Table 5. — Electivity indices for seven species of zooplankton 

 which are retained by the 0.363-mm mesh net as adults. The 

 range for the mean is calculated from the replicate samples from 

 the proportion of the numbers and the proportion of the calcu- 

 lated organic weight present in the sample and in the stomach of 

 the ctenophores. Refer to the text for further details. 



Table 6. — Diel variation in the percentage of ctenophores which 

 have empty stomachs. The numbers in parentheses are the 

 numbers of specimens examined per sample. 



empty stomachs at midnight and midday were 

 small, the medians being 71% and 68%, respec- 

 tively (Table 6). These medians are not 

 significantly different as determined by a two- 

 tailed U-test (P>>0.20). It is concluded that no 

 day-night differences exist in the proportion of 

 the postlarval ctenophores feeding, at least at the 

 time of this study. 



The prey categories most frequently found dur- 

 ing the diel study in both day and night stomach 

 contents were copepods and cladocerans — A. 

 tonsa, unidentified copepods, P. parvus, copepod 

 nauplii, Clausocalanus spp.,E. tergestina, C. ang- 

 licus, Oithona spp.,£J. acutifrons, Oncaea spp., and 

 unidentified material. In these samples A. tonsa 

 was over 50% of all prey by numbers. The species 

 which were present in stomachs of P. bachei 

 captured at night include the same groups cap- 

 tured during the day, the larger and deeper-living 



FISHERY BUM tTlN: VOl 72. NO. 2 



copepods, euphausiids and other crustaceans 

 being absent in the stomachs. If redundancy in the 

 presence of prey species day and night (a qualita- 

 tive aspect) occurs at other times of the year, then 

 the prey species of Pleurobrachia could be ade- 

 quately described by stomach analyses of 

 ctenophores captured during the daytime. How- 

 ever, in quantitative aspect diel variations of per- 

 centages of each species may vary. For two of the 

 species most frequently present, A. tonsa and P. 

 parvus, the results are different. As determined by 

 a two-tailed U-test, there is no significant differ- 

 ence in median percentage Acartia of the total 

 number of prey for day vs. night samples (P>0.20); 

 however, the same test ior Par acalanus indicates 

 significant day-night differences (P<0.05), there 

 being more frequent occurrences at night than 

 during the day. Further investigation of prey 

 selection by Pleurobrachia in relation to 

 time-space distributions of prey and predators is 

 important for understanding the ecology of P. 

 bachei but beyond the scope of the present study. 



The stomach contents (on the basis of numbers 

 and mass of organic carbon) of postlarval 

 Pleurobrachia in 10 size classes over the period 8 

 March 1970 to 2 June 1971 indicated some pat- 

 terns in the frequency distribution of prey 

 categories (Table 7). The patterns or trends exist 

 as four types: (I) decreasing frequency with in- 

 creasing ctenophore size, (II) increasing frequency 

 with increasing ctenophore size, (III) little change 

 in frequency with increasing ctenophore size, and 

 (IV) non-systematic change and low frequency of 

 occurrence for all ctenophore sizes. Examples of 

 each pattern type are: (I) E. acutifrons, copepod 

 eggs,^. spinifera; (IDA. tonsa, L. trispinosa; (III) 

 Oithona spp., C. anglicus, P. parvus, copepod 

 nauplii; (IV) Rhincalanus nasutus, euphausiid 

 calyptopis, brachyuran zoea, Sagitta euneritica. 



These results are subject to several sources of 

 bias, three of which are: 1) the occurrence of the 

 stomach contents of ctenophore prey in the 

 stomachs of ctenophores, 2) the numbers of obser- 

 vations per ctenophore size category and the 

 number of total occurrences per prey category, and 

 3) seasonal variations in the length- 

 frequency distributions of ctenophores and their 

 co-occurrences with prey. The diatom and dino- 

 flagellate prey categories may be biased toward 

 higher frequencies of occurrence if some of these 

 types of organisms which occur in the stomachs of 

 herbivores are released into the gut of a 

 ctenophore during digestion. Fortunately, these 



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