FISHERY BULLETIN: VOL. 73, NO. 1 



and every 3 or 4 h thereafter, at least 10 fish 

 were killed and the copepods present were 

 counted. Evacuation of newly hatched Artemia 

 nauplii was measured by allowing unfed larvae 

 to feed to satiation on high prey densities (0.3 

 to 3.0 nauplii/cm^), placing them in a food-free 

 environment, and then periodically removing 

 larvae for determination of remaining gut con- 

 tents. Sampling of both copepod- and Artemia -fed 

 fish continued periodically from the time of 

 feeding until more than one-half of the fish had 

 empty tracts. 



Linear regression equations of log-transformed 

 data were used to describe the evacuation process 

 (Peters et al. 1974). The equations were of the 

 form: 



logioC = A + Bt 



where C = 1 + the mean number of copepods 

 or Artemia present in the gut 



t = time 

 A +B = regression terms 



Instantaneous evacuation rates were calculated 

 from the equation — = 2.303 EC (Peters and 



Kjelson in press). 



Daily rations were calculated using information 

 on diel periodicity of gut content and instantan- 

 eous evacuation rates. Previous calculations of 

 fish rations (Bajkov 1935; Seaburg and Moyle 

 1964) have assumed a constant evacuation rate, 

 but more recent data (Tyler 1970; Elliot 1972) 

 indicate that digestive rate changes with the 

 quantity of food in the digestive tract. 



Our method of calculating daily ration (Peters 

 and Kjelson in press) accounts for changes in 

 evacuation rate which accompany diel changes 

 of feeding intensity. To calculate the rations, we 

 first determined an average instantaneous evac- 

 uation rate (in copepods per hour) for each of 

 the 4-h sampling periods in the diel cycle. This 

 average rate was the geometric mean of the 

 instantaneous evacuation rates at the beginning 

 and end of the period. Since each period lasted 

 4 h, the estimate of food evacuated during the 

 period was four times the average instantaneous 

 hourly evacuation rate. The total food evacuated 

 per day was achieved by summing the six 4-h 

 evacuation estimates, and is an estimate of the 

 daily ration, because average ingestion rate must 



equal the rate at which material leaves the gut 

 whether by assimilation or expulsion. 



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 rela- 

 tionship: 



W = 6.274L - 0.153 



where W = dry weight in micrograms 



L = copepod length, based on Heinle's 

 (1966) data for all stages of 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 zooplank- 

 ton, and were compared to 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 during 

 March, based on microbomb calorimeter mea- 

 surements of mixed estuarine zooplankton 

 (Thayer et al. 1974). 



FOOD PREFERENCES 



The larvae we collected were feeding primarily 

 upon copepods, a common food source for both 

 freshwater and marine fish larvae (Werner 1969; 

 May 1970). Copepods composed 99% (by volume 

 and number) of the gut contents of larval spot, 

 pinfish, and menhaden (Table 1). Four copepod 

 taxa (Centropages, Temora, Acartia, and Harpac- 

 ticoida) were dominant. Diatoms, amphipods, 

 barnacle larvae, crab zoea, and ostracods, al- 

 though present in some larvae, were rare. 



Table 1. — Relative (percent) composition by number of the 

 major taxa in the total gut contents of three species of larval fish. 



138 



