BAILEY ET AL,: ZOOPLANKTON ABUNDANCE AND FEEDING HABITS OF FRY 



the spring when fry are migrating. Assuming an 

 equal flow of seawater with plankton density of 

 24,000 organisms per cubic meter into Traitors 

 Cove from Behm Canal and surface entrainment 

 near the constriction, we calculated that 16.5 x 10^ 

 organisms would be brought daily into Traitors 

 Cove by freshwater-driven circulation. Dividing 

 the number of organisms by 544 (the high estimate 

 of organisms eaten by one fry daily) yields a con- 

 servative estimate of 30 million fry that could be 

 fed by an amount of food added daily by circula- 

 tion. Although it is naive to assume that all of the 

 plankton brought into Traitors Cove as a result of 

 circulation would become available to the fry, the 

 upwelling and thorough mixing that occur at the 

 constriction between the two bays result in a con- 

 tinual resupplying of zooplankton to the upper 

 meter of depth where grazing apparently takes 

 place. Field observations did indicate that the larg- 

 est concentrations of fry were consistently found 

 in eddies near the constriction, lending some 

 credence to the theory that upwelling of deep 

 water created a favorable supply of food in this 

 area. 



The effects of tidal circulation and freshwater- 

 runoff-driven circulation are often additive. 

 Therefore, we calculated the influx of food or- 

 ganisms by tidal exchange. We assumed that the 

 surface waters were flushed completely by the 

 outgoing tide; that complete mixing of incoming 

 water with water present occurred on each tide; 

 and that all zooplankton in the upper meter had 

 been consumed before the waters were mixed. The 

 influx of new food can then be estimated from the 

 tidal prism as 



F = [T/V]xP 



where F is the net influx of new food organisms as 

 zooplankters per cubic meter per tide; T is the 

 volume of the tidal prism; V is the volume of the 

 outer bay; and P is the density of zooplankton 

 outside the bay. (We used P = 24x lOVm^ because 

 we assumed that abundance was the same outside 

 the bay as it was inside.) 



The resulting calculation assuming a mean tidal 

 range of 4.11 m (McLain 1968) and a mean depth of 

 90 m gives for the net influx of organisms per 

 tide: 



F = 4.11m /tide x 24 x lO^/m^ = 1.09 x 10^ 

 90 m 



zooplankters per cubic meter per tide. 



Only those in the upper meter are available, and 

 since there are two tides per day, the calculated 

 quantity of new food available to salmon fry per 

 day is: 



Q = 2xFxrarea of bayjxl m = 2 tides/ 

 day (1.09 x 10^ zooplankton per cubic 

 meter per tide) x (7.6x lO^m^x Im) 

 = 16.6 X 10^ zooplankters per day. 



This number will feed 30x10^ fry per day 

 (16.6 x 10^ zooplankters -i- 544 zooplankters per day 

 per fry). The estimate is high because mixing is 

 not complete, as implied by the calculations. By 

 adding fry that could be fed from the effects of 

 freshwater runoff to fry that could be fed by tidal 

 action, we get an upper estimate of carrying 

 capacity of 60 million fry. 



The numbers of fry that could theoretically be 

 fed by the two sources of zooplankton, i.e., stand- 

 ing crop in the surface water and plankton in the 

 net circulation, are not strictly additive. Although 

 some plankton in deep seawater would be con- 

 tinuously entrained upward to flow seaward on the 

 surface, some would never reach the surface of the 

 bay and a portion of the surface stock would be 

 removed from the bay by outflow. Therefore, it 

 would seem prudent to consider that populations 

 numbering more than 30 million pink and chum 

 salmon fry might cause reduced growth of fry 

 (because of the competition for food). Also, such 

 large populations might stimulate a more rapid 

 migration of fry through the estuary to areas 

 where food organisms were more abundant. 



On the basis of available spawning grounds, it 

 seems unlikely that Traitors Cove has ever had to 

 support more than 20 million pink and chum salm- 

 on fry, although it is possible that 11 to 60 million 

 fry could be supported in years when food abun- 

 dance equaled or exceeded that observed in 1966. 

 The release of 50 to 100 million additional hatchery 

 fry into this estuary would probably exceed the 

 carrying capacity of the area. Competition for 

 food, especially if zooplankton production were 

 lower than average, and increased potential in- 

 fection by disease, parasitism, and predation could 

 theoretically result in increased mortality, slower 

 growth, or accelerated movement of fry out of the 

 estuary. Further, a great increase in numbers of 

 salmon fry in Traitors Cove could deplete plank- 

 tonic food and planktonic larvae required to sup- 

 port other fisheries. We have used Traitors Cove to 

 discuss carrying capacity of estuaries only because 



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