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

GALVAN DE; SWEETING CJ; REID WDK

MARINE ECOLOGY PROGRESS SERIES

Inter-Research Science Center

Lugar: Oldendorf/Luhe; Año: 2010 vol. 407 p. 271 - 278

0171-8630

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 d15N for inshore coastal fishes, approximately 60% were non-significant. This study tests whether non-significant trends reflect a true feeding pattern, a lack of power or a function of both. Results suggest none of these tests, for which correlation coefficients were available, achieved power to rigorously assess observed slopes. The sample size required to detect 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 size range, < 10% of non-significant trends had the power to detect a slope corresponding to a 0.25 change in TP over 80% of maximum length (Lmax) throughout life. Therefore non-significant trends in d15N are predominantly the result of poor power and no conclusion can be drawn as to associated fish trophodynamics. Analysis here suggests that size based feeding studies should aim to sample over 40% Lmax range of the population, as the sample size required to maintain power diminishes almost exponentially with size range. Researchers should strive to minimise other sources of variation as linear increases in dispersion lead to virtually linear increases in sample size necessary to maintain power. Finally a key component of hypothesis testing for size based feeding should be not just whether a trend is significant or not, but whether sampling is adequate to detect a minimum ecologically relevant threshold slope. This will separate tests which lack power from those where size independent feeding is a true feeding mode.