Development and evaluation of standard weight equations for bridgelip suckers and largescale suckers

Fisheries assessment tools originally created for use with game fish populations are now helping to assess individual nongame species and to evaluate fish communities. Length and weight measurements of individuals within a species can be summarized into condition indices that give insight into the health and condition of the individual and the aquatic community. Such data (weight [g] and total length [TL, mm]) were obtained for 105 populations of bridgelip suckers Catostomus columbianus and 135 populations of largescale suckers C. macrocheilus from three northwestern states. These data were used to develop standard weight (W-s) equations via the regression-line percentile (RLP) and empirical percentile (EmP) methods. Length constraints were 130-460 mm TL for bridgelip suckers and 170-640 mm TL for largescale suckers. The data set was limited by TL constraints and split into development and validation sets. The RLP method yielded the following W-s equations: log(10)W(s) = -5.01699 + 3.02648.log(10)TL for bridgelip suckers and log(10)W(s) = -4.94684 + 2.99720.log(10)TL for largescale suckers. Equations based on the EmP method were log(10)W(s) = -5.09721 + 3.04947.log(10)TL for bridgelip suckers and log(10)W(s) = -5.2586 + 3.12136.log(10)TL for largescale suckers. For both species, the relative weights produced from RLP and EmP equations differed by less than 10%. However, differences were inconsistent over the length range, indicating a length-related bias. Based on this assessment, use of EmP W-s equations for bridgelip suckers and largescale suckers is recommended. Further, the EmP method should be used to establish new W-s equations.