OMORI and GLUCK: LIFE HISTORY OF SERGESTES SIMILIS 



mencement of spawning and the survivorship of 

 larvae are closely related to the ambient tempera- 

 ture rather than the quantity of food available. 

 This study showed that: 1) S. lucens started 

 spawning in June, immediately after the tempera- 

 ture exceeded 18°C at 20-50 m, and the number of 

 larvae increased with increasing vertical thick- 

 ness of the optimum temperature zone for the 

 growth of larvae (18°-25°C), 2) the population size 

 of S. lucens was determined by the abundance of 

 larvae during the first half of the breeding season, 

 June- August, and 3) the abundance of larvae was 

 often related to the fluctuation in vertical width of 

 the optimum temperature zone. During midsum- 

 mer the warming of surface waters above to 25°C 

 and the shoaling of cold water < 18°C restricted 

 the optimum temperature zone, and consequently 

 the mortality of protozoeal larvae increased. 



As with S. lucens , a rise in temperature may 

 trigger the commencement of spawning of the 

 Sergestes similis population in the northern sub- 

 arctic waters where surface temperature is < 10°C 

 during most of the year. However, this seems not 

 to be the case for the southern California popula- 

 tion where favorable temperatures were available 

 year-round in some stations between 50 and 100 

 m. Yet, the spawning began abruptly when the 

 temperature around 100-m depth began to rise. 

 Abundance of larvae was greatest during the 

 period when the vertical thickness of the optimum 

 temperature zone was the greatest, and spawning 

 activity almost ceased both when the ambient 

 temperature was lowered by coastal upwelling 

 and when warm surface water subsequently ap- 

 peared. Thus, the spawning season of S. similis is 

 not always positively correlated with the upwell- 

 ing which causes environmental enrichment and 

 subsequent increase of plankton biomass in the 

 southern California eddy. The correlation of 

 spawning to coastal upwelling in Euphausia pa- 

 ct fica , another very abundant species of the 

 California Current zooplankton assemblage, is 

 the most striking difference affecting the spawn- 

 ing seasons of that organism and S. similis . Simi- 

 lar to S. similis, the southern California popula- 

 tion of E. pacifica seems to be adapted for larval 

 development between 12°and 16°C, but its spawn- 

 ing is highest when coastal upwelling is strongest 

 in May-June (Brinton 1976). Although true 

 mechanisms remain unexplained, we theorize 

 that the distinctive spawning season of S. similis 

 in southern California is based mainly on the 

 adaptation of this species to the vertical thickness 



of optimum temperature. The vertical thickness of 

 the optimum temperature zone was also corre- 

 lated with the abundance and survival of larvae of 

 S. similis. The cumulative depths of the optimum 

 temperature ranges for S. similis from January to 

 March were 220, 318, 311, and 380 m from 1951 to 

 1954 whereas the average numbers of protozeal 

 larvae occurring from January to April were 129, 

 218, 224, and 543 individuals/1,000 m^, respec- 

 tively. In 1951, zoeal larvae were found in the 

 lowest numbers when the cumulative depth was 

 the smallest. 



One possible interpretation of the irregular 

 small pulse of spawning of S. similis in seasons 

 other than winter and early spring is that shrimp 

 which reproduce during these periods are carried 

 from northern offshore waters, i.e., subarctic 

 North Pacific, to the study area. If temperatures of 

 9°-10°C in the habitat of S. similis really trigger 

 the commencement of spawning, those living in 

 subarctic waters would start spawning later than 

 July in most areas. The yearly mean velocity of the 

 eastward component of the North Pacific Drift is 

 about 3 cm/s at the surface in the areas lat. 45°N 

 west of long. 150°W. On the other hand, a strong 

 south-flowing current, which flows at the velocity 

 of 5-10 cm/s but occasionally >20-30 cm/s is ob- 

 served throughout the year both at the surface and 

 at 200-m depth off the west coast of the United 

 States ( Wyllie 1966; Stidd'*). If part of the popula- 

 tion of S. similis near lat. 47°N, long. 140°W, e.g., 

 where tremendous numbers are eaten by baleen 

 whales (Omori et al. 1972), is carried southeast- 

 ward by the currents, the shrimp can easily reach 

 the southern California coast within 2 yr at a 

 mean speed of 5 cm/s of flow. The spawning occurs 

 in the summer off California due to continuous 

 recruitment of such northern populations. An 

 electrophoretic study of S. similis population may 

 help to answer this question, although, due to di- 

 verse trophic regimes, genetic variability of the 

 southern California population may be too large to 

 distinguish it from the subarctic population (see 

 Valentine and Ayala 1976). 



Another possible interpretation is that the 

 phenomenon is caused by the adaptation of the 

 local population to mid latitude irregularities in 

 oceanographic and trophic conditions. It has been 

 observed for several penaeids, sergestids, and 

 euphausiids in temperate and tropical regions 



^Stidd.C.K. 1974. Ship drift components: means and stan- 

 dard deviations. SIO Ref. 74-33, 57 p. 



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