HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEI IN LA JOLLA BIGHT 



Table 11. — The life table for Pleurobrachia bachei during 1 

 May- 18 June 1970 based on laboratory growth and reproduction 

 data at 15°C and calculated mean rates of mortality for this 

 period. The symbols dx.lx. bx andx represent the age interval in 

 days, survival to the beginning of the age interval, the numbers 

 of live births during the age interval, and the pivotal age, respec- 

 tively. 



mean age distribution in field samples was 66.7% 

 eggs, 20.0% larvae, 8.7% 1- to 2-mm postlarvae, 

 and 4.7% all other stages. The field age distribu- 

 tion is unlike the stable age distribution in that 

 the proportions of eggs and larvae are reversed 

 and the proportion of late stages is sevenfold 



higher than in the stable age distribution. The 

 population growth rates (r) in other time periods 

 predict decreases of population abundance from 8 

 March to 2 April (r = -0.105) and increases in 14 

 July to 21 August (r = 0.020) and in October (r = 

 0.0115). Eggs and larger ctenophores were again 

 more frequent, and larvae less frequent in the field 

 during July- August than calculated for the stable 

 age distribution. 



For postlarvae the mean rate of net production 

 per day for all stations located 1.6 km from the 

 shore (Table 13) followed the seasonal variation in 

 the standing stocks. The maximum rate of net pro- 

 duction on 13 August, 212 mg organic matter m'^ 

 day , was about 20% of the standing crop. About 

 two-thirds of ANP occurred during August. The 

 variance of the mean ANP is quite large, but since 

 the confidence limits for the mean are determined 

 by standard deviations, the 95% confidence inter- 

 val for the mean ANP is 4,200-6,280 mg organic 

 matter m~^ yr \ 



Mean ANP of postlarvae at stations 1, 3, and 5 

 (1.6 km from shore) and at station 6 (10 km from 

 shore) are given in Table 14. Note that "annual" 

 net production at the stations 1, 3, and 5 are for 

 0.956 yr and at station 6 for 0.84 yr; these values 

 were not corrected to a full year by proportion, 

 because statistical tests based on variances would 

 not be valid. Tests for differences of variances 

 (F-ratio) and means (f-tests) between stations 1, 

 3, and 5 were made. The variance of station 3 was 

 significantly different from that of stations 1 and 5 

 rP<0.01), but the variances of stations 1 and 5 

 were not different from each other (P>0.05). 



The difference between means of all pairs of 

 contrasts for stations 1, 3, and 5 are significant 

 (P<<0.01). The net production at station 6 located 

 10 km from shore off Scripps Institution was about 



Table 12. — Summary of population parameters for P. bachei during four time periods in 1970. The 

 symbols /Jo,'", T,B, b,d andCx refer to net reproduction, instantaneous rate of population growth, 

 generation time, finite birth rate, instantaneous birth rate, instantaneous death rate and stable 

 age distribution respectively. The percentages of eggs (E), larvae (L), 1-2 mm, and s 3 mm stages 

 are given in that order for the stable age distribution. 



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