Power of stable isotope techniques to detect size-based feeding in marine fishes

Stable isotope techniques are now the most frequently applied method to investigate size-based variation in fish trophic position (TP), and data suggest changes in TP with body size are common. However, of 131 collated investigations of the relationship between body size and δ15N for inshore coastal fishes, approximately 60% were non-significant. The present study tests whether non-significant trends reflect a true feeding pattern, a lack of statistical power, or a function of both. Results suggest none of these studies for which correlation coefficients were available, achieved enough statistical power to rigorously assess observed slopes. The sample size required to detect statistically significant trends of different magnitude, given assumptions of dispersion and observed data on body size range sampled, was then assessed. Even under lenient assumptions of below average dispersion and uniform sampling effort across the sampled body size range,