BRINTON: POPULATION BIOLOGY OF EUPHAUSIA PACIFIC A 



lar\'ae. Large and medium-sized spawners were 

 substantial contributors to this recruitment. The 

 latter were predominant and continued to be 

 throughout 1955. This differed from 1953, 1954, and 

 1956 when small or large spawners were 

 predominant during at least part of the year. 



In September a brief increase in larvae closely 

 followed the year's peak in potential egg produc- 

 tion, as observed during four September cruises 

 closely spaced in time. This was at the time of 

 maximum water temperatures (Figure 4b). The 

 November peak in eggs, to which small spawning 

 females contributed importantly for the only time 

 in 1955, led to a slight increase in recruitment in 

 December. This November activity was associated 

 with residual upwelling that was significantly 

 more intense than the 20-yr November mean. 



1956 



Upwelling began early in February as in 1953. 

 February spawning was also high, as in 1953 and 

 differing from 1954-55. Spawners were small and 

 medium-sized females (Figure 4d). Larvae peaked 

 during the same month. Following a March decline 

 in eggs and larvae, April spawning returned to the 

 February level associated with the usual spring 

 appearance of large spawners. This egg maximum 

 was followed in May by a small peak in larvae. In 

 June, egg production reached a peak for the 4-yr 

 period (13,000 eggs/1,000 m'^) at the same time as a 

 4-yr peak in the upwelling index which, however, 

 was not confirmed by the observed temperature 

 minima (Figure 4a, b). In July 1956, larvae showed 

 strong survival from the June spawn with a 

 density of 17,000/1,000 m\ While the upwelling 

 index continued to be well above average through 

 August, CalCOFI sampling did not resume until 

 October. Therefore, August-September recruit- 

 ment was not recorded. High numbers of larv^ae 

 observed from 28 September to 5 October 

 (5,000/1,000 m^) together with record numbers of 

 8-11 mm juveniles appearing in November- 

 December (Figure 10) indicated that August-Sep- 

 tember spawning was heavy and greater than the 

 substantial August-September spawn of 1953. 



An increase in egg production in November 1956 

 resulted in little recruitment in December, after 

 upwelling had stopped. An explanation may be 

 inferred from the fact that, though zooplankton 

 biomass had peaked earlier (May-July, 5505-07), 

 the euphausiid part of the biomass became ex- 

 tremely high (24-41 g/1,000 m^) only in 



November-December, consisting largely of 8-12 

 mm juveniles and young adults (Figure 5b). Eu- 

 phausia pacijica then made up a larger proportion 

 than ever before of the total biomass (15-20%), 

 indicating a diminished amount of organisms of 

 other taxa, such as salps and copepods. These, like 

 larval euphausiids^ depend heavily upon primary 

 production for food. Their reduced numbers sug- 

 gest diminished phytoplankton food (unless their 

 mortality was not food related), hence the poor 

 December survival of E. pacijica larvae emerging 

 from the November spawn. Additional evidence of 

 diminished food in November-December will be 

 seen in the negligible rate of growth during 

 November-December of the massive population of 

 8-12 mm E. pacijica. Alternatively, this population 

 may have consumed the November larvae as well 

 as their food, but this presumption is not support- 

 ed by its low growth rate. 



Recruitment Efficiency and 

 Spatial Aggregation of Eggs 



The relationship of spawning potential (density 

 of ripe eggs) to larvae subsequently recruited is 

 irregular, although a trend (Figure 6) indicated 

 that eflSciency of recruitment from available eggs 

 was better during spring and summer (March- 

 September) than during fall and winter (October- 

 February). In 1953 the spring-summer peaks in 



RELATION OF NO OF LARVAE RECRUITED IN GIVEN MONTH 

 TO NO OF RIPE EGGS OBSERVED IN PREVIOUS MONTH 

 • SPWNG-SOmUCR (MARCH EGGS VS APRIL LARVAE, Tt«OUGH AUGUST EGGS VS SEPT LARVAE) 

 o AUTUMN- WINTER (SEPT EGGS VS OCT LARVRE.Tt«OOGH FEB EGGS VS MARCH LARVAE) 



13.000 



5000 10,000 



NO OF LARVM (PER lOOOm") 



Figure 6.-Density of <4.5 mm Euphausia pacijica larvae in 

 given month in relation to ripe unspawned eggs observed 

 previous month, 1953-56 data. Regressions (Bartlett's test) for 

 spring-summer (March-September) and autumn-winter 

 (October-February) data are not significantly different. 



743 



