FISHERY BULLETIN: VOL. 72. NO. 2 



Table 7. — Continued 



taxa made up less than 2% of any one category by 

 numbers and less on a mass basis because of their 

 small size. The number of observations per 

 ctenophore size category are similar for classes 

 1-6, but thereafter they decrease sevenfold. There 

 are few observations in the last two size classes, 

 because these sizes are relatively infrequent and 

 occur in high numbers only during July- August. 

 Some of the larger copepods (e.g., Calanus, Met- 

 ridia, and Rhincalanus) occur relatively infre- 

 quently in stomachs, perhaps because of their rel- 

 ative rarity and spatial separation from the 

 ctenophores. Other infrequent groups such as fish 

 eggs, cirriped nauplii, euphausiids, etc., may not 

 be spatially separated but are perhaps rare, not 

 selected as prey or are unavailable because of 

 temporal separation during different seasons. 

 Whatever the reasons for the infrequent occur- 

 rence of these groups in the stomachs of 

 Pleurobrachia, the data for these prey are much 

 less reliable and many more observations are re- 

 quired to establish patterns of occurrence with 

 size of the ctenophore predator. The potential ef- 

 fect of seasonal variations in size-frequency dis- 

 tribution of ctenophores and co-occurrence of prey 

 on patterns of stomach contents is suggested when 

 the annual data are examined separately by sea- 



sons. The results indicate that some prey are very 

 seasonal in occurrence, while many are present 

 throughout the year. The seasonal data are given 

 (Table 8) for only those categories which showed 

 strong seasonal variations. The first two prey 

 were most frequent in summer-fall, E. tergestina 

 in summer-winter, the next four in fall and the 

 last one in winter-spring. Note the differences in 

 occurrence of three species of Evadne regarding 

 seasonal separation and predation by different 

 sizes oi Pleurobrachia. 



When all stomach content data are grouped to 

 include all sizes of postlarval Pleurobrachia and 

 prey categories are ordered by rank of occurrence, 

 the results show that A. tonsa and £. acutifrons 

 account for nearly one-half of all prey items 

 (Table 9). Thereafter, the percentage contribu- 

 tion from each category decreases to less than 1% 

 by the sixteenth category, at which point the 

 cumulative percentage is 94.2%. On a mass basis 

 Calanus, Labidocera, and Sagitta join Acartia as 

 the main large prey items. While these larger 

 items may afford good growth to a few individu- 

 als, most of the ctenophore population is being 

 nourished by A. tonsa, E. acutifrons and sev- 

 eral other species of copepods and cladocerans. 



Variations in the standing stock of food avail- 



318 



