FISHEKY BULLETIN VOL. 77. NO 



However, after reaching a total of 61-66% hatched, 

 the rate decreased sharply with hatching continu- 

 ing at a low, constant rate to a final total of 90-91% 

 at day 16. The latter pattern resembles that found 

 for resting eggs (13°, 9° C spawn) hatched at sum- 

 mer temperatures (Figure 5A, B). A comparable 

 56% hatch also occurred at 15° C within the first 

 2.5 days. However, development then ceased until 

 temperature was increased to 21° C. Thus, at all 

 three temperatures, approximately 60% of the 

 eggs behaved as nondormant summer eggs, while 

 the remaining 40% had characteristics of resting 

 eggs. 



Dormancy increased from 409i at 15"C to 98% at 

 5° C (Figure 7), presumably as a result of dor- 

 mancy induced by low temperatures in otherwise 

 nondormant eggs. This result is similar to that 

 seen for summer eggs hatching at the lower tem- 

 peratures (Figures 5C, D; 6). 



Egg viability during short- and long-term dor- 

 mancy was determined by increasing temperature 

 to 21° C on days 9, 11, 15. and 102 for the 15 , 13°, 

 9°, and 5° C hatching treatments, respectively (Fig- 

 ure 7). Mortality was low during the 9-15 day 

 incubation period at 15°, 13°, and 9° C with a final 

 cumulative hatch of 85-91% . In contrast, only 45% 

 of the dormant eggs incubated for 102 days at 5° C 

 completed development into NI nauplii following 

 temperature elevation. Probably few dormant 

 summer-type eggs survived the long holding 

 period at 5° C, given that about 40% of the eggs 

 exhibited characteristics of resting eggs in 21°, 

 17°, and 15° C water. This conclusion is supported 

 by the high mortality ( 99-100% ) found for summer 

 eggs incubated at 5° C for 120 days in the October 

 experiment i Figure 6). 



Hatching of Resting Eggs 

 Collected in the Field 



Salinit) Expc-riment 



The effect of salinity on hatching of resting eggs 

 collected from field sediments was initially deter- 

 mined at 17" C, a temperature favorable for egg 

 hatching (Figure 5) and population growth of A. 

 californiensis (Figures 2, 3). Results presented in 

 Figure 8 are the combined data for both A. califor- 

 niensLs and A. clausi. since their respective over- 

 wintering eggs could not be separated. 



Hatching occurred at all salinities from 23.5"'im 

 to 0%o (Figure 8). Rates decreased only slightly 

 with decreasing salinity from 23.5%o (37%'day ') 



IncuboTion Time Idoys) 



Figure 8. — Hatching succe,s.s of field-collected resting eggs of 

 Acartia spp. as a function of salinity (0-23.5'Kki) at 17° C. 



to 12.5%o(31%-day'; day 2.5). Final hatch in this 

 salinity range was 91-96%. Initial hatching rates 

 below 12.5%ii decreased markedly, ranging from 

 26% -day"' at' 10%,, to 1.8% -day' at 0%„. These 

 latter rates, while reduced, were nevertheless 

 substantial over a 2-wk span. Overall hatching 

 success by day 12 was 21%, 61%, and 88% of the 

 resting eggs at 0"/oo, 5%o, and 8%o, respectively. 

 Furthermore, hatching was continuing, even in 

 freshwater, as indicated by the slopes of the 

 curves, when the experiment was ended. Ii: com- 

 parison, resting eggs of Tortanus forcipattis do not 

 hatch in freshwater when temperature is favor- 

 able (Kasahara, Onbe, and Kamigaki 1975). 



The retarding effect of low salinity on hatching 

 appeared to be limited to the actual process of 

 naupliar escapement from the eggshell. Em- 

 bryogenesis proceeded at similar rates (subjective 

 observations) in all treatments (0-23. 5%o), appar- 

 ently independent of external salinity concentra- 

 tion. Developmental arrest, when present, nor- 

 mally occurred after the fully formed nauplius was 

 visible inside the eggshell. 



Exposure to low salinity (0-5"/,)o) for varying 

 periods during dormancy was not fatal for the 

 majority of prehatch nauplii. In each case, high 

 rates of successful hatching (Figure 9) quickly fol- 

 lowed a salinity increase to 23.5%o. However, pre- 

 hatch mortality of "holding" nauplii increased 

 substantially as a function of salinity reduction 

 when compared at equal time. This is seen in a 

 final hatch of 79%, 71%, and 59%, following a 

 salinity increase (day 12) to 23.5%ofor eggs previ- 

 ously incubated at 5%o, 2.5%o, and 0%o, respec- 

 tively. Thus, an approximate 10% increase in mor- 

 tality occurred for each 2.5%n decrease in salinity 

 below 5%(,. 



576 



