Coastal Cutthroat Trout as Sentinels of Lower Mainland Watershed ...

28practices. Life-history profiling is likely where management effort would best be directedon coastal cutthroat trout in the short-term, especially in the Lower Fraser basin wheregenetic fitness of various stocks could inadvertently be compromised by fish culturalpractices. Use of micro-chemical analyses of samples of scales, fin rays or otoliths andsupport by radio/sonic tracking should discern the ranges of migratory life histories thatare dominant among adult cutthroat from Fraser basin tributaries. Molecular geneticsampling is also apt to detect differences within the Lower Fraser Basin, particularly ifcutthroat are sampled from large tributary basins including the Pitt, Harrison and LillooetRivers (R. Taylor pers. comm. 2005). In the interim, in advance of more detailed geneticanalysis work, stocks of different life histories and morphological features may be thebest indicators of genetic differences that control complex migratory behaviors. Toensure conservation, it should be assumed that different profiles have a strong hereditarybasis and should not be mixed in fish culture practices. Mixing discrete stocks may be ofgreater consequence than most contemporary hatchery practices, although unintentionalshifts in the modal timing of spawning (Chilcote et al. 1986) and family inbreeding arealso risks, particularly if hatchery fish are recycled.Another concern, directly related to fish culture, is hybridization of coastal cutthroat troutwith steelhead/rainbow trout, which is readily detectable using molecular geneticsampling and analyses (Costello and Rubridge 2004). Reproductive isolation betweensteelhead/rainbow and coastal cutthroat is maintained in the wild by habitat differences inpreferred stream selection as well as some temporal differences in spawning (Hartmanand Gill 1968). However, recent research indicates hybridization is much greater thanpreviously thought. Hybridization was identified in 29 of 30 streams on VancouverIsland that supported sympatric populations of the two species, with hybridization ratesranging from 3 to 88 %, or 20 % on average (Bettles 2004). In two streams, Chase Riverand Cougar Creek, “hybrid swarms” appeared to occur, with introgression rates of up to54 %. Relatively high rates of hybridization have also been detected in the Keogh Riverwhere 10 % of the steelhead smolts were hybrids and 26 % of cutthroat parr/smolts werehybrids (D. McCubbing pers. comm. 2004).A suggested reason for high rates of hybridization is “residualism” of hatchery steelheadand cutthroat, combined with poor survival of steelhead in the ocean. A significantincidence of non-migratory precocious males among stocked steelhead smolt groups mayincrease the incidence of cross-mating. Many of these residual steelhead can be similarin size to maturing cutthroat, especially the resident form (Slaney and Harrower 1981).Release of smolts near river mouths or tide-water reduces this risk, but these “residuals”have been documented to disperse upstream for two kilometers at the Keogh River(Slaney and Harrower 1981). In a parallel study of stocked cutthroat trout at LittleCampbell River, large hatchery steelhead (6000, 124 g) and cutthroat smolts (2000, 112g) were stocked three, five and seven km upstream of a counting fence. Only 57 % and42 %, respectively, migrated seaward and a high incidence of residualism was detected.In two sampled stream sections (4 % of the stream length), residuals comprised 6 %(unstocked section) and 62 % (stocked section) of the total salmonid biomass (such largeprecocious-prone smolts are no longer cultured and released in Region 2). This wasconfirmed more recently by Bates (2000) by a comparison of life history characteristics