HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEI IN LA JOLLA BIGHT 



Table 8. — Seasonal variations in the stomach contents of ctenophores 

 for eight prey categories. All values are expressed as the number of 

 occurrences per 1,000 stomachs. The seasons spring (SP), summer 

 (SU), fall (F) and winter (W) are groups of three months starting with 

 February 1970 and ending in January 1971; the following spring 1971 

 is also included. The dashed lines indicate absence of data. 



able to Pleurobrachia from 18 June 1970 to 2 June 

 1971 showed a twentyfold range, with high val- 

 ues in May to early November and low values 

 from mid-November to mid-March (Table 10). 

 The food concentrations were about 10-30 mg 

 C/m^ during the summer-fall maxima in num- 

 bers and standing stocks of Pleurobrachia 

 postlarvae. The decrease in abundance of 

 Pleurobrachia during November and December 

 (see Figure 13) was associated with a fivefold de- 

 crease in the standing stock of prey. The winter 

 increase of P. bachei occurred while food con- 

 centration doubled from the minimum in De- 



cember. The higher food concentrations in May 

 and June do not seem to cause increases in 

 ctenophore abundance. 



DISCUSSION 



The vertical distribution of P. bachei in La Jolla 

 Bight is related to the diel light-dark cycles, but 

 in reverse to the pattern for most migrating zoo- 

 plankton; the pattern is the result of one or more 

 causes of differing selective advantage to the tem- 

 poral persistence of this species. Four potential 



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