LOUGH: LARVAL DYNAMICS OF DUNGENESS CRAB 



locomotion, they swarm to the surface again and 

 are congregated by the prevailing currents usu- 

 ally in a band w^ithin 5 miles of the coast. If the 

 late zoeal larvae do in fact reside near the sea 

 bottom, the onshore drift current within 10-20 m 

 of the bottom would prevent them from being 

 transported offshore. Bottom flow in waters less 

 than 40 m deep is towards the coast in the direc- 

 tion of wave travel throughout the year (Gross et 

 al. 1969). The behavior of the larvae within the 

 water column in relation to the hydrological fea- 

 tures of the nearshore area under usual cir- 

 cumstances tends to restrict dispersal of the larvae 

 to any great degree. 



1971 Season 



The sparseness of C. magister late zoeal larvae 

 and megalopae during the 1971 season implies 

 that a mass mortality occurred in the early zoeal 

 stages. This apparent mortality was associated 

 with sea surface temperature and salinity in 

 analyses of covariance, but larval survival pre- 

 dicted through response surface methodology and 

 gut-fullness analysis did not substantially ex- 

 plain their sparseness. The lack of highly suppor- 

 tive evidence leads to further speculation as to 

 the causes of larval mortality in the plankton. 



Hypothesis 1: Direct Effects of Temperature 

 and Salinity 



Sea surface temperature, and salinity to a les- 

 ser degree, were important environmental factors 

 in explaining the difference in yearly larval popu- 

 lation means of C. magister by analyses of multi- 

 ple covariance. However, the statistical impor- 

 tance of these factors in determining larval 

 abundance may be misleading. A wide tempera- 

 ture gradient during a larval season, i.e., a steep 

 slope, could be statistically significant, but the 

 range of temperatures may be well within the 

 tolerance limits of an organism. In contrast, the 

 salinity gradient during the same larval season is 

 usually narrow resulting in a statistically non- 

 significant slope, which may still occur outside 

 the range tolerated by the larvae. Also, the errat- 

 ic surface temperature and salinity fluctuations 

 that occurred during the summer upwelling may 

 cancel the effect of a signiflcant gradient that oc- 

 curred earlier in winter and spring. 



Cancer magister larvae were reared by Reed 

 (1969) under various temperature-salinity com- 



binations and he concluded that these factors, as 

 they normally occur off the Oregon coast, would 

 not significantly affect survival. Response surface 

 techniques, using Reed's data, predicted about 

 45% survival under the extreme temperatures 

 and salinities that occurred during February and 

 March 1971. The sea surface temperatures and 

 salinities used in the analysis probably represent 

 the most extreme long-term conditions that the 

 larvae could have experienced in the field. Larvae 

 several meters below the surface may be pro- 

 tected from the more extreme fluctuations of 

 temperature and salinity, but some degree of ex- 

 posure seems certain in view of the fact that ex- 

 tensive wind mixing occurs in shallow waters 

 along the coast. The North Pacific is charac- 

 terized by heavy precipitation during the fall and 

 winter seasons resulting in considerable land 

 drainage and river runoff along the nearshore 

 area. Larvae along the coast, particularly near 

 the mouths of bays and rivers, may lie in the 

 low-salinity plume waters before sufficient 

 mixing occurs. Harder ( 1968) reported that many 

 planktonic organisms tend to accumulate near 

 density interfaces that frequently occur in 

 natural waters. Some species of copepods were 

 observed under laboratory conditions to react to 

 extremely small changes in density. Whether C. 

 magister larvae have the ability to avoid these 

 low-salinity surface waters that may be detri- 

 mental to them is not known. The early zoeal lar- 

 vae would seem most vulnerable to low surface 

 salinity as their behavioral response directs them 

 to the surface and their swimming ability is 

 slight compared to the megalops stage. Early lar- 

 val ability to avoid low-salinity surface waters 

 would have to be sufficient to overcome the in- 

 creased storm-induced mixing during this season. 

 The mortality rate of C magister larvae reared in 

 the laboratory under optimum conditions was 

 constant and minimal throughout development 

 (Reed 1969). Mortality increased greatly for lar- 

 vae reared at 20L salinity; early zoeal larvae 

 were killed within a short period in salinities 

 less than 20%.. In addition, both the lower range 

 of salinities and temperatures used in his ex- 

 periments increased the duration of the larval in- 

 stars where survival could be monitored for a suf- 

 ficient time period. 



It is difficult to evaluate the extent to which 

 results from laboratory studies approach reality 

 in order to understand how environmental vari- 

 ables may affect survival. Larvae reared at sub- 



367 



