Conochilus in Lake Washington

The rotifer Conochilus unicornis appeared in Lake Washington sporadically and usually in small numbers during a total of 28 years of observation since 1933. Conochilus hippocrepis was present even less frequently until the 3 year period 1977–1979, when it became extraordinarily abundant. The abundances of food organisms and known predators have been examined.The only consistent correlation found was between the seasonal maximum of Leptodora and the decrease of C. hippocrepis from its peak abundance each year. Leptodora was generally less abundant during 1977–1979 than during most of the preceding years and the years following the C. hippocrepis episode. No such correlations were found with Epischura, Cyclops or edible algae.In July 1985, there was a small resurgence of C. hippocrepis preceded by a small population of C. unicornis. Leptodora was less abundant than in the immediately preceding years. Observations were made on stomach contents of both animals and colony size of C. hippocrepis. Recommendations are made for programs of new field studies and laboratory experiments to establish the actual requirements for successful populations of Conochilus.