Rainbow Trout Origin 



Fin erosion, scale analysis, and tetracycline marks were compared for individual rainbow 

 trout captured by anglers to discern their origin (wild or hatchery). Results were 

 inconsistent between methods. The estimated proportion of rainbow trout of hatchery 

 origin creeled during the survey period ranged from 47% by scale analysis to as low as 

 3% by tet-mark. 



Dorsal fin erosion is a subjective indicator of hatchery origin. In 177 rainbow trout 

 examined for fin erosion, 28% were classified as hatchery fish. In contrast, Byorth and 

 Weiss (2002) report fin erosion rates of 16% and 23% in rainbow trout captured in gill 

 nets in May 2000 and May 2001, respectively. 



Scale checks analyzed with the methods of Oswald et al. (1990) led to the highest 

 estimated proportion of hatchery rainbow trout in the creel. Of 1 77 rainbow trout 

 examined in the creel, scale samples indicated 47% were of hatchery origin. Oswald et 

 al. estimated that 69% of rainbow trout sampled in 1989 were of hatchery origin. Scale 

 analysis is quite subjective as well considering that only 75% of the classifications of the 

 same scales concurred with Oswald et al. (1990). 



Table 3 summarizes a comparison between classification of 177 rainbow trout by fin 

 erosion and scale analysis. Agreement between methods was achieved in 41% of the 

 samples for wild fish and 1 7% of samples for hatchery fish. The highest discrepancy was 

 that 31% of samples classified as wild by fin erosion were classified as hatchery by scale 

 analysis. Only 12% classified by hatchery by fin erosion were classified as wild by scale 

 analysis. 



Table 3. Matrix of rainbow trout classifications as hatchery (H) or wild (W) according to 

 fin erosion and scale analysis, collected in the Hebgen Reservoir creel survey June 12, 

 2000 -June 10,2001. 



41 

 17 

 31 



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Tet-marks are the most certain indicator of hatchery origin, although marking efficiency 

 is apparently low. Of 90 rainbow trout vertebrae examined, only 3 expressed a tet-mark, 

 indicating that 3.3% of the sample was known to be of hatchery origin. Each of these 

 marks was on a rainbow trout classified as hatchery fish by both fin erosion and scale 

 analysis. Of the same 90 rainbow trout, a total of 29 had dorsal fin erosion. If fin erosion 

 is assumed to be a reliable indicator, then tet-marking efficiency would be assumed to be 

 around 10% (i.e. only 3 of 29 known hatchery fish actually recorded a tet-mark). 



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