PREDATION BY THE ESTUARINE SHRIMPS CRANGON FRANCISCORUM STIMPSON AND PALAEMON MACRODACTYLUS RATHBUN

¹ Hydrobiology Laboratory, Department of Land, Air and Water Resources, University of California, Davis, California 95616

1. Feeding studies were conducted on the caridean shrimps Crangon franciscorum and Palaemon macrodactylus of the Sacramento-San Joaquin Estuary, California. Foregut contents from 352 specimens of C. franciscorum and 198 specimens of P. macrodactylus collected over six diel cycles in 1976 were itemized.

2. C. franciscorum and P. macrodactylus are mainly carnivorous. Animal material always exceeded plant fragments in frequency of occurrence in foreguts, being respectively 46 to 86% and 7 to 61% in C. franciscorum and 74 to 92% and 0 to 59% in P. macrodactylus. The most frequent prey item identifiable in both predators was the mysid shrimp Neomysis mercedis. Both predators selected for N. mercedis of intermediate lengths (5 to 9 mm), and selected against those 2 mm or > 10 mm long.

3. Day and night differences in the number of individuals of N. mercedis per foregut were significant only for C. franciscorum, and then only in late September and late November, with greater means at night in each case.

4. The percentages of N. mercedis standing crops consumed daily by C. franciscorum and P. macrodactylus respectively, ranged from 0.1 to 6.2% and 0.0 to 4.8% in terms of individuals, and from 0.1 to 5.1% and 0.0 to 3.7% for biomass. The percent of their respective population biomasses that specimens of C. franciscorum and P. macrodactylus respectively consumed daily as N. mercedis ranged from 1 to 3% and 1 to 4%. Similarities between these predators suggest the possibility of competition.