LILLELUND and LASKER: PREDATION BY MARINE COPEPODS 



I5r 



g J^ 10 



in o 



lO o 



I Q. 



o ui 



n 



< UJ 



id 



o — 

 z ^ 



< 



10.9 



oo 

 o 



3.1 



ego 



o 



8 

 8o 



oc' 



,c»' 



de 



0' 



_Q- 



.-^^ 



=5^^ 



&^ 



^' 



-d?' 



,cje' 



.^0' 



l>5 



Figure 16. — Predation by Pontellopsis occidentalis on 

 anchovy larvae to 3.5 days old. Each horizontal line 

 indicates the mean value of the experiments shown with 

 open circles ; 30 larvae were provided to 1 to 6 copepods 

 in 3500 ml at the beginning of each experiment. 



measure the rate of ascent of anchovy eggs to 

 determine the maximum depth at which spawn- 

 ing could occur and yet insure the presence of 

 yolk-sac larvae at the surface. In La Jolla sea- 

 water, salinity 33^;, and 17° C, anchovy eggs 

 rise 5 cm/min or 3 m/hr. Thus, with time from 

 spawning to hatching at 2 days, eggs spawned 

 as deep as 144 m would hatch at or near the 

 surface of the ocean, although spawning that 

 deeply is rare (Ahlstrom, 1959). During de- 

 velopment and, as they use up their yolk, an- 



chovy larvae become almost neutrally buoyant 

 and start to sink very slowly in laboratory con- 

 tainers. Even so, after 2 days of development, 

 50 ""f of the laboratory-reared larvae were still 

 above the 30 to 40 cm depth. 



Ahlstrom (1959) reported closing-net cap- 

 tures of anchovy larvae at a variety of stations 

 and depths in the California Current off Cali- 

 fornia and Baja California. He has kindly pro- 

 vided us with length distributions of anchovy 

 larvae taken at two stations, a night station, 

 5206-90.28, where over 500 larvae were taken, 

 and a day station, 5504-120.50, where over 5000 

 lai'vae were captured. The length of the larva 

 is roughly indicative of its age (Kramer and 

 Zweifel, 1971), and we have tabulated the depth 

 distribution of anchovy lai'vae at these stations 

 by length and age (Table 2) . 



The depth distributions by age and length of 

 anchovy larvae indicate that 50 Cr or more of 

 anchovy larvae up to 3 weeks old are above 10 m 

 in depth. Fifty percent of the youngest class, 

 3 to 4.5 mm and 1 week old or less, were above 

 3.5 m during the day, and were slightly more 

 than 2 m deep in the night. Ahlstrom's data 

 also suggest that larvae of the Pacific sardine, 

 Sardinops caendea, Pacific mackerel. Scomber 

 japoniciis. and jack mackerel, Trachunis sym- 

 mefricus. all pelagic fish of the California Cur- 

 rent, may be similarly distributed. 



Labidocerans are not diurnal vertical migrants 

 and seem to be confined near the surface of the 

 sea. Oblique tows with a plankton recorder 

 (Longhurst et al., 1966) were taken in an area 

 where Labidocera trispinosa and anchovy larvae 

 are known to occur. The results are shown in 

 our Table 3. The volume of each discrete sample 

 at a particular depth was small (4-6 m°) and the 

 zeros may simply indicate relatively low abund- 

 ance below the surface. Both tows, taken a day 

 apart in close pro.ximity to one another, showed 

 that L. trispinosa was mainly present above 

 10 m, as were anchovy larvae. Simultaneously 

 10-min neuston tows were taken which filtered 

 463 m" no deeper than 30 cm at the surface of 

 the sea (Table 4). The large number of Labi- 

 docera trispinosa and anchovy larvae in these 

 tows suggests that the upper 30 cm of the ocean 



665 



