FISHERY BULLETIN: VOL 77. NO, I 



spawning takes place around 50 m and using data 

 on both the sinking velocity of the eggs and the 

 development time of eggs at 10°C, we can estimate 

 that the eggs sink to about 220-m depth before 

 hatching. The ambient temperatures which eggs 

 may encounter during their descent are 7°-13°C. It 

 is probable that some eggs are laid deeper than 50 

 m. However, like the population off the Oregon 

 coast (Pearcy and Forss 1969), S. similis is seldom 

 distributed over the continental shelf off southern 

 California. Therefore the majority of eggs would 

 not sink to the bottom but remain within the water 

 column. 



A comparison of vertical distribution patterns 

 at all stations confirms the following hypotheses: 



1) the occurrence of larvae is restricted to water 

 <140 m where the temperature range is 9°-16°C, 



2) the larvae often appear in the 0-20 m level at 

 night but rarely in the daytime, and 3) the larval 

 distribution is more restricted inshore than 

 offshore to a limited vertical range. The descent of 

 eggs and ascent of naupliar larvae are well 

 documented in the oceanic euphausiid £Mp/ia(/sia 

 superba and Meganyctiphanes norvegica 

 (Mauchline and Fisher 1969). Presumably the 

 nauplii of S. similis rise from 200 m or deeper to 

 layers where the temperature is usually > 10°C. In 

 this manner, the nauplius, which is probably 

 highly vulnerable to predation, develops in the 

 less hazardous layers which are deeper than the 

 following larval stages. Protozoeal and zoeal lar- 

 vae stay mostly in the shallower environment 

 which is relatively rich in food (phytoplanktonand 

 microzooplankton). They perform daily vertical 

 migration starting PZl, and their downward mi- 

 gration at daytime becomes more marked with 

 each stage. This hypothesis is further supported 

 by the positive phototaxis in N3 to PZl larvae and 

 negative phototaxis after PZ2 observed in the 

 laboratory (Omori 1979). 



According to Omori (1974), the larvae of pelagic 

 shrimps can be classified into several types on the 

 basis of their ontogenetic migration. The first 

 group is composed of the species living in the 

 epipelagic and upper mesopelagic zones. Their 

 larvae perform migration within the euphotic 

 zone. Sergestes similis belongs to this group, hav- 

 ing a similar pattern to that described for Sergia 

 lucens (Omori 1974), but the negatively buoyant 

 eggs of Sergestes similis differ from Sergia lucens 

 eggs which have density similar to seawater. 



Adult Sergestes similis were abundant inshore 

 off Oregon during the winter, but they tended to 



shift to an offshore distribution during the sum- 

 mer (Pearcy and Forss 1969). This inverse rela- 

 tionship between nearshore and offshore stations 

 indicates a horizontal ontogenetic migration of 

 this species by active swimming with the help of 

 subsurface currents. The movement by a species 

 to nearshore regions for spawning is a characteris- 

 tic behavior among several sergestid shrimps 

 (Omori 1974). 



Relationship Between 

 Spawning Season and Environment 



Larvae of S. similis, in particular PZl, were 

 more abundant inshore than offshore, which indi- 

 cates that the spawning of S. similis is taking 

 place mainly close to shore above the continental 

 slope off southern California (but not as far in- 

 shore as the continental shelf). The assumption by 

 Pearcy and Forss (1969) that S. similis in the 

 Oregon population spawns during most of the year 

 with a seasonal minimum occurring during the 

 summer was partially true in the southern 

 California population as there were small pulses 

 of spawning in summer and autumn. However, the 

 southern California adult population appears to 

 be recruited largely from the local population 

 spawned from late December to early April. 



One may argue that the decrease of larvae in the 

 study area in summer and autumn was caused 

 merely by the seasonal change off the southern 

 California gyre. It would be interesting to compare 

 our data with samples from stations outside of the 

 northward flowing path of the gyre. However, we 

 do not think that such an extreme absence of lar- 

 vae in summer and autumn is taking place with 

 the year-round spawning of S. similis. At least 

 some larvae should have successfully remained in 

 the study area to yield noticeable recruitment dur- 

 ing those months. Incidentally, females having 

 fully developed ovaries (Omori 1979) were sel- 

 dom found in the IMKT collection from summer 

 and autumn. Genthe (1969) assumed that 

 maximum reproductive activity of S. similis from 

 the Santa Barbara Channel was in summer and 

 autumn, but his assertion that juveniles collected 

 in August of 5.0-6.5 mm CL are 11 or 12 mo old is 

 misleading. Shrimp of this size are more likely to 

 be of the 6-7 mo class. 



Omori et al. (see footnote 7) studied the relation- 

 ship between environments and reproductive be- 

 havior of another sergestid, Sergia lucens. in 

 Suruga Bay, Japan, and found that the com- 



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