Natural selection should favour host defenses that reduce a host’s exposure to parasites or reduce their negative effects. One strategy that resolves the substantial costs of host immunity and/or tolerance is to avoid infective stages altogether. For fish, behavioural avoidance is well-known for defense against aquatic predators, but it is poorly known for defense against parasites. I used a model system that is amenable to experimental manipulation to test the behavioural avoidance hypothesis for fathead minnows exposed to the larvae of two of their common flatworm parasites. First, I showed that minnows exposed to a liver encysting trematode, Ornithodiplostomum sp. showed an increase in lipid peroxidation, an indicator of oxidative stress, persisting through worm development. Three lines of evidence provided support for the behavioural avoidance hypothesis. First, shoal area decreased in groups of minnows exposed to O. ptychocheilus cercariae compared to those exposed to cues from other aquatic threats. Second, average worm numbers were 50 % lower in fish confined to artificial shoals compared to non-shoaling minnows, indicating that shoaling reduces risk of exposure. The third experiment showed that minnows within the centre of shoal reduced their risk of infection by 67%. Taken together, these results demonstrate a cost of trematode infection on minnows, that minnows can detect infective larvae within the water column, and that social living reduces a hosts’ risk of exposure. / xi, 84 leaves : ill. ; 29 cm

Avoidance to a blend of four metals (relative proportions: 1.00 copper, 0.54 chromium, 1.85 arsenic, 0.38 selenium) was determined for schools of fathead minnows (<i>Pimephales promelas</i>) in a laboratory avoidance chamber, an artificial stream and in a natural stream setting on a seasonal basis during continuous exposure to low levels of the metal blend for up to 9 months. Laboratory avoidance responses were determined seasonally during 12 months of laboratory holding for unexposed (control) fish and two levels of metal blend exposure in a steep gradient laminar flow chamber. Toxicity of the metal blend was determined for laboratory and field, control and metals acclimated fish. Unexposed fish avoided very low levels of the blend (29 ug/L total metals). Fish exposed to low levels of the blend (49 ug/L total metals) for 3 months failed to avoid levels equal to 5X holding exposure levels. Fish exposed to higher levels of the blend (98 ug/L total metals) preferred elevated levels (3X holding exposure) after 3 months exposure, mildly avoided 5X holding levels after 6 months exposure and were not responsive to levels approaching 10X holding exposure after 9 months continuous exposure. Activity was not affected by long term exposure. Field avoidance responses were determined seasonally during 7 months of field laboratory holding in New River water for unexposed (control) fish and exposed (98 ug/L total metals) fish in a modified artificial stream supplied with raw New River water. Unexposed fish avoided 71.1 and 34.3 ug/L total metals in spring (3 months holding) and summer (6 months holding) respectively. After 3 months exposure in New River water, fish did not respond to metal blends as high as 1,470 ug/L total metals. In-stream avoidance responses were determined in the summer for unexposed (control) and exposed I (98 ug/L total metals) fish in a Adair Run, a second order tributary to the New River. Unexposed fish avoided 73.5 ug/L total metals in Adair Run. After 3 months exposure, in New River water fish did not respond to metal blends as high as 2,940 ug/L in Adair Run. Water hardness, turbidity and physical setting are implicated as possible causative factors for differences between control fish responses tested in different seasonal and locations. Fish exposed to the high level exposure in the laboratory had a 96-hr LC50 value 1.25X higher than laboratory control fish. Laboratory control fish avoided metals levels at 0.4% of their 96-hr LC50. Fish exposed to the metals blend in the field had a 96-hr LC50 value 1.41X higher than field control fish. Field control fish avoided metals levels between 0.7 and 2.5% of their 96-hr LC50 depending upon test location and season. There was no difference between the 96-hr LC50s of laboratory vs field control fish or between laboratory vs field exposed fish. Optimum statistical methods for analyzing avoidance behavior in schooling fish were developed. / Ph. D.