KJELSON ET AL.: FEEDING ECOLOGY OF POSTLARVAL FISHES 



of the three larval fish species were fed concen- 

 trated amounts oi Artemia (> 0.3 nauplius/cm^) 

 and allowed to feed until their digestive tracts 

 were full. Each species then was transferred to 

 food-free containers and separated into two 

 groups, one handled roughly and another handled 

 gently. Fish were sampled immediately and 

 every 2 h thereafter. The rough treatment was 

 similar to that used to study the influence of 

 handling on gut content. The gently handled 

 fish were sampled by dipping them carefully 

 out of the tank with a beaker and anesthetizing 

 them prior to dissection. The similarities of the 

 regression coefficients (Table 6) for fish of the 

 same species under the two treatments indicate 

 that evacuation rates were not affected by rough 

 treatment. The higher B value for roughly 

 handled menhaden was not significantly dif- 

 ferent. Thus, our use of laboratory evacuation 

 data to represent the normal evacuation in 

 nature appears reasonable. 



The regression coefficients ior Artemia nauplii 

 evacuation were larger (B values ranging from 

 -0.18 to -0.34) than those for copepod evacua- 

 tion (5 values ranging from -0.08 to -0.17) for 

 all three species (Tables 5, 6). This was expected 

 since the Artemia nauplii were estimated to be 

 only one-half the volume of copepods ingested 

 by the larvae. 



Food quality may also affect evacuation rate. 

 Rosenthal and Hempel (1970) working with 

 herring larvae found that Artemia nauplii were 

 not digested as completely as copepods. We also 

 observed that copepods become transparent in 

 the posterior gut, whereas Artemia nauplii re- 

 mained opaque. 



The variation in the numbers of prey per 

 larva between individual menhaden and pinfish 



larvae increased with each successive sampling 

 period (0, 2, 4, 6, and 8 h after feeding stopped), 

 but fluctuated in spot. The increasing variation 

 in menhaden and pinfish may be explained by 

 differences in individual evacuation rates. Food 

 densities and gut capacities were relatively con- 

 stant for the individual larva and thus, the initial 

 numbers of prey per larva were similar. Varied 

 individual evacuation rates would influence the 

 amounts present in the tracts of the fish sampled 

 at later times and therefore increase the varia- 

 tion. Individual fish may have significantly dif- 

 ferent evacuation rate constants as has been 

 shown for juvenile pinfish (Peters and Hoss 1974). 



DAILY RATIONS 



The estimated daily rations for the three larval 

 fish species varied between 3.5 and 9.0% of the 

 mean wet weight of the fish or from 38 to 99 

 copepods/fishday. The daily ration estimate for 

 menhaden larvae (Table 7) was corrected by a 

 factor of 2.5 to account for the fact that men- 

 haden larvae lose approximately 60-68% of their 

 gut contents during capture and subsequent 

 handlings (Tables 2, 4). Since pinfish and spot 

 larvae did not lose food from their gut when put 

 under the stress, no correction factor was used. 



Two estimates of daily ration, based upon both 

 the 1972 and 1973 haul seine-dip net collections 

 (Figure 1), are provided for spot larvae (Table 7). 

 The two spot rations (4.3% and 9.0% of the body 

 weight) differ considerably, probably due to dif- 

 ferences in food availability. 



Measurements of larval metabolic expenditures 

 based on O2 consumption (D. E. Hoss and W. F. 

 Hettler, Jr., Atlantic Estuarine Fisheries Center, 



Table 7. — Daily rations calculated from feeding studies and O2 con- 

 sumption measurements at 15°-17°C for larval Atlantic menhaden, pinfish, 

 and spot in the Newport River estuary, N.C. 



143 



