Population Abundance and Stock Assessment of Westslope Cutthroat Trout in the Oldman River Watershed


Jason Blackburn


Westslope cutthroat trout currently occupies no more than 20% of its historical distribution in Alberta. The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) has designated the species as 'Threatened'. In the Upper Oldman River drainage, cutthroat trout still occupies most of its historical range; however, detailed information on the current status of the population is insufficient for fisheries management. We conducted a basin-wide stock assessment to facilitate collection of a rigorous database and to evaluate the need for a catch-and-release fishery. We divided the drainage into four strata based on stream size and electrofished a total of 126 stream sections between 2006 and 2007. We calculated fish density and abundance per sample site and then projected values to the entire drainage area using capture-mark-recapture and bootstrapping methods.

We captured 9,266 cutthroat trout, the majority of which (95%) were larger than 70 mm FL. Of the total catch, 52% were of spawning size (> 149 mm fork length), with less than 3% of legal harvest size (> 300 mm total length). We estimated the drainage population of trout (> 70 mm) to be 296,981 individuals, consisting of 125,479 and 2,996 individuals of spawning and legal harvest-sized fish, respectively. A comparison of fish density, abundance and population structure between the Livingstone River (catchand- release fishery) versus the Oldman River (allowable harvest fishery) indicated that the mainstem Oldman River had more than three times as many fish as the Livingstone River. However, the Livingstone River had more than four times as many legal harvest-sized fish than the Oldman River. In addition, the catch from the Livingstone River had proportionately more legal harvest-sized fish than that from the Oldman River. Angling harvests may have altered fish population structure and size in the Oldman River, despite hooking damage in the Livingstone River mainstem being more than double that of the Oldman River mainstem (52% vs. 23%). In addition, data from a 2004 creel survey suggested there were more than twice as many anglers in the Upper Oldman River drainage, at 7,185 ± 13.9%, than legal-size cutthroat trout, leading to potentially high incidences of hooking damage in the stock.

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