THAYER ET AL.: CALORIC MEASUREMENTS OF ORGANISMS 



been the result of increased lipid content prior 

 to spring spawning. We feel that the large var- 

 iation observed for the ctenophores was partially 

 the result of high energy food in the gut contents. 

 February and March are periods of high zoo- 

 plankton abundance in the Newport River estu- 

 ary, and ctenophores may exert heavy predation 

 pressure on zooplankton (Hyman, 1940; Her- 

 man, Mihursky, and McErlean, 1968). Conse- 

 quently, high energy zooplankton (Comita and 

 Schindler, 1963) in the gut may have caused the 

 high caloric values obtained during this period. 



The Osteichthyes showed trends in their cal- 

 oric content which were associated with life 

 history stages and feeding habits. Adult fishes 

 did not show the temporal variation in caloric 

 values which was observed for many of the other 

 taxonomic groups (Appendix Table 1). Post- 

 larval and juvenile stages generally had higher 

 caloric (per unit dry weight and per unit ash- 

 free dry weight) and lower ash contents than 

 their adult stage. The higher caloric content per 

 unit dry weight is of course partly due to the 

 lower ash content. The higher caloric content 

 per unit ash-free dry weight is probably the re- 

 sult of consumption of high energy food such as 

 zooplankton (Comita and Schindler, 1963) by 

 postlarval and juvenile fishes. Adult Brevoortia 

 tyrannus, a planktonic feeder, tended to have 

 have higher (but not significant) caloric values 

 (mean 6.018 kcal/g ash-free dry weight) than 

 adult carnivore-omnivore feeding types (mean 

 5.748 kcal/g ash-free dry weight) . The higher 

 values obtained for menhaden are not surprising 

 since up to 28.7% of the wet weight of these fish- 

 es may be fats (Perkins and Dahlberg, 1971). 



On the basis of limited data Slobodkin and 

 Richman (1961) argued that the frequency dis- 

 tribution of energy (per gram ash-free dry 

 weight) in a "haphazard collection of species" 

 is skewed right, i.e., the modal frequency is at 

 the lower end of the energy range. They explain 

 this distribution by stating that ". , . there has 

 always been selection for maximum number of 

 reproducing progeny, but only sporadic selection 

 for high energy content/gm." Even if there al- 

 ways has been selection for maximum number of 

 reproducing progeny, the analysis of energy con- 

 tent relative to spawning time will afl^ect the re- 



sults. The argument also ignores the evolution 

 of other life history strategies resulting in fewer 

 offspring with higher survival rates (Mac 

 Arthur and Wilson, 1967; Gadgil and Bossert, 

 1970) where there may be long-term storage 

 of energy in individual organisms. Cummins 

 and Wuycheck (1970) presented a frequency 

 distribution similar to that of Slobodkin and 

 Richman (1961) and stated that the distribution 

 resulted from the predominance of plant values 

 in their data. We suggest, with Paine (1964), 

 that it may have been premature for Slobodkin 

 and Richman to recognize a particular type of 

 distribution. By combining the results of 15 

 species of invertebrates with Slobodkin and Rich- 

 man's data on 17 species, Paine observed a more 

 symmetrical distribution. 



If, as indicated by our samples, there is an 

 evolutionary trend toward increased energy con- 

 tent, the predominance of structurally more ad- 

 vanced species in our sample would result in a 

 frequency distribution skewed left as shown in 

 Figure 2. The predominance of advanced spe- 

 cies in our samples is consistent with available 

 check lists for the Beaufort area (Duke Uni- 

 versity Marine Laboratory, 1953; Turner and 



40 « 4 4 8 5 2 S.e 6.0 6.4 6.8 7.2 7.6 8.0 8.4 

 KCAL/G ASH-FREE DIY WII6HT 



Figure 2. — Frequency distribution of energy in a system 

 of shallow estuaries. The values are means for the 51 

 species presented in Appendix Table 1. 



293 



