Limnological control of brine shrimp population dynamics and cyst production in the Great Salt Lake, Utah

In the Great Salt Lake of Utah, the brine shrimp Artemia franciscana Kellogg is an important food resource for birds and they produce dormant cysts that are harvested and used extensively in the aquaculture industry. We analyzed the limnological factors controlling Artemia growth and cyst production over 12 months in 1994 and 1995. Laboratory experiments showed that inter-brood intervals were highly dependent on temperature and slightly on food level. At optimal temperatures and nutritious food, juveniles reached reproductive size within 7 d in the laboratory. In winter when temperatures were less than 3 degreesC, Artemia were absent from the lake, phytoplankton abundance was high (greater than or equal to13 Chl a mug l(-1)), and the dominant grazers were ciliated protozoans. In the spring, cysts hatched when phytoplankton was abundant (15-30 mug Chl a l(-1)), and the Artemia grew and produced large clutches of ovoviviparous eggs. Estimated naupliar production from these eggs was 80 l(-1) from April to May. Despite the high production of nauplii, Artemia densities declined to 8 l(-1)by June and the growing shrimp population grazed down the phytoplankton resource to <1 mug Chl a l(-1). With the depleted phytoplankton food resource during the summer, Artemia growth slowed, lipid indices decreased, clutch sizes declined, and females switched primarily to oviparous cyst production. During the summer, there was limited production of ovoviviparous eggs, and limited recruitment of juveniles, probably due to low food. Although oviparous reproduction began in June, more than 90% of the cysts were produced after July when female densities had declined to 1.5 l(-1), but nearly all of them were producing cysts. Estimated cyst production was 650 000 m(-2), or 4.54 x 10(6) kg dry weight for the entire lake. The reported commercial harvest took 21% of the 1994 cyst production. That harvest had little impact on the subsequent year's population, as Artemia densities were ultimately controlled by algal production in the lake.