Body condition indices are commonly used in the management of fish populations and are a surrogate to physiological attributes such as tissue-energy reserves. Relative condition factor (K_n) describes the condition of species relative to populations in a geographic area. We developed models to allow for the calculation of K_n in Montana, USA by using the weight–length data collected by Montana Fish, Wildlife & Parks. We generated log₁₀weight–log₁₀length relationships to obtain Montana specific parameter estimates for relative condition equations (W′) for 51 species and three subspecies. We developed separate models by water type (e.g., lotic and lentic) and sex for five species due to varying growth based on sexual dimorphism and varying ecosystem types. Relative condition offers the advantage of describing body condition relative to species in Montana, provides a condition index for species that do not have standard-weight models developed for relative weight (Wr), and affords more information for the global database on weight–length relationships of fishes.

This study aimed to understand the ecological relationship among burbot Lota lota , brown trout Salmo trutta and lake trout Salvelinus namaycush , with a focus on burbot, a species of greatest conservation need in Wyoming. While we hypothesised a reciprocal intraguild predation dynamic, where competition and predation occur between predators based on size or age structure, our findings provided limited support for this hypothesis. Both dietary overlap and trophic position were minimal among burbot, brown trout and lake trout. Instances of reciprocal predation were rare; no predation between burbot and lake trout was observed; and brown trout was the only species consumed by all predators (burbot 0.02 mean proportion by weight; lake trout 0.09 mean proportion by weight). Predation by brown trout on burbot was negligible, contributing only 0.01 to the mean proportion by weight and frequency of occurrence. Additionally, both burbot and brown trout were less piscivorous than expected, with fish comprising 0.36 and 0.17 of their mean proportion by weight in their overall diets, respectively. Overall, our findings suggest that these predators coexist with limited competitive or predatory interactions, likely due to differences in prey selection.