Evaluation of Mean RANK Shift for Discerning Changes in Reservoir FISH Assemblages

Mean Rank Shift (MRS) is a recently proposed, but here to fore unevaluated, approach for detecting changes in assemblages derived from long-term monitoring studies.  We evaluated the sensitivity of MRS (as proposed) for detecting temporal changes in reservoir fish assemblages.  We also constructed alternative formulations to use the approach in a before/after context, including restricted subsets of the data to focus on “core” species.  A simulated dataset based on 14 years of reservoir gill-net sampling was used to evaluate the performance and sensitivity of the MRS formulations based on four ranking metrics including number, weight, index of relative importance (IRI) and the prey-specific index of relative importance (PSIRI).  The original MRS was not sensitive to directional changes resulting from simulated perturbations (i.e., reductions or increases of 10%, 25%, and 50%) of subsets of species.  Two novel MRS-derived formulations based on the initial assemblage were useful for detecting directional changes in a management relevant time frame of 4 to 8 years when PSIRI was the ranking metric.  The modified versions of MRS were most sensitive when using PSIRI-based ranks and offer managers a new tool for quantifying changes in the structure of reservoir fish assemblages in response to perturbations.