FISHERY BULLETIN: VOL. 12. NO. 2 



NUMBER PER m^ 



20 10 



TEMPERATURE, °C 



Figure 10. — The vertical distributions o{ Pleurobrachia and 

 temperature during the study of 22 May to 1 June 1970. Profiles 

 are during the evening of 26 May and the following morning. The 

 times of sampling, the depth of the water, and the numbers of 

 ctenophores per square meter of sea surface are also indicated. 



night at about 13°C. In addition, individuals may 

 have been displaced from each other horizontally 

 during the migration by currents moving at dif- 

 ferent velocities at different depths (Hardy, 1935). 

 A physical process of this nature superimposed on 

 the biological activity of diel vertical migration 

 may help account for the observed variations in 

 numbers of ctenophores per square meter. Profiles 

 taken from 0900 to 1530 h 5 November and from 

 0155 to 0350 h 6 November indicated abundances 

 threefold or fourfold higher than at other times. It 

 is apparent that following a target population 

 with drogues will have limited success over in- 

 creasingly longer time periods, even for cases in 

 which vertical migration is restricted to shallow 

 depths. 



The vertical distribution of Pleurobrachia on 

 the evening of 26 May 1970 and the following 

 morning (Figure 10) showed patterns similar to 

 those observed in November 1969, but with more 

 clearly defined vertical migration from about 40 m 

 at night to about 10 m the next morning. Note that 

 very low abundances occurred below 65 m day or 

 night. The size frequency distribution of 

 ctenophores in these samples was more rectangu- 

 lar than in the previous November, with a range of 

 1-12 mm and the 5-9 mm sizes being most fre- 

 quent. Again no larvae or eggs were found in these 

 bongo samples, and it is suspected that these smal- 

 ler, more delicate stages may have been broken 

 during sampling or they did not occur in sufficient 

 abundance to be counted in small subsamples. 



1000 



100 



E 



o 

 o 



q: 0- 



DEL MAR 



100 — 



10 



S.I.O. 



100 -' 



10 



J L 



_L 



cc 



UJ 

 QD 



3 



1000 — 



TORREY PINES 

 100 



X 



OCEAN ,,„,. 

 BEACH V'^^^ 



100 



PACIFIC BEACH 

 100 



.t. 



ot 



^ 



10 — 



■t 



POINT 

 LOMA 



I 



-t 



10 10 

 DISTANCE OFFSHORE, km 



10 



Figure 11. — The horizontal, offshore distribution oi Pleuro- 

 brachia postlarvae on 6 November 1969 at stations along six 

 transects perpendicular to shore. Note the tenfold greater abun- 

 dance off Del Mar and Torrey Pines and the break of scale in 

 order to account for absence data. The vertical bar represents the 

 range of values for the replicate samples, and dots indicate that 

 the range is less than the size of the dot; the open symbols off 

 Point Loma without any vertical bar are single observations. 



Thermal stratification was more pronounced than 

 in November 1969, and these ctenophores experi- 

 ence a 7°C average temperature differential dur- 

 ing the course of diel migration. 



The horizontal, offshore distribution of postlar- 

 valP. bachei, as deduced from three field studies, 

 indicated higher abundances at the shoreward 

 stations and lower abundances offshore (Figures 

 11 and 12). Note that: 1) data are presented on 

 semilogarithmic plots to the same linear scale of 

 distance offshore, 2) there is a tenfold lower abun- 

 dance scale shift for stations located south of 

 Scripps Institution in Figure 11, and 3) breaks 

 occur in the scale of abundance to account for 



308 



