Changes in numbers and growth of Ligula intestinalis in the spottail shiner (Notropis hudsonius), and their roles in transmission.

Eighteen species of fishes were collected during the open water (1985-1987) and winter seasons (1985-1986) from Dauphin Lake, Manitoba, Canada (51 degrees 17'N, 99 degrees 48'W) and examined for plerocercoids of Ligula intestinalis (L.). Plerocercoids were most prevalent (5.3%) in spottail shiners (Notropis hudsonius), the major fish host for Ligula in Dauphin Lake. Detailed analysis of the spottail shiner-Ligula host-parasite system revealed that the number of plerocercoids differed between years and among habitats but there was no statistically significant seasonal pattern; recruitment of new worms was highest in young fish and decreased with age, and infected spottails had reduced gonad development. Analysis of host and parasite growth revealed that the soma of male spottails infected with Ligula weighed more but had a reduced growth rate. Growth rate of females was unaffected by Ligula but somatic weight was slightly increased. This appears to be related to the greater metabolic stress on infected males. Based on increased mean somatic weight and skewness of the weight distribution for infected fish, we propose that infected spottails are subjected to size-selective mortality. Some of the contradictions in the literature may be attributed to underestimated prevalence due to increased numbers of spawning fish in the spring, mixed age-classes of Ligula in separate age-classes of fish, and differential effects on growth due to infection in male versus female hosts.