Graduate Thesis Or Dissertation
 

An Evaluation of the Effects of Hatchery and Management Practices on Geomagnetic Orientation Behavior in Juvenile Pacific Steelhead Trout (Oncorhynchus mykiss)

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  • Numerous populations of salmon and trout undergo extensive migrations, migrating from freshwater to the sea and throughout ocean basins. Years later they return to their natal streams with remarkable accuracy and precision. The outward migration is accomplished without the benefit of following experienced migrants or prior individual experience. Evidence suggests that Pacific salmon and trout use the earth's geomagnetic fields when undertaking these migrations. My study evaluated current hatchery and management practices that may influence the ability of Pacific steelhead trout (Oncorhynchus mykiss) to perceive or respond to geomagnetic fields. Magnetized coded wire tags (CWT) were implanted into Alsea river steelhead pre-smolts (juveniles that have not undergone the physiological transition to prepare for oceanic life stage) during the fall of 2013. Test subjects were exposed to simulated magnetic fields at the latitudinal periphery of their oceanic foraging range in the North Pacific. Orientation responses were recorded and compared to non-tagged individuals. However, due to fish illness or other factors, control test subjects did not exhibit the north/south orientation responses observed during previous studies and results were inconclusive. Magnetic susceptibility, total magnetic field intensity, and median destructive field were assessed by analyzing 48 CWTs from five injection units. The total field intensity was consistent among machines and tags within a single machine. The strength of the fields produced by the tags relative to the known magnetic sensitivity of steelhead trout suggest that they are unlikely to pose major problems for large-scale navigation in fish. Sex differences in geomagnetic orientation response were evaluated based on work that documented early movement downstream by female steelhead. Experiments documented no significant difference in geomagnetic orientation responses based on genetic sex, though these results may also have been confounded by the same problems associated with the CWT orientation experiments. Lastly, effects of rearing juvenile steelhead in concrete raceways (subject to magnetic distortion due to iron rebar) were examined following similar "simulated magnetic displacement" protocols. These juvenile steelhead exhibited random orientation responses. The inability of fish reared in raceways to distinguish between fields at the periphery of their oceanic foraging range could pose potential problems for homing and navigation in hatchery fishes. The duration of these affects is unknown and warrants further study. Based on these findings, there is evidence to suggest that some current hatchery and management practices may influence the ability of juvenile salmon and trout to perceive geomagnetic fields. In turn, this may influence successful navigation to foraging grounds in the Pacific, subsequent return rates, as well as homing and straying behavior.
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