Abundance and Distribution of Westslope Cutthroat Trout in the Castle River Drainage, Alberta, 2008-2009


Jason Blackburn


In the Castle River drainage the population status of Westslope Cutthroat Trout is uncertain. Past introductions of non-native Rainbow Trout have resulted in widespread hybridization restricting pure Cutthroat Trout to headwater tributaries. I initiated a drainage-scale stock assessment to estimate the population size and distribution of Westslope Cutthroat Trout, (considered ‘threatened’ by The Committee on the Status of Endangered Wildlife in Canada (COSEWIC)), Rainbow Trout, and their hybrids in the Castle River drainage.

I divided the drainage into five strata based on stream size and electrofished a total of 86 randomly selected, and 7 systematically selected sampling sites in 2008 and 2009. Fish density and abundance was calculated per sampling site and then projected to the entire watershed area using a beta-distribution of capture-probabilities from capturemark-recapture estimates, and bootstrapping methods.

A total of 2,260 fish belonging to the genus Oncorhynchus were captured using backpack, tote-barge and raft electrofishers. I identified 61% of the catch as Cutthroat Trout (n=1,375), 26% as hybrids (n=595), and 13% as Rainbow Trout (n=290). Of the total catch, 9% (n=198) were legal-harvest-sized fish (>285 mm fork length), of which 63% were Cutthroat Trout (n=124), 20% were hybrids (n=41), and 17% were Rainbow Trout (n=33).

Using stream surface area, the estimated total Oncorhynchus trout population was 145,286 (90% CI=90,976–229,564), of which 4,915 (90% CI=2,900–8,023) were legalharvest- size. The estimated Cutthroat Trout population was 108,596 (90% CI=65,804– 173,411), of which 3,743 (90% CI=2,015–6,411) were legal-harvest-sized. Hybrid and Rainbow Trout abundances were 27,567 (90% CI=9,729–55,083) and 7,883 (90% CI=3,669–14,250) respectively, with legal-harvest-sized populations of 727 (90% CI=341– 1,296) and 479 (90% CI=227–849).

A between drainage comparison indicated Cutthroat Trout abundance in the Castle River drainage is roughly one third that of the population estimated for the upper portion of the Oldman River drainage in 2006 - 2007. Cutthroat Trout were most dominant in the upper reaches of the Castle River drainage. Our data also suggest that Cutthroat Trout densities decrease with increased stream size (strata). Drainage abundance estimates calculated by stream area were slightly but consistently greater than those calculated by stream length because of the added dimension of stream width.

The proportion of Cutthroat Trout streams that were uncompromised by hybrids and Rainbow Trout was highest in the upper part of the drainage at 100% uncompromised in stratum 1 and 84% in stratum 2, and declined in a downward drainage progression to 0% in the lowest reaches (stratum 5). Consequences of hybridization appear to be the elimination of legal-harvest-sized Cutthroat Trout from the Castle River mainstem (stratum 5), which is now dominated by Rainbow Trout. Our drainage estimates represent the maximum potential abundance of remaining pure Westslope Cutthroat Trout in the Castle River drainage.

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