Ruffe Gymnocephalus cernuus length–weight data from 141 data sets were obtained from a variety of waters across Europe and the Laurentian Great Lakes for summarization and development of standard weight (Ws) equations. The mean slopes of the length–weight relationships from all populations and for all percentiles except the 95th percentile were not different from 3.0, suggesting that ruffe growth is generally isometric. The Ws equations developed using the regression line percentile, linear empirical percentile (EmP), and Froese's methods exhibited length-related biases. The quadratic EmP Ws equations for the 75th and 50th percentiles did not exhibit length-related biases and thus can be used to compute the relative weight of ruffe. The EmP 75th percentile Ws equation was log10(Ws75) = −2.5800 + 0.6210·log10(total length, TL) + 0.6073·[log10(TL)]2, and the 50th percentile Ws equation was log10(Ws50) = −3.3524 + 1.3969·log10(TL) + 0.4054·[log10(TL)]2 when constrained to ruffe between 55 and 205 mm TL. We propose that the minimum TL for a five-cell length categorization system used to compute stock indices for ruffe be 55, 90, 120, 140, and 175 mm TL. These results provide a method for computing relative weight indices for typical (50th percentile) and above-average (75th percentile) ruffe; this method will allow comparisons of body condition across time, among habitats, among bodies of water, and among length categories
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