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 copep od Acartia spp. (hereafter Acartia) at 0.5 to 15 copepods 1-1 and on t he diatom Chaetoceros gracilis at 1.7 to 14.8 mug chl a 1-1 were measu red to provide the first quantitative information on carnivorous feedi ng rates for this krill. Predation rates on Acartia 10.004 to 0.144 Ac artia (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 sm all size, algae like C gracilis may be suboptimal food for N. australi s. We used a Holling type III model to describe the predation function al response. A Michaelis-Menten model best described the grazing funct ional response. N. australis ingested Acartia at significantly lower r ates when algae were present. This effect was independent of algal con centration: 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 t he handling time of, Acartia. Presumably the increased time spent proc essing (capturing, handling, ingesting) C. gracilis reduced the time a vailable to search for Acartia. Krill ingested pigment faster at high copepod concentrations. Analysis of gut pigment in N. australis feedin g on Acartia alone showed that significant quantities of pigment can b e derived from either the stomachs of ingested prey or copepod faeces. The implications of this finding are that herbivorous feeding rates m easured in situ by the gut fluorescence technique may be biased for om nivorous zooplankton.