KJELSON and JOHNSON: FEEDING ECOLOGY OF POSTLARVAL PINFISH AND SPOT 



values on a per unit weight basis are typical for 

 larvae of different species. Such similarity, how- 

 ever, may not exist for all species and size classes. 



Measurements of postlarval metabolic expendi- 

 tures based on oxygen consumptions at 15°C 

 using a Gilson respirometer (Hoss, pers. com- 

 mun.) and a flowing water respirometer (W. F. 

 Hetter, Jr., pers. commun., Atlantic Estuarine 

 Fisheries Center) are shown in Table 5. In both 

 cases, fish were deprived of food for 24 h prior to 

 measurement of their oxygen consumption, and 

 the oxygen content of the water was near air sat- 

 uration. Both Hoss and Hettler consider their 

 measurements to be routine oxygen consumption 

 as defined by Fry (1971), i.e., the mean rate ob- 

 served in fish whose metabolic rate is influenced 

 by random activity under experimental condi- 

 tions in which movements are presumably some- 

 what restricted and the fish are protected from 

 outside stimuli. Postlarval pinfish and spot in the 

 flowing water respirometer were confined in an 

 11-liter chamber identical to that used for our 

 laboratory current- feeding experiments described 

 earlier and therefore were able to move about 

 considerably. 



A major problem exists in most measurements 

 offish oxygen consumption due to the uncertainty 

 as to the animals state of activity (Altman and 

 Dittmer 1971). Furthermore, measurements of 

 fish respiration under natural conditions, termed 

 normal respiration, have been unattainable; and 

 although many investigators have estimated 

 normal respiration by doubling routine me- 

 tabolism, such a process is felt to be too subjec- 

 tive by Hoss and Peters (in press). 



Considering the requirement for information 

 on fish metabolic needs under natural conditions, 

 it appears that our method of estimating the daily 

 rations of postlarval fishes has potential value. 

 Our estimates of daily rations were higher or 

 equal to those rations estimated from oxygen con- 

 sumption measurements. The observed differ- 

 ences in rations (Table 5) are reasonable if we 

 assume that oxygen consumption measurements, 

 particularly those of Hoss, are closer to routine 

 respiration than to normal. The Hoss data have 

 the lowest values, followed by the Hettler data. 

 These differences, although probably not sig- 

 nificant, are reasonable because the less restric- 

 tive system provided in the flowing water res- 

 pirometer allowed the fish to move about in a 

 manner similar to that in natural water. The lack 

 of feeding activity by fish during respiration mea- 



surements and the respective decrease in oxygen 

 consumption (Warren and Davis 1967) also 

 should account for a lesser daily ration. 



Based on earlier metabolic measurements, 

 Thayer et al. (1974) estimated a daily ration of 

 1.04 cal/fish per day for larval fishes in the New- 

 port River estuary during January and February. 

 They indicated that, with larval energy require- 

 ments of this magnitude and a 90% assimilation 

 efficiency, the larvae would be required to graze 

 on an average of 10% of the zooplankton popula- 

 tion per day. Furthermore, they suggested that 

 this need may indeed have accounted for de- 

 creases in zooplankton observed in the estuary 

 during spring. Our daily rations, based on feeding 

 periodicity and evacuation (Table 5), are some- 

 what larger and tend to support the conclusion, 

 assuming larval densities similar to those pre- 

 sented by Thayer et al. that larval fishes may 

 have a significant impact on copepod populations 

 in this system. 



ACKNOWLEDGMENTS 



We express our sincere appreciation to Ronald 

 L. Garner and Jerry D. Watson for their technical 

 assistance during the entire study. 



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