HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEl IN LA JOLLA BIGHT 



From 8 March to 1 May 1970 sampling for the 

 seasonal variations in the coastal plankton was 

 done at stations located between Del Mar and 

 Scripps Institution 1-3 km from shore. Results of 

 the study of current velocities during 22 May to 1 

 June 1970 indicated that the plankton were ad- 

 vected northward on the order of tens of kilome- 

 ters per week. Therefore from 18 June 1970 to 2 

 June 1971 the sampling stations for seasonal vari- 

 ations in the coastal plankton were changed to 

 those six stations in Figure 4. Five stations are 

 located 1.6 km from shore about 8 km apart be- 

 tween Oceanside and Scripps Institution, and the 

 sixth station is located about 8 km beyond the 

 station off Scripps. Replicate samples were taken 

 at stations 1-5 with each of the two kinds of ring 

 nets described above. Samples with only the 

 0.363-mm mesh net were taken at station 6. 

 Analysis of the 0.363-mm mesh net samples, as 

 described below, was carried out on samples at all 

 stations from 8 March through 29 August 1970 

 (this period includes the seasonal maximum in 

 abundance). From 29 August 1970 to 2 June 1971 

 mainly samples from stations 1, 3, 5, and 6 were 

 analyzed once it became apparent that the varia- 

 tions between the five stations parallel to shore 

 could be about as well accounted for by variations 

 at stations 1, 3, and 5. 



Samples were taken on two occasions for special 

 studies related to the diel variation in predation 

 by Pleurobrachia, their stomach contents in net 

 tows of short duration and their selectivity of prey 

 species with which they co-occur. During a field 

 study from 23 to 27 July 1971, five sets of tripli- , 

 cate samples were taken with the 0.5-m net of 

 0.363-mm mesh in the upper 50 m off Del Mar. 

 Three of these sets were taken at midnight and 

 two sets at midday. All samples were sorted and 

 counted as described below. The ctenophores were 

 measured and the stomach contents identified to 

 determine whether diel variations exist in: 1) the 

 proportion of ctenophores which contain prey and 

 2) the numbers and kinds of prey which occur in 

 stomachs during the day and at night. This study 

 is important because all other information about 

 the stomach contents of P. bachei during the sea- 

 sonal study were derived entirely from samples 

 taken between 0900 and 1600 h. On 25 August 

 1970 a pair of samples was taken at the surface 

 with the 0.363-mm mesh net on station 5 at 

 1500 h. The tows were for 60 s duration and the 

 maximum period which prey were at risk is 95 s. 

 Samples were sorted and counted and the stomach 



contents of ctenophores identified for: 1) compari- 

 son of these prey species to other data from field 

 samples of longer sample durations and periods at 

 risk, and 2) calculations of the electivity indices of 

 prey on a numerical and organic weight basis. 

 Counts were made of all zooplankters in 2.5% sub- 

 samples of each net tow, and the proportions of 

 prey in stomachs and in the net samples were used 

 to calculate electivity indices (Ivlev, 1961). 



Whole samples of each of two replicates per sta- 

 tion taken with the 0.363-mm mesh net were 

 sorted at 6-12x magnification under a dissecting 

 microscope, and all postlarval ctenophores were 

 counted and measured in polar diameter with an 

 ocular micrometer. These procedures apply to all 

 field samples taken for the offshore distribution, 

 seasonal distribution, and special sets of samples 

 taken for the analysis of diel variations in feeding 

 and prey selectivities. For the sets of samples 

 taken during the seasonal study, postlarvae of one 

 or the other replicate sample selected at random 

 were dissected and the contents of stomachs iden- 

 tified and given one of four scores described above. 

 All specimens were examined if there were less 

 than about 100/sample, but during a few periods of 

 high abundances subsamples of about 50 speci- 

 mens were taken. For each of 30 sampling dates 

 between 8 March 1970 and 2 June 1971, data on 

 stomach contents of about 100 specimens were 

 obtained, except on those dates with very few 

 specimens captured in all samples lumped to- 

 gether. For these same ctenophores which were 

 measured and dissected, counts were also made of 

 the numbers of larval and early juvenile stages of 

 the facultative endoparastic amphipod, H. 

 mediterranea. Also enumerated in these samples 

 of the seasonal study were the numbers of adult 

 and late juvenile//, mediterranea, which were not 

 attached to ctenophores, and the numbers of 

 Bero'e sp. 



Each replicate sample of the eggs and larvae 

 of P. bachei taken at station 5 with the 0.035 

 mm mesh net was concentrated to 400 ml by 

 settling overnight, siphoning off the excess 

 water and transferring it to a graduated cylinder. 

 Each of two subsamples of 20 ml was removed 

 by Stempel pipet, examined under 12-25x 

 magnification and the numbers of eggs and lar- 

 vae counted. Numbers per square meter of sea 

 surface were calculated as ten times the total 

 numbers in both subsamples times the ratio of 

 the sample depth in meters to the volume of 

 water filtered in cubic meters. 



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