FISHERY BULLETIN VOL 77, NO :l 



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Table 2- — Nondormant egg development time for A cart la tonsa 

 and A. clausi as a function of temperature, and time that un- 

 hatched eggs of A. californiensis were initially held at the same 

 experimental hatching temperature (October expenmentl. 



Incubation lime (days) 



spawning at 12° C and sorted into replicate 

 batches of 50 eggs each. Hatching rate and success 

 weredeterminedat2r'. 17", 15', 13°, 9\and5°C in 



'Data based on prediction of Beletiradek function (Zillioux and Gonzalez 

 1972) 

 ^Data Irom experimental observations of Landry (1975b) 

 ^Temperatures at wtiicti A californiensis eggs were spawned 



25\i) water. Other procedures were as described 

 above. 



HATCHING OF RESTING EGGS 

 COLLECTED IN THE FIELD 



A series of preliminary experiments (J. K. 

 Johnson, unpubl. data) had demonstrated that 

 resting eggs of A. californiensis and A. clausi 

 occur in similar numbers in the surface sediments 

 in the vicinity of Station 39 (Figure 1). Unfortu- 

 nately, the resting eggs of the two species are not 

 distinguishable from each other on the basis of 

 diameter, shape, or color. Therefore, the following 

 hatching experiments include resting eggs of both 

 species at unknown ratios. Much information and 

 insight were gained in spite of this serious ex- 

 perimental limitation. 



Salinity Experiment 



Resting eggs were obtained by collecting mud 

 with an Eckman-Birge grab 500 m upstream of 

 Station 39 on 7 February 1976 during low tide. 

 The upper 1-2 cm of sediments were saved for later 

 screening. Temperature and salinity values near 

 the bottom were 9.8° C and 4.1%o. An accompany- 

 ing series of plankton tows from Station 21 to 

 Station 45 verified that no copepodite stages of A. 

 californiensis were present in the upper estuary, 

 as expected from earlier field work. Acartia clausi 

 was present in low numbers only at Stations 21 

 and 29 (<50-m"^). It does occur at Station 39 

 during January to April but only at extremely low 

 densities (ca. 5-20 • m ') during periods of high 

 tides (Zimmerman 1972). Thus, few recently 

 spawned eggs were likely to be present in the 

 sediments collected at Station 39. 



Sediment samples were maintained at field 

 temperature (10° C) during transport to the 



570 



