Detection of systematic change in Adélie penguin foraging trip duration: consequences of high inter-annual variability and usefulness of ice cover as a covariate

Power analyses were carried out using a 12 year data set from the Béchervaise Island Adélie penguin colony with the aim of determining minimum sample sizes required to detect systematic temporal change in CEMP Parameter A5 (penguin foraging trip duration). Two different types of systematic change were investigated: 1) change occurring at a constant rate after a certain point in time and 2) a sudden step change in parameter values from one average level to another. Modelling showed that change of the latter form could be more quickly and powerfully detected at a range of effect sizes than could consistent rates of change occurring over longer periods of time. Sample sizes of 50 penguins carrying out three foraging trips each were required to detect a 35% step increase in foraging trip duration after three years or alternatively, with equal power, a 3.75% annual increase in trip duration after 12 years from the onset of change (55% overall increase). Inclusion of ice cover as a covariate in the analyses enhanced detection of change for birds monitored in the guard stage of chick rearing. However, caution must be exercised when incorporating covariates into power analyses, as inclusion of covariates to explain part of the natural interannual variability in a monitored parameter is only useful if the covariate itself is independent of the factors causing the systematic change.