Date of Completion

Spring 5-6-2012

Thesis Advisor(s)

Eric Schultz


Biology | Marine Biology


Glacial retreat during the Pleistocene caused landlocking of anadromous Alaskan threespine stickleback, Gasterosteus aculeatus, furnishing a natural 'experiment' in osmoregulatory divergence. The objective of this study was to assess the effects of individual acclimation and population divergence on salinity preference. Full-sibling families of marine, anadromous, and freshwater-landlocked populations of stickleback were reared in common environments until 3 weeks post-hatch, then were split and acclimated to low or high salinity. At 6 to 8 weeks of age the six experimental groups were tested for salinity preference in a tank that offers fish a choice of compartments with different salinities arranged in a gradient from fresh to sea water. We observed significant population and acclimation effects. Anadromous fish preferred sea water and avoided fresh water, whether acclimated to low or high salinity.

Landlocked fish showed a strong acclimation effect, avoiding salt water when acclimated to fresh and avoiding freshwater when acclimated to salt, while showing no preference for their acclimation salinity. Fish from the marine population showed little preference for fresh or sea water regardless of acclimation salinity. After restriction to fresh water for more than five thousand generations, landlocked fish have evolved weaker preferences in response to a salinity gradient compared to their anadromous ancestors.