KJELSON and JOHNSON: FEEDING ECOLOGY OF POSTLARVAL PINFISH AND SPOT 



A = amount ingested + 1 

 B = evacuation rate constant 

 t = time. 



By adding 1 to the amount ingested and to gut 

 contents we were able to include empty gastroin- 

 testinal tracts in our calculations. From the 

 above equation, with log base 10: 



2.303 (log A + Bt) 



and the instantaneous evacuation rate 



dC 2.303 (log A + Bt) 



— = 2.303 Be 

 dt 



or 



dC 

 dt 



2.303 fiC. 



Feeding Periodicity 



Diel periodicity of digestive tract contents indi- 

 cated the intensity and chronology of feeding by 

 the fish. Our purpose was to refine the feeding 

 chronology curve (Kjelson et al. 1975) by taking 

 samples more frequently than in our previous 

 study. Ten fish of each species were collected at 2-h 

 intervals between 0600 and 1800 and at 2100 and 

 2400 h. Fewer samples were taken at night be- 

 cause our past observations have shown that lar- 

 val fish cease feeding during darkness. All fish 

 were measured, the copepods they contained 

 counted, and a geometric mean for copepods per 

 fish calculated for each sample. 



Daily Rations 



One objective of this research was to re-estimate 

 the daily ration of larval fish for comparison with 

 our earlier study. Daily rations were calculated by 

 the same technique (Kjelson et al. 1975 ) using new 

 information on diel periodicity of gut contents and 

 refined measurements of instantaneous evacua- 

 tion rates. Our method of calculating daily ration 

 accounts for changes in evacuation rate which ac- 

 company diel changes in feeding intensity. 



To calculate daily ration, we first estimated the 

 average evacuation rates (in copepods per hour) 

 for each of the 2-, 3-, or 6-h sampling periods in our 

 feeding chronology study. This average rate was 

 the geometric mean of the instantaneous evacua- 



tion rates at the beginning and end of each period. 

 The estimate of food evacuated during any period 

 is equal to the number of hours in the period mul- 

 tiplied by the respective average hourly evacua- 

 tion rate. The total food evacuated per day was 

 computed by summing the nine respective evacua- 

 tion estimates, and is an estimate of the daily 

 ration because the average ingestion rate must 

 equal the rate at which material in the gut is 

 assimilated or defecated. 



Daily rations were calculated initially as 

 copepods per fish per day and then transformed to 

 percent of the larval body weight and calories per 

 fish per day. Dry weights of ingested copepods 

 were estimated from the length-weight relation- 

 ship: W = 6.274L - 0.153 where W is the dry 

 weight in micrograms andL is the copepod length 

 in millimeters, based upon Heinle's (1966) data for 

 all stages oi Acartia tonsa. Copepod dry weights 

 were converted to wet weights using a factor of 9.1 

 based upon our measurements of the wet-dry ratio 

 for zooplankton and were compared with wet 

 weights of the fish to compute the daily ration as a 

 percent of live body weight. Daily caloric intake 

 was computed using our estimation of 0.555 cal/ 

 mg wet weight of an average size copepod, based 

 on micro-bomb calorimeter measurements of 

 mixed estuarine zooplankton (Thayer et al. 1974). 



RESULTS AND DISCUSSION 



Effects of Current Speed on 

 Feeding Intensity 



Pinfish and spot larvae collected along the shore 

 had more copepods present in their digestive 

 tracts than those collected in midchannel (Table 

 1). Previous observations (Kjelson et al. 1975) in- 

 dicated that neither pinfish nor spot regurgitate or 

 defecate food under the stress of capture or han- 

 dling. Thus, differences in collecting techniques 



Table l. — Digestive tract contents of larval fishes collected 

 at midday at midchannel and shore stations in the Newport 

 River estuary, January to February 1974. 



'No current was observed along the shore on any sannple date. 

 2LF = late flood. IVIE = nnid ebb, HS = high slack. 



425 



