Summarizing quantile estimates of weight at length to compare condition: Geographic and temporal variation in Arctic Grayling.

This article presents a novel, concise method for summarizing variation in quantile regression estimates of fish weight at length across multiple grouping factors, demonstrated using Arctic Grayling (Thymallus arcticus) populations. By applying percentile summaries (maximum, 75th, 50th, 25th, and minimum) of predicted quantiles across 28 geographic locations and multiple years within two populations, the approach simplifies complex distributions into quartiles, facilitating comparisons of fish condition while accounting for heterogeneity in allometric exponents. The study reveals substantial spatial and temporal variation in Arctic Grayling condition, with annual variation within populations often exceeding half the variation observed among locations. The authors provide statistical code and reference tables that can replace traditional standard weight equations, avoiding their inherent biases and enabling more comprehensive diagnostic assessments of fish condition across populations and time.