HIROTA: NATURAL HISTORY OF PLEUROBRACHIA BACHEI IN LA JOLLA BIGHT 



terest, a general equation to calculate equal trans- 

 fer efficiency for n transfers is 



ETE ^(NPL/NPP)^'^ 



(5) 



In the equation ETE is the equal transfer 

 efficiency, NPL is the net production of trophic 

 level L (which is equal to ingestion by level L + 1), 

 NPP is the net primary production, and n is the 

 number of transfers from primary producers (the 

 zeroth trophic level) through trophic level L. This 

 equation was derived from the works of Schaefer 

 (1965) and Ryther (1969). The equal transfer 

 efficiency calculated in this manner for n>2 says 

 nothing about the efficiency of a given trophic 

 level, but only the equal efficiency of all trophic 

 levels from primary producers through trophic 

 level L. 



The equal transfer efficiency from primary pro- 

 ducers through P. bachei was calculated using es- 

 timates of annual net primary production in the 

 coastal waters of southern California, ANP of P. 

 bachei, and the weighted mean number of trans- 

 fers from primary producers through P. bachei. 

 The mean number of transfers was calculated 

 from the percentage contribution from each of 21 

 prey categories to the total numbers of prey (these 

 21 categories are 989c of the total numbers of prey 

 in the stomachs of P. bachei over a year) and best 

 guesses as to the number of transfers from prim- 

 ary producers to each of the 21 prey species. The 

 mean and range of ANP by all stages of P. bachei 

 extrapolated to 365 days (5,700 and 8,300-2,500 

 mg organic matter) were converted from units of 

 organic matter into organic carbon by taking 50% 

 of the organic matter as organic carbon. The mean 

 and range of annual net primary production (400 

 and600-200gC m yr ) were estimated from the 

 mean and range of the rates per day in southern 

 California coastal waters (Eppley, Reid, and 

 Strickland, 1970; W. Thomas, pers. commun.) and 

 multiplication by 365. The expectation of the 

 number of transfers from primary producers 

 through P. bachei is 2.3 with an upper limit of the 

 estimate equal to 2.5. These fractions occur be- 

 cause species of animals often do not fall into a 

 single trophic level, and this is in fact the case 

 with P. bachei; some of its prey organisms are 

 herbivorous and some are themselves carnivor- 

 ous. The equal transfer efficiency was calculated 

 and presented in a matrix for the means and 

 ranges of the three variables stated above (Table 



Table 15. — Calculations of the equal transfer efficiency in per- 

 cent for different numbers of transfers from primary producers 

 through Pleurobrachia. given the observed range and mean of 

 annual net production of P. bachei and the estimated range and 

 mean of annual net primary production in g C m^ yr"'. In each 

 group of three values, the first is for the highest value of 

 ctenophore net production (4.1 g C m^ yr '), the second is for the 

 mean (2.8 g C m^ yf ) and the third is for the lowest value ( 1 .2 g C 



15). Note that the range of equal efficiency is found 

 on the diagonal from the lower left to the upper 

 right of the table. The overall central value is an 

 equal efficiency of 11%. This efficiency of transfer 

 involves phytoplankton, herbivores, and those 

 primary carnivores on which P. bachei feeds, P. 

 bachei, and the predators and parasites of P. 

 bachei. This efficiency equals the nth root of the 

 ratio of ingestion by predators of P. bachei to net 

 primary production. 



DISCUSSION 



Life table parameters of P. bachei show adap- 

 tive value in the interdependence of the schedule 

 of births and the rates of development and mortal- 

 ity on population growth. Early reproduction 

 makes a very important contribution to net repro- 

 duction and population growth rate, but only in 

 relation to the rates of development and mortality. 

 The larvae have relatively high rates of mortality 

 and lower rates of growth compared to other 

 stages. The 1- to 2-mm postlarvae have the low- 

 est rate of mortality and grow slowly, but they are 

 able to reproduce at an early age and thereby 

 contribute an important fraction of net reproduc- 

 tion. The 3- to 7-mm stages have very rapid tis- 

 sue growth (instantaneous rates of 0.21-0.47) but 

 do not contribute many young to the population. 

 Instead, these larger stages are important to net 

 production of organic matter because of their 

 rapid growth and high abundance in summer. The 

 stages larger than 8 mm are able to produce 

 enormous numbers of young, but few survive to 



329 



