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316 THE AMERICAN MIDLAND NATURALIST 110(2) its isolation prior to the operation of the steam plant, it has not been affected by selenium pollution and its fish fauna has remained intact (Cumbie, 1978). Study organisms. -Bothriocephalus acheilognathi is a pseudophyllidean tapeworm first described in Japan by Yamaguti (1934). It has since been reported throughout Asia, New Zealand, Europe and, as indicated by Hoffman (1980), was introduced into the United States in grass carp in the early 1970s. Since that time it has spread throughout the mid-South and southeastern states. It was introduced into Belews Lake in the mid-1970s; it has not, however, been observed in Charlie's Pond. The life cycle of Bothriocephalus acheilognathi is typical for pseudophyllidean tapeworms. After eggs are released from the definitive host, and a brief period of em- bryonation, amotile coracidium emerges and swims free. Coracidia are ingested by cyclopoid copepods which, in Belews Lake, include Cyclops bicuspidatus thomasi, Mesocyclops edax and Tropocyclops prasinus. The larvae penetrate the gut wall, enter the hemocoel and develop into a procercoid. When the infected cyclopoid is ingested by an appropriate definitive host, the procercoid matures (strobilates) and becomes an egg- shedding adult. The entire life cycle can be completed in 15 days under proper en- vironmental conditions (Hoffman, 1976). Within Belews Lake, three definitive host species have been identified; these in- clude mosquitofish, red shiners and fathead minnows. The primary focus for the pres- ent study is the mosquitofish and its interactions with Bothriocephalus acheilognathi. The biology of Gambusia affinis has been well described by Krumholz (1948). Structure of Gambusia affinis populations. - Mosquitofish were sampled monthly from March 1980 through February 1982 within Charlie's Pond and an ambient temperature site and a thermally altered station in Belews Lake. The Belews Lake sites were immediately adjacent to those used in other parts of the study as reported by Granath and Esch (1983a, 1983b). Random samples of 60 G. affinis were collected with a dip net, anesthesized and fixed in 10% buffered formalin. In the laboratory, the fish were washed in running water, measured for standard and total length, sexed and preserved in 70% ethanol. Immature mosquitofish were sexed by the method of Turner (1941). Laboratory studies on survivorship and mortality. -Random samples of 10, 200 or 400 mosquitofish were obtained from Belews Lake during the winter and placed in 4-, 95- or 208-liter aquaria, respectively; aquaria were maintained at temperatures of 20, 25 or 30 C f 1. Each aquarium was aerated and filtered constantly, and fish were fed Tetra-Min ad lib. All aquaria were inspected daily. Fish which died were necropsied and, when present, parasites counted. Additional evidence of parasite-induced host mortality was obtained experimental- ly. Random samples of 200 mosquitofish were collected during the winter and placed in aerated and filtered 95-liter aquaria where they were fed Tetra-Min ad lib. In order to stimulate growth and development of Bothriocephalus acheilognathi, water temperature was raised approximately 1 C per day from ambient to 25 C. Temperature was then held constant and the aquaria were examined daily. Fish that had died were necropsied and the parasites counted. Data analysis.-The Stastistical Analysis System (SAS) (SAS Institute, Inc., Raleigh, N.C.) was used for several tests. Contingency chi-square analysis was per- formed on sex ratios of mosquitofish collected in Belews Lake and Charlie's Pond. Analysis of variance (ANOVA) was used to compare mean lengths of male or female mosquitofish collected monthly at the three sampling stations; Duncan's multiple range procedure was applied when a significant one-way ANOVA resulted. Pearson correla- tion and multiple regression analyses were performed to compare the relationship be- tween parasite density, total length of mosquitofish and survivorship time. Further analysis of survivorship was conducted using the PIL (Breslow and Mantel Cox tests) of the P-series of the Biomedical Computer Programs (BMPD), Health Sciences Com- puting Facility, University of California, Los Angeles.