HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEl IN LA JOLLA BIGHT 



production over a given period as the square of the 

 interval; therefore, sampling should be carried out 

 on a regular basis without long intervals between 

 dates unless previous information is available on 

 the seasonal distribution of production and the 

 relationship between means and variances. 



Three parameters can influence the magnitude 

 of net production by P. bachei in addition to errors 

 in the estimation of abundance: 1) water tempera- 

 ture, 2) food supply, and 3) parasitism. In the first 

 two cases, it would appear that for the observed 

 range of temperature and food supply in the study 

 area temperature is more important than food 

 supply. A 10°C range of surface temperature over 

 the year or the temperature change experienced 

 during a postulated 20-m vertical migration in 

 August (see Figure 8) should affect the rate of 

 growth in length and weight and survival. Data 

 are not sufficiently good for quantitative state- 

 ments about the effect of vertical migration dur- 

 ing August on rates of tissue growth, net produc- 

 tion, and population growth. The case for the effect 

 of food supply on rates of net production is some- 

 what better than for temperature. Rates of growth 

 in the laboratory at 15°C are essentially the same 

 for ctenophores cultured on Acartia at 35 /ug 

 C/liter and 500 yug C/liter. The rates of growth in 

 length and weight at 14.5'C and 1-2 ^;g C/liter of 

 mixed natural food organisms indicated that the 

 postlarvae grow about as well as at tenfold higher 

 food concentrations. From field samples in which 

 the calculated food concentration was about 10 Mg 

 C/liter growth rates in the laboratory and from 

 the field size frequency distributions agree within 

 ±20% of the mean rate. It appears therefore that 

 P. bachei postlarvae are very efficient at the ex- 

 traction of prey from the water at very low con- 

 centrations. The estimates of gross growth 

 efficiency showed that over one-half of the food 

 ingested was incorporated into somatic tissues. 

 Perhaps the "passive" feeding mode of these car- 

 nivores allows them to have a very low threshold 

 for the commencement of feeding activity, espe- 

 cially since the area of the tentacles is very large 

 in relation to the bodily size of the ctenophore, and 

 relatively low metabolic expenditure is generated 

 while waiting for prey to contact the tentacles. 



The estimates of ANP by P. bachei are also 

 inaccurate, because no corrections were made for 

 effects of parasitism hy Hyperoche on rates of tis- 

 ' sue growth and duration of development. Since 

 the occurrence of parasitism and high rates of 

 production both were in August (two-thirds of the 



ANP was in August), correction may not be triv- 

 ial. It is not possible to make a quantitative esti- 

 mate of the error based on any data, but the follow- 

 ing sources of error must be considered: 1) the 

 standing stock, B,, was overestimated by the 

 amount of ctenophore tissue in the volume that 

 the parasites occupy; 2) the instantaneous rate of 

 tissue growth, G,, was overestimated by the dif- 

 ference between the instantaneous growth rates of 

 nonparasitized and parasitized ctenophores; 3) the 

 instantaneous rate of mortality, M, , was overes- 

 timated by the difference between the durations of 

 development of nonparasitized and parasitized 

 ctenophores (see Equations 1 and 4) the total loss 

 of ctenophore tissue (due to mortality of all types 

 and to ingestion of tissue by parasites that does 

 not result in mortality) in one time increment was 

 underestimated by that fractional amount of tis- 

 sue removed from the mean standing crop during 

 the time increment by parasitism. Overestima- 

 tion of the standing stock of ctenophores due to 

 presence of parasites is believed to be negligible, 

 especially since only one or two parasites were 

 present in 927c of all cases (Table 4). For given 

 values of standing stock and rates of growth and 

 m.ortality(e.g.,fi, = 100mg/m2,G, =0.2,andM, 

 = 0.5), the effect of additional tissue loss due to 

 parasitism on the rate of net production is rela- 

 tively small (ca. 107c) for instantaneous rates of 

 parasitism up to 50% of the rate of mortality. The 

 mean net production per time interval was overes- 

 timated. The actual extent of the overestimate can 

 not be evaluated without more information on the 

 effect of parasitism on ctenophore growth. 



The ratio of net production per day to mean 

 standing crop during the day for all postlarvae is 

 the biomass-weighted mean instantaneous rate 

 of tissue growth (Allen, 1971), assuming that 

 growth and mortality rates are exponential. The 

 similarity of mean values between stations 1, 3, 

 and 5 is due partly to the bias of having used only 

 the growth rates at 15°C throughout the year, but 

 the range between stages of the mean exponential 

 growth rate is at least tenfold. Some of the consis- 

 tency in ratios of production to mean standing 

 stock is due to similarity in the length frequency 

 distributions between stations and relative con- 

 tribution of different stages to the total crop. The 

 overall annual mean production to standing stock 

 value of 0.202 indicates that net production per 

 day is 20% of the mean daily standing stock. This 

 value is within the range of values summarized by 

 MuUin (1969), but is quite high considering the 



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