Patterns of Distribution and Abundance of Tubifex tubifex and Other Aquatic Oligochaetes in Myxobolus cerebralis Enzootic Areas in Pennsylvania

Recent outbreaks of whirling disease (caused by the myxosporean parasite Myxobolus cerebralis ) among wild rainbow trout Oncorhynchus mykiss have called attention to deficiencies in the knowledge of aquatic oligochaete ecology in rivers and streams of North America. Revealing the patterns of distribution and abundance of Tubifex tubifex , an obligate host in the life cycle of M. cerebralis , is critical to understanding local and regional differences in the epizootiology of this disease. To gather fundamental information on the ecology of T. tubifex among streams in Pennsylvania, we surveyed oligochaetes and the features of their habitat. We used indirect and direct ordination techniques to search for patterns in oligochaete assemblages and their relationships to the environment. In general, higher‐gradient streams with coarse substrate contained low‐density oligochaete assemblages dominated by the family Enchytraeidae. Lower‐gradient streams with fine‐sediment habitat were dominated by Limnodrilus hoffmeisteri and contained either Ilyodrilus templetoni or T. tubifex . A key finding to emerge from this survey was that high oligochaete density, T. tubifex occurrence, and T. tubifex density were associated with proximity to obvious sources of organic enrichment. Limits on the distribution of T. tubifex in Pennsylvania relative to distributions in M. cerebralis –affected systems in the Intermountain West may be a factor in explaining why whirling disease has not been observed to cause population declines among wild rainbow trout in this region and should be further investigated.