FISHERY BULLETIN VOL 77, NO :i 



portion of the fall population decline. Some field 

 evidence of this is seen in the weak pulse of 

 copepodites found in late October, indicating that 

 substantial hatching did occur below 15° C earlier 

 in the month (Figures 2, 31. 



Termination of Diapause and 

 Population Reappearance 



Hatching of field-collected resting eggs ofAcar- 

 tia spp. at various temperature-salinity combina- 

 tions indicates that termination of dormancy is 

 essentially temperature dependent. However, 

 hatching rates and survival are regulated by sa- 

 linity. My conclusions with respect to A. cali- 

 fornienf:is, based on the often disparate observa- 

 tions, are: 



1. Appro.\imate!y half of the experimental rest- 

 ing eggs were A. californiensis. 



2. Embryogenesis and hatching occurred at all 

 salinities tested at 12..5'-15' C (5-257oo) and 17= C 

 (0-23.5%"). 



3. Diapause persisted at 10' C over the range of 

 10-25%o, while embryogenesis occurred at 5"U. 



4. Hatching was retarded at low salinities, par- 

 ticularly below 10"/oo. 



5. Developmental arrest in low salinity nor- 

 mally occurred at the last stage of embryogenesis. 

 Viability of "holding" prehatch nauplii was limit- 

 ed to 1-2 mo, depending on temperature. 



6. Mortality losses were increasingly severe 

 below 10""" for hatched nauplii and substantial for 

 "holding" eggs. 



7. No nauplii survived to reach the copepodite 

 stages at salinities below 15%" at 12.5" C or at any 

 salinity at 10"" C. 



Exposure to low temperatures over a prolonged 

 period (comparable to cold stratification for seeds 

 of many temperate, deciduous plants) is unneces- 

 sary forthe release of diapause in A. californiensis 

 overwintering eggs. This is probably the normal 

 condition for most Acartia species (see Zillioux 

 and Gonzales 1972 and Uye and Fleminger 1976). 

 It is not universal, however, as the resting eggs of 

 Po/!/e//amrarfi, a neritic species, require a chilling 

 period before hatching can occur (Grice and Gib- 

 son 1977). A similar requirement is implied but 

 not conclusively demonstrated for resting eggs of 

 P. mediterranea and Centropages ponticus 

 (Sazhina 1968). Overwintering eggs of some 

 freshwater calanoids (e.g., Diaptomus oregonen- 



s(.s, Cooley 1971 ) are also known to require expo- 

 sure to low temperatures prior to hatching. 



While chilling is unessential for termination of 

 diapause in A. californiensis resting eggs, some 

 effects from chilling were observed. For example, 

 all field-collected eggs were found to commence 

 development at 12.5° C (Figure 11) while labora- 

 tory-spawned eggs terminated diapause only 

 above 15° C (Figures 5, 6, 7). Thus, exposure to 

 winter temperatures appears to lower the hatch- 

 ing threshold. The time required is unknown but 

 may be quite short. Newly spawned resting eggs 

 initially hatched at vei-y low rates when incubated 

 at 17°or2rC(Figure5). However, after Hand 14 

 days of chilling, hatching rates approached those 

 of summer eggs when placed in favorable temper- 

 atures. 



The "holding" phenomenon induced by low sa- 

 linities represents a second type of short-term 

 quiescence in A. californiensis resting eggs. It dif- 

 fers from the temperature-induced quiescent state 

 seen in summer eggs in that quiescence does not 

 set in until the final stage of embryogenesis. In 

 addition, salinity-induced quiescence is much 

 weaker, since hatching continues at low levels. In 

 these aspects, it closely resembles the dark inhi- 

 bition of summer egg hatching of A. clausi 

 (Landry 1975a). Development of A. c/af/.s: eggs in 

 the dark proceeds to the prehatch naupliar stage 

 before "holding" occurs. Viability of eggs in this 

 darkness-induced quiescence is even shorter than 

 that of A. californiensis in low salinity-induced 

 quiescence. Uye and Fleminger (1976) and my 

 own data (unpubl.) indicate it is 20-25 days. 



On the basis of field collections in February, 

 experimental results and the temperature and sa- 

 linity cycles in the field, significant hatching (or 

 embryogenesis followed by holding) of resting 

 eggs must occur over much of the year. Low oxy- 

 gen tension in sediments, while important in in- 

 hibiting hatching (Kasahara, Onbe, and Kami- 

 gaki 1975), is probably not a critical factor, since 

 resting eggs in the bottom sediments are continu- 

 ally exposed to oxygenated surface layers by 

 turbulence and erosion. Termination of diapause 

 does not always coincide with the presence of 

 favorable environmental conditions for naupliar 

 growth. Those resting eggs which undergo 

 development and then either enter quiescence or 

 hatch during the winter-spring months, a period of 

 very low salinities, must be soon lost. Such a pro- 

 cess would partially account for the seasonal de- 

 cline in resting egg numbers in the sediments with 



582 



