute food for grazers, 

 the smallest of which (zooplankton) are consumed by small 

 fish, which in turn become food for larger carnivores higher 

 in the food chain. 



The amount of chlorophyll contained in the phytoplankton 

 is an index of the stock of phytoplankton. Productivity is 

 the increase of the stock in relation to time. Our ocean- 

 ographei-s measured basic productivity by injecting radio- 

 active isotopes of carbon-14 into bottles of sea water. After 

 a specified time, the samples wei'e analyzed with a Geiger 

 counter. The amount of carbon-14 assimilated by the phyto- 

 plankton provided an index of productivity. 



Maximum productivity in 1966-67, as indicated by utili- 

 zation of nutrients, was in the spring; minimum productivity 

 was in the winter. Chlorophyll in the late winter south of 

 the Aleutian Islands increased from north to south and was 

 highest in the warmer, central Pacific Ocean. Productivity 

 in summer was highest in the Alaskan Stream and on either 

 side of the area of vertical upwelling (Ridge Area) which 

 brought up nutrients from the ocean depths. 



MANAGEMENT OF SALMON STOCKS 



The eflicient use of salmon stocks requires a knowledge 

 of where, when, and how many to harvest. Is it more effi- 

 cient to catch smaller, younger, more numerous salmon far 

 out in the ocean before many of them die from natural causes 

 or are eaten by predators? Or, is their growth and survival 

 — between their exposure to high-seas and coastal fishing • — 

 such that the total weight of the stock is greater when larger. 



older, but fewer fish are caught inshore? How many salmon 

 die after escaping from the gill nets of the Japanese fleet, 

 and what is the eff'ect of the j^hysiological stress from their 

 struggles? Biometric analysis, knowledge of the relative 

 efficiency of nets, growth and mortality studies, physiological 

 research, and techniques to identify races or stocks may fur- 

 nish some of the answers. 



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