Elsevier JEM. 298 I OF INGEST1 CARBON ASSIMILATION AS A FUNCTKI N RATE IN

Abstract: During the larval stage, clupeoids and other fishes are characterized by straight, relatively undifferentiated guts. Where it has been studied in detail, the evacuation rate in these larvae appears to be directly related to ingestion rate, which is in turn related to food concentration. Qualitative assessments have suggested that the degree of digestion of food particles is inversely related to the rate of evacuation. In the present study, early larvae of the Pacific herring, Clupea harengus pallasi Valenciennes, were fed for 1 h upon 14C-labelled rotifers, Brachionusplicatilis Muller, and Artemia nauplii at high densities. At the end of 1 h, larvae were transferred to six food densities, ranging from 0 to 10 unlabelled prey. ml- I. After 22 h, larvae were removed from the feeding tanks and sacrificed. The labelled carbon remaining in the larvae as a function ofthe percent consumed after the I-h feeding interval was used as an index of carbon assimilation. The percent carbon retained decreased significantly with increasing food concentration. Larval herring thus decrease carbon assimilation from individual food particles at high food densities. The magnitude of increasing ingestion, however, more than compensates for the decreased carbon assimilation, and larvae gain greater total energy under conditions of high food concentration. The results support the suggestion that clupeoid larvae are adapted to utilize high food concentrations associated with plankton patches in the pelagic environment