UW Aquatic & Fishery Sciences Quantitative Seminar

Erik Schoen

UW School of Aquatic & Fishery Sciences

Bias and precision in diet analysis: A contest between methods



Diet analysis, one of the lowest-technology techniques in fisheries biology, produces data that are critical to research in aquatic community ecology, conservation biology, invasive species ecology, and multispecies stock assessment.  Two methods are commonly used to analyze diet data for use in energy flow and predator-prey studies: 1) Diet proportion by weight (Wi), in which a single diet proportion is calculated from the combined contents of all stomachs, and 2) Mean diet proportion by weight (MWi), in which separate proportions are calculated for each stomach and these values are then averaged.  These methods can produce starkly different diet proportions from the same data.  In this talk, I use a simulation modeling framework informed by empirical data to evaluate the accuracy and precision of each method under a range of conditions.  I used an operating model to generate a set of “true” diet data for a population of predators, sampled a subset of these data, analyzed the subset using both methods, and compared the results to the true values.  This process was repeated across a range of operating model scenarios.  The Wi method was accurate under all scenarios, approaching the true diet proportion with a large enough sample size. The MWi method was biased under most conditions, typically producing diet proportions for large prey 10-70% below the true population value.  MWi was most biased when the size difference between prey types was large and when large prey were rare.  However, the MWi method was more precise than Wi, reaching a stable (but biased) value with fewer samples.  The main results were consistent for both generalist and specialist predators.  I encourage caution in the use of the MWi method, which is likely to be accurate only under special conditions.  I propose an empirical test of these model results and discuss implications for studies using stomach or stable isotope data to estimate diet.

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