Establishing realistic length standards for Cutthroat Trout, Rainbow Trout, and Mountain Whitefish

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Abstract Objective Proportional size distribution (PSD) is a simple index used to summarize length data that is easily communicated, provides insight on fish population dynamics (e.g., growth, recruitment), and is often used to establish management objectives. However, the current length categories for Cutthroat Trout Oncorhynchus clarkii and Rainbow Trout O. mykiss were based on the all-tackle world records (an atypically large adfluvial Cutthroat Trout and a steelhead [anadromous Rainbow Trout]). Consequently, the length categories (stock [S], quality [Q], preferred [P], memorable [M], trophy [T]) are so large for these two species that the index is of limited use for most populations throughout western North America, particularly for large length-groups. In addition, length standards have not been proposed for Mountain Whitefish Prosopium williamsoni. As such, we sought to provide new length standards for Cutthroat Trout, Rainbow Trout, and Mountain Whitefish that allow fisheries scientists to calculate useful PSD values. Methods We compiled data from nearly 3 million fish across the western United States. A percentile-based approach was used to identify the individual total length that would form the basis of estimating length standards. We placed each fish into its respective 10-digit hydrological unit code, obtained the maximum length (all lengths are total length) for each 10-digit hydrological unit code by species, and then identified the 99th percentile of the maximum lengths. The species-specific 99th percentile value was then used to estimate potential length standards for each length category. For Mountain Whitefish, we used the current world-record length as the basis for generating length standards. We compared PSDs calculated using existing and revised length standards to provide insight on potential issues with the proposed values. Results We propose the following minimum total lengths for PSD calculations: 18 cm (7 inches; S), 28 cm (11 inches; Q), 35 cm (14 inches; P), 43 cm (17 inches; M), and 53 cm (21 inches; T) for Cutthroat Trout; 20 cm (8 inches; S), 30 cm (12 inches; Q), 40 cm (16 inches; P), 50 cm (20 inches; M), and 60 cm (24 inches; T) for Rainbow Trout; and 15 cm (6 inches; S), 25 cm (10 inches; Q), 30 cm (12 inches; P), 38 cm (15 inches; M), and 48 cm (19 inches; T) for Mountain Whitefish. Revised values provided more realistic estimates of PSD for Cutthroat Trout and Rainbow Trout than existing values. The proposed length standards for Mountain Whitefish resulted in PSD values that were realistic and useful for characterizing length structure. Conclusions The length standards presented here provide fisheries scientists with tools to aid in the study and management of Cutthroat Trout, Rainbow Trout, and Mountain Whitefish populations.