FISHf;RY BULLETIN VOL 77. NO .! 



The population range extended from Station 21 to 

 Station 57 (Figure 1) during the months of 

 maximum abundance (August, September). How- 

 ever, few adults or copepodites were ever present 

 at these two boundary stations. The bulk of the 

 population was restricted to the region from the 

 vicinity of Station 29 to Station 45 with an approx- 

 imate population center at Station 39. Location of 

 the population center was not static at Station 39 

 because of the diurnal tides. It was occasionally 

 found during sampling cruises downstream at 

 Station 29 or upstream at Station 45. 



The maximum density of A. californiensis ob- 

 served on a given sampling day was considered to 

 be the center of the population, regardless of loca- 

 tion in the upper estuary. A plot of maximum 

 density values for adults and copepodites I-V on 

 successive sampling dates represents the seasonal 

 cycle of the population (Figure 2). The persistent 

 alternation between peak abundance of adults and 

 copepodites is evidence that successive genera- 

 tions remain distinct. The fact that successive 

 density estimates at the population center show 

 such a refined feature creates confidence that the 

 approach is valid. 



Physical factors such as temperature and salin- 

 ity can be correlated with population growth and 

 decline on the premise that optimal conditions at a 



adults 

 Copepodites I-V 



aUG SEPT 



1972 



given time coincide with maximum spatial popu- 

 lation abundance. Thus, given the population con- 

 centration from Station 29 to .Station 45, mean 

 temperature and salinity data (surface and bottom I 

 at Stations 29 and 45 (Figure 3) can be considered 

 to represent the lower and upper range of 

 the most favorable physical conditions possible for 

 growth of A. californiensis in the estuary at any 

 given time. The means closely reflect the entire 

 water column ( 4-6 m depth) as the upper estuary is 

 well-mixed (Type D; Burt and McAlister 1959) 

 during June to October with a maximum vertical 

 gradient of l"-2' C and 1-3'U. The fortnightly 

 periodicity seen in the data is an effect of the 

 progression of tidal stage upon twice-weekly sam- 

 pling conducted during the fixed hours from 0900 

 to 1400. A sharp decline in salinity during late 

 November and December (Figures 3, 4) occurred 

 as a result of the beginning of the winter rains and 

 resulting heavy runoff Salinity in the upper es- 

 tuary remained extremely low (Figure 4) until the 

 following spring when rainfall and runoff de- 

 creased, and the salt wedge intruded up the bay. 

 The appearance in early June of A. californien- 

 sis copepodites occurred when water temperatures 

 were between 15 and 18' C and .salinities were 

 10-20%o (Figure 3). Abundance gradually in- 

 creased throughout June and much of July. A 

 population explosion began in late July and con- 

 tinued throughout August, a period when water 

 temperature and salinity were 17°-22' C and 20- 

 31"/oo. The subsequent population crash in early 



Figure 2. — Seasonal population cycle o^ Acartia californiensis 

 in Yaquina Bay ( I972i based on maximum abundance of adults 

 and copepodites (I-Vi observed on successive sampling days. 



JUNE JULY AUG SEPT OCT NOV 



1972 



Figure 3.-~Temperature lAl and salmity (B) profiles at Sta- 

 tions 29 and 45 during June to November 1972. Points represent 

 means of surface and bottom values. Envelopes correspond to 

 general range of most favorable physical conditions for Acartia 

 californiensis population growth at a given time. 



572 



