BRINTON: POPULATION BIOLOGY OF EUPHAUSIA PACIFIC A 



all sizes after maturity, applicable to b) and c), or 

 because females grow slower and live longer, also 

 applicable to b) and c); and the bias is an artifact of 

 reduced net avoidance by ripe females and of 

 observations during seasons when some males 

 mature relatively late, resembly females for a 

 time, applicable to c). Evidence supports each of 

 the above. With regard to the increasing 

 female/male ratio with body size, there are par- 

 ticularly strong indications of relatively slow 

 growth in females, apparently leading to better 

 survival than in males at given sizes and ages. 



DISCUSSION 



The predominance of E. pacifica among zoo- 

 plankters off southern California appears related 

 to the spring-early summer upwelling regime, 

 which coincides with heaviest spawning. Recruit- 

 ment consistently crested during May-July fol- 

 lowing annual surface temperature minima in 

 April or May. Although this species may range 

 southward along California and Mexico because of 

 currents and the cool (10°-18°C) water, sole 

 dependence on temperature effect in the southern 

 California area for reproductivity is not likely 

 because the area as a whole is coolest during 

 December- April and the most substantial recruit- 

 ment is later. 



That the dependence is partly effected by food, 

 as indicated by the seasonal pattern in availability 

 of nutrients (plants), was shown in charts of the 

 California Current region for 1969 for nutrients 

 (Thomas and Seibert 1974) and for chlorophyll a 

 (Owen 1974). The assumption is made that timing 

 of the seasons in 1969 agrees enough with 1953-56 

 so that the April-June buildup in upwelling applies 

 to both periods. Off southern California nutrient 

 concentrations intensified in April and peaked in 

 May in a patchy distribution corresponding to the 

 areas of low surface temperature. For example, 

 PO^-P (integrated through 0-50 m depth) was in 

 the range of 10-40 mg-at./m- during January- 

 February and August-December but increased to 

 40-60 mg-at./m- during April-June. Silicate-Si 

 peaked at 400-1,000 mg-at./m- during April-June; 

 during the other months concentrations >400 

 mg-at./m- were rare. 



Correspondingly, during the main upwelling 

 period, April-September, chlorophyll a in the 

 surface waters inhabited by newly hatched eu- 

 phausiid larvae showed the patchy pattern of 

 extreme concentrations shown also by the nu- 



trients. Values peaked at 3.0 mg/m' during April- 

 September compared with 2.0 mg/m^ for Jan- 

 uary-March and 0.5 mg/m-^ for October-December. 

 The possible importance of shallow (12-19.5 m), 

 intense (to 50 iug/liter) chlorophyll maxima- 

 particularly those containing the dinoflagellate 

 Gymnodinium splendens-to first feeding of an- 

 chovy larvae was put forward by Lasker (1975). 

 These maxima were found during March-April 

 1974 within 15 km of the southern California coast. 

 Such layering of food particles could have broad 

 significance to feeding and survival of zooplankton 

 larvae. 



Most larvae of E. pacifica are found in nearshore 

 areas described above as recruitment refuges 

 where upwelling prevails and currents are slug- 

 gish. Similarly, off Oregon (Smiles and Pearcy 

 1971), more larvae were in nearshore upwellings 

 than in offshore water characterized by a summer 

 productivity minimum typical of the region. Also 

 working off Oregon, Peterson and Miller (1975) 

 found no relationship between year-to-year 

 (1969-71) intensity of summer upwelling and 

 abundances of euphausiid eggs and larvae (not 

 identified to species). 



Evidence that larvae occupying southern 

 California waters are produced locally is seen in 

 the time of the upwelling season along the coast. 

 Upwelling peaks off southern Baja California in 

 February-March. Progressing northward, its 

 maximum off Oregon is during August-Sep- 

 tember. Hence maximum spawning and recruit- 

 ment, if upwelling induced, should develop along 

 the same northerly track, counter to the direction 

 of main flow in the California Current during this 

 period of relatively consistent northeast winds. 

 This is the case: recruitment off mid-Baja Califor- 

 nia, lat. 27°-29°N, is mainly February-April 

 (Brinton 1967b, 1973), in Monterey Bay it is both 

 spring and summer (Barham 1957), off southern 

 California it is mainly May-July, and off Oregon, 

 August-December. 



Although ripening of ovaries, spawning, and 

 recruitment reach maxima as consequence of 

 upwelling-associated events, the southern 

 California population includes ripe females and 

 newly hatched larvae year-round (Figures 4c, d; 

 19). Off Oregon, E. pacifica also includes some 

 larvae at all times (Smiles and Pearcy 1971); while 

 in the Sea of Japan (lat. 40°-50°N) and south of 

 Kamchatka, Alaska (lat. 50°-55°N), in areas en- 

 riched by winter mixing of the water column, E. 

 pacifica possesses ripe gonads in May-June, the 



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