Threespine stickleback (Gasterosteus aculeatus) and juvenile chinook salmon (Oncorhynchus tshazvytscha) co-occur during high tide in tidal channels of the Fraser River estuary. Given the high density of resident stickleback, there is the potential for strong interactions within and between the two species. Inter- and intra-specific interactions were tested by means of laboratory experiments, with support from field studies. Laboratory experiments placed stickleback and chinook in mixed and single species groups. The levels of aggressiveness were quantified, along with prey choice between surface (Drosophila), midwater (Artemia), and benthic (Tubifex) prey; microdistribution was also recorded. The experiments demonstrated that stickleback were highly aggressive towards chinook, and would drive them away from optimal feeding territories. Chinook consumed surface prey only when tested with stickleback, exploiting benthic and midwater prey when feeding alone. Stickleback demonstrated no significant difference in diet between single and mixed species trials, which is consistent with the supposition of strongly asymmetrical competition for food and space. Field data lend further support to this premise; a marked difference observed in diet suggests microhabitat partitioning between the two species, with stickleback feeding on benthos and chinook largely consuming surface prey. This thesis proposes that interactive segregation is an important process between sympatric stickleback and juvenile chinook in estuarine tidal channels and might have important implications for Fraser chinook stocks. / Science, Faculty of / Zoology, Department of / Graduate