FISHERY BULLETIN: VOL. 69. NO. 1 



the mouth to measure the amount of water filt- 

 ered. 



The first 20 samples were from oblique tows, 

 and the other 154 were from vertical tows. This 

 change to vertical tows was made to ensure equal 

 sampling at all depths throughout a tow. Com- 

 parison of the catches of several oblique and 

 vertical tows taken during the same night indi- 

 cated little difference in the number and size 

 of E. pacifica per unit volume filtei'ed. 



Because euphausiids may avoid nets in the 

 daytime, all tows were taken during nighttime 

 when visual avoidance would be minimal (Brint- 

 on, 1967) . This is also a period when E. pacifica 

 presumably has migrated into the upper 100 m 

 of the water column. E. pacifica captured in 

 several 6-ft Isaacs-Kidd midwater trawls were 

 measured to see if large eui:)hausiids that were 

 possibly avoiding the small vertical meter net 

 could be captured. There was no indication that 

 the maximum size of trawl-caught was larger 

 than meter net-caught euphausiids. 



The maximum depth of our tows was usually 

 200 m. Because Ponomareva (1963) suggested 

 that E. pacifica adults inhabit the 200-500-m 

 layer in their second winter and no longer mi- 

 grate daily to the surface, tows were taken to 

 1000 m with both the midwater trawls and 

 vertical meter nets. These deeper tows, how- 

 ever, did not contain any larger animals. Twelve 

 vertical meter net samples from de])ths of 200 

 m or 1000 m to the surface did not show appre- 

 ciable differences in size structure. Therefore, 

 we assumed that a representative sample of the 

 E. pacifica population was caught in the upiier 

 200 m at night. 



The entire plankton sample was preserved at 

 sea in neutralized 10 'r Formalin. In the lab- 

 oratory ashore, all euphausiids were removed 

 from each sample unless the number of euphau- 

 siids was large (more than 200 individuals). 

 In such cases the samj^le was usually divided 

 in half with a Folsom plankton splitter (Mc- 

 Ewen, Johnson, and Folsom, 1954), and euphau- 

 siids were sorted from only one-half the sample. 

 Males and females were not differentiated. 



The length of each individual E. pacifica was 

 measured to the nearest 0.1 mm from behind 



the eye to the posterior margin of the carapace, 

 and each animal was then assigned to a 0.3-mm 

 size-group. Total lengths (from the posterior 

 of the eye to the tip of the telson) were also 

 measured from randomly selected individuals 

 of various lengths to enable comparisons of our 

 data with those of others. A least squares fit 

 of 146 comparisons gave the equation: 



Y = 2.54 X + 0.66 



where Y = total length and X = carapace 

 length. The variance was 248.19. Our measure- 

 ments are all given as total lengths. 



RESULTS 

 RECRUITMENT AND ABUNDANCE 



Although larval E. pacifica occurred during 

 almost all months of the year, definite trends 

 in abundance were evident over the 4-year per- 

 iod ( Fig. 1 ) . Larvae were usually most abun- 

 dant between October and December. During 

 some years recruitment began as early as June 

 and was also prominent in the summer months. 

 No major concentrations of larvae were found 

 during winter or spring. 



These larval forms of E. pacifica were f urcilia 

 of about 7 mm or less, agreeing with Boden's 

 (1950) size measurements and description of E. 

 pacifica furcilia. Furcilia are found 16-18 days 

 after spawning, usually within the upper 100 m 

 of the water column (Ponomareva, 1963; Brint- 

 on, 1967). 



Catch curves (Fig. 2) show the average num- 

 ber of different size-groups of E. pacifica col- 

 lected during the entire study. All sizes of E. 

 imcifica were much more abundant i)er m^ in- 

 shore over the continental shelf than in oceanic 

 offshore waters. Individuals larger than 15 mm 

 were rare at station 65 miles or farther offshore. 

 Our finding that larvae were less abundant at 

 offshore than inshore stations agrees with 

 Brinton (1962b), who also noted that E. pa- 

 cifica was more abundant inshore than oflFshore 

 of California. Thus, despite the wide oceanic 

 distribution of E. pacifica, the density of near- 



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