YCT Multi-state Assessment February 10, 2003 



Appendix D. Genetic Considerations for Fish Managers 



Factors that influence hybridization and introgression between introduced non-native trout 



and indigenous westslope cutthroat trout: Genetic considerations and management 



implications 



Matthew Campbell 



Fishery Geneticist, Idaho Department of Fish and Game 



Introductions of non-native trout for fisheries management purposes have occurred throughout 

 the range of westslope cutthroat trout for more than 100 years. It has been well documented that 

 these introductions have often led to hybridization and introgression, a potentially serious, on- 

 going genetic hazard throughout much of the species present range (Weigel et al. 2002, Sage et 

 al. 1992, Leary et al. 1995). However, there is also research that has failed to show evidence of 

 hybridization and introgression within populations even though non-native trout have been 

 previously stocked (Williams et al. 1996, Mays 2001). 



There are many factors that determine whether non-native trout (e.g. rainbow trout, Yellowstone 

 cutthroat, golden trout) introductions will result in hybridization (i.e., the interbreeding of 

 introduced non-native trout with indigenous westslope cutthroat trout) and introgression (i.e., the 

 incorporation of genes of non-native trout into the gene pool of a westslope cutthroat 

 population). 



One or more of the following factors may influence levels of hybridization and introgression: 



• The number of non-native trout stocked; 



• The number of times stocked, time of year stocked, time since last stocking, age/size at 

 stocking, strain or subspecies stocked, survival of stocked fish, size of the indigenous 

 westslope cutthroat population, and fishing pressure on stocked streams; 



• Presence/ Absence of isolating mechanisms (both pre-mating and post-mating 

 mechanisms). For instance, the presence or absence of isolating mechanisms may depend 

 on whether rainbow trout are stocked on westslope cutthroat populations that are 

 naturally sympatric with native populations of O. niykiss, or whether they are stocked on 

 westslope populations that have not previously lived in sympatry with O. mykiss); 



• Dispersal patterns and reproductive success of introduced trout and hybrids; 



• Ecological conditions can influence many aspects of stocked rainbow trout sur\'ival, the 

 presence/ absence of isolating mechanisms, fitness of hybrids, gene flow between 

 populations, as well as the geographical distribution of introduced non-native trout, 

 native trout, and hybrids within an area. 



There are also numerous complicating factors that determine whether the percentage of non- 

 native alleles within a population, the number of hybrids in a population, or the number of 

 hybridized populafions will increase, decrease, or remain unchanged over fime. The fate of non- 

 native trout alleles introduced into a westslope cutthroat trout population depend on the extent to 

 which introduced trout and westslope cutthroat trout hybridize, the subsequent reproductive 

 fitness of hybrids and the extent to which the hybridizing populafions depart ft-om Hardy- 

 Weinberg expectations of an ideal populafion. 



For example, if 20 rainbow trout (breeding adults) are introduced onto a cutthroat populafion (80 

 breeding adults, no other individuals), before any mating, the sample offish is composed of 20% 

 rainbow trout (RBT) alleles and 80% westslope cutthroat trout (WCT) alleles. If the introduced 



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