QUANTITATIVE-ANALYSIS OF CARNIVORY IN THE KRILL NYCTIPHANES-AUSTRALIS, WITH AN EXAMINATION OF THE EFFECT OF NONPREFERRED PHYTOPLANKTON ALTERNATIVE PREY

The functional responses of Nyctiphanes australis feeding on the copepod Acartia spp. (hereafter Acartia) at 0.5 to 15 copepods 1-1 and on the diatom Chaetoceros gracilis at 1.7 to 14.8 mug chl a 1-1 were measured to provide the first quantitative information on carnivorous feeding rates for this krill. Predation rates on Acartia 10.004 to 0.144 Acartia (mg acetone-extracted dry weight, E(dw))-1 h-1] were comparable to those of more carnivorous krill and were up to 32 times higher than grazing rates on C. gracilis. Ingestion rates on C gracilis were very low (0.45 to 3.45 ng pigment mg-1 E(dw) h-1), but because of their small size, algae like C gracilis may be suboptimal food for N. australis. We used a Holling type III model to describe the predation functional response. A Michaelis-Menten model best described the grazing functional response. N. australis ingested Acartia at significantly lower rates when algae were present. This effect was independent of algal concentration: ingestion rates of Acartia at 3.1, 7.5 and 17.0 mug chl a l-1 were not significantly different. Parameter estimates showed that the presence of C gracilis reduced the attack rate on, and increased the handling time of, Acartia. Presumably the increased time spent processing (capturing, handling, ingesting) C. gracilis reduced the time available to search for Acartia. Krill ingested pigment faster at high copepod concentrations. Analysis of gut pigment in N. australis feeding on Acartia alone showed that significant quantities of pigment can be derived from either the stomachs of ingested prey or copepod faeces. The implications of this finding are that herbivorous feeding rates measured in situ by the gut fluorescence technique may be biased for omnivorous zooplankton.