FISHERY BULLETIN: VOL. 72. NO. 2 



Table 9. — Rank order in frequency of occurrence by numbers of 

 all developmental stages per prey category in all sizes of 

 Pleurobrachia postlarvae and the corresponding estimates of 

 total carbon mass per prey category. 



Table 10. — Seasonal variations in the calculated standing 

 stocks of prey for Pleurobrachia at station 3. 



Prey category 



Frequency Percent ug C 



Percent 



'The frequency value refers to the nunnber of groups of copepod eggs 

 and the value in parenthesis below it refers to the total number of eggs. 



advantages of migration to P. bachei are the abil- 

 ity to: 1) seek locations and depths with favorable 

 food types and concentrations, 2) seek locations 

 and depths with few predators and parasites, 3) 

 avoid lethal or near-lethal surface temperature 

 after the downward migration at night and in- 

 crease the rate of development by living in 

 warmer water during the day, and 4) maintain the 

 pattern of high abundance close to shore and de- 

 crease the chance of drifting offshore. No data 

 have been collected on quantitative changes in the 

 abundance of prey and predators with P. bachei 

 during vertical migration; such information 

 would enable qualitative evaluation of the effect 

 of these changes on the pattern of vertical dis- 

 tributions. Limited data from the study of diel 

 changes in the species composition of prey in 

 stomachs of ctenophores indicated no major 

 changes between day and night, although 



Sample date 



Mean 

 (mg C/m3) 



Range 

 (mg C/m^) 



18 June 1970 

 14 July 

 13 August 

 29 August 

 22 September 

 5 November 

 18 November 

 16 December 



11 January 1971 

 8 February 



12 March 

 4 May 



2 June 



Paracalanus occurred more frequently in 

 stomachs of ctenophores captured at night. 

 Another major study would be required to quan- 

 tify changes in the temporal and spatial 

 co-occurrence of Pleurobrachia with their prey 

 and predators. The data on vertical and seasonal 

 distribution of P. bachei and the thermal 

 stratification of water; the failure of laboratory 

 cultures at 20°C; and growth experiments in the 

 deep tank facility at 14.5° and 19.5°C are inter- 

 preted to indicate that vertical migration for this 

 ctenophore is beneficial for survival and would 

 optimize the rates of development and increase of 

 bodily mass. A constant temperature of 20°C was 

 detrimental to survival and growth of bodily 

 weight (Figure 1), relative to conditions at 15°C. It 

 is suspected that in August when the ctenophore 

 abundance is highest and the maximum thermal 

 stratification occurs, vertical migration from the 

 surface to 20-m depth increases the chance of 

 survival by lowering the ambient temperature at 

 night by nearly 10°C. In addition, the detrimental 

 effect of high temperature on somatic growth may 

 be decreased and the ctenophores develop at some 

 rate intermediate to the rate at 12° and 22°C. 

 Growth experiments using the deep tank facility 

 in which stratification of temperature is made to 

 simulate conditions in nature might support some 

 of these speculations. Alternatively, laboratory 

 growth experiments could be made in which 

 temperature is varied with a semidiurnal period. 

 Another complication in these experiments, if 

 they are to simulate conditions in the field, is the 

 co-occurrence of the parasite Hyperoche and its 

 possible temperature-dependent effect on the 

 growth and survival of P. bachei. 



A consequence of diel vertical migration in the 



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