Stock density indices: Development, use, and limitations

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Abstract
The purposes of this paper are to review the development and assess the utility of stock density indices. Stock density indices, specifically proportional stock density (PSD) and relative stock density (RSD), were developed to quantify length‐frequency data. Length categories for standardized determination of stock density indices were based on percentages of world‐record length for each fish species; five‐cell length categories have been proposed for many warm‐ and coolwater fishes, but few coldwater fishes. Both seasonal patterns in sampling data and gear‐related biases can affect length‐frequency data used to determine stock density indices. Stock density indices have been correlated with population dynamics (recruitment, growth, and mortality), relative abundance, and condition for many fish species; coefficients of determination typically are low, and much of the variability in the relations is unexplained. Stock density indices for predator and prey fish populations tend to be inversely related; however, inverse relations are more likely to be present in small (<50 ha) impoundments. We recommend that development of five‐cell length categories for additional fish species continue to be based on established percentages of world‐record length.