THE EFFECT OF GREBE PREDATION ON A BRINE SHRIMP POPULATION

Investigations were performed at Mono Lake, California, USA, to determine if the marked autumnal decline in the brine shrimp (A. monica Verrill) population was related to the annual arrival of hundreds of thousands of eared grebes (Podiceps nigricollis Brehm). Grebes and brine shrimp were censused through later summer and autumn of 1980 and 1981, and grebes were collected for stomach analyses. Passage times of Artemia through grebe stomachs were estimated by feeding dyed Artemia to marked grebes and collecting these grebes at intervals after they had been fed. The mean number of Artemia eaten per grebe per day ranged from c. [circa] 8000-70,000. The mean proportion of the Artemia population eaten per day tended to increase throughout the autumn, ranging from 0.5% in Aug. to 22% in late Oct. 1980 and from 0.02% in Aug. to 6.6% in late Nov. 1981. Grebe predation could account for 55-83% of the decline in Artemia density in 1980 and 8-27% of the decline in Artemia density in 1981. In both years, grebe predation could account for substantial amounts, but not all, of the observed declines in Artemia levels, and most Artemia mortality was due to other causes in 1981.