FISHERY BULLETIN: VOL. 72. NO. 2 



this size in nature. Regulation of population 

 growth rate would be very sensitive to changes in 

 mortality rates during production of the first 

 50-100 young and again during the production of 

 the several hundred young by later stages. 



The seasonal occurrence of//, mediterranea and 

 the frequency distribution of single and multiple 

 infections and of the percentage of cases for differ- 

 ent stages of hosts show two kinds of patterns that 

 are related to life history episodes: 1 ) the parasites 

 do not often attack the 1- to 2-mm stage postlar- 

 vae which are important to net reproduction as 

 discussed above and 2) the parasites occur mainly 

 as one or two individuals per host and most fre- 

 quently in 6- to 8-mm postlarvae. The early 

 stages of parasites infect the larger hosts more 

 frequently than the smaller hosts because of sea- 

 sonal availability and perhaps also because of the 

 ability of the larger hosts to better accommodate 

 the extra metabolic burden. The "strategy" of the 

 parasites appears to be infection of larger hosts 

 with few young to provide sufficient food and shel- 

 ter during their development, but not overexploit 

 each host with too many parasites. The larger 

 stages of hosts are buffered against local extinc- 

 tion by adult parasites, because suitable hosts be- 

 come more difficult to locate the faster they die. 

 The total ctenophore population has some protec- 

 tion from overexploitation of postlarvae by para- 

 sites and other predators in the presence of rela- 

 tively large numbers of eggs and larvae and the 

 ability of young postlarvae to reproduce soon after 

 development to 1-mm size. 



The calculated population growth rates of P. 

 bachei indicate that the minimum time for a popu- 

 lation doubling is about 35 days (0.693/0.02). This 

 suggests that rapid increases of Pleurobrachia 

 observed on a time scale less than a month are 

 probably due to gross advective change if refer- 

 ence of a "bloom" is made to total abundance of all 

 stages. However, the growth in bodily size of 

 Pleurobrachia from 2 mm to 6-7 mm diameter 

 may occur in about 2 wk, and this may account for 

 the visual impression of a bloom. Regarding indi- 

 vidual and potential population growth rates the 

 salp Thalia democratica as another macrozoo- 

 plankter, is much faster than P. bachei (Heron, 

 1972a, b). 



The statistical treatment of variances for mean 

 net production per day describes precision of the 

 estimates, which probably is not the same as inac- 

 curacy in the estimates. For example, it is ques- 

 tionable whether growth rates in the laboratory 



under constant temperature, food concentration, 

 and food type are accurate estimates of the rates in 

 nature. Variation during a day in ambient condi- 

 tions appear to be at least as important or more 

 important than the average condition (e.g., temp- 

 erature). The rates of tissue growth and mortality 

 both depend on the duration of development 

 within a stage, and they are not fully independent 

 variables although they are treated as such in 

 Equation (2). Another error ignored in the statis- 

 tical treatment is the variance of the standing 

 stock calculated for each replicate sample. I as- 

 sumed in the calculation of the net production per 

 day for each replicate sample at one station that 

 the variance for the best estimate of the crop is 

 negligible compared to the deviations between the 

 best estimates from the regression equations for 

 each sample. The net production per day for each 

 replicate sample is based on the mean rates of 

 growth and mortality and the best single estimate 

 of the standing crop. 



The variance for the mean value of the ANP 

 depends on the variance of the mean net produc- 

 tion per day and the square of the time interval 

 over which the rate is linearly applied (Equation 

 (4)). Assuming that the data on net production per 

 day would have a Poisson distribution (variance 

 equals the mean), reasonably small 95% 

 confidence limits for the annual net production 

 (ANR±ANP/10) are obtained with repHcate sam- 

 ples if each of ten sampling dates is spaced 

 evenly during the year. The limits are relatively 

 insensitive to whether the seasonal distribution of 

 production is rectangular and continuous, rectan- 

 gular and discontinuous, or triangular and discon- 

 tinuous. The important considerations to 

 minimize the confidence limits for the mean an- 

 nual production are: 1) the number of observations 

 per sampling date, 2) the number of sampling 

 dates, and 3) the time interval between sampling 

 dates in relation to the seasonal maximum abun- 

 dance and rate of production. The number of ob- 

 servations per sampling date is determined by the 

 number of replicate samples and the number of 

 stations. More stations and replicate samples im- 

 prove the accuracy in estimating the mean and 

 should decrease the variance of the overall mean 

 for a given sampling date. The number of sam- 

 pling dates minus one is the number of degrees of 

 freedom for the ^-statistic which is multiplied by 

 the standard deviation of the mean to give one tail 

 of the confidence limit. The time interval between 

 sampling dates will affect the variance for the net 



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