Quantifying prey availability using the foraging plasticity of a marine predator, the little penguin

1. Detecting changes in marine food webs is challenging, but top predators can provide information on lower trophic levels. However, many commonly measured predator responses can be decoupled from prey availability by plasticity in predator foraging effort. This can be overcome by directly measuring foraging effort and success and integrating these into a measure of foraging efficiency analogous to the catch per unit effort (CPUE) index employed by fisheries. 2. We extended existing CPUE methods so that they would be applicable to the study of generalist foragers, which introduce another layer of complexity through dietary plasticity. Using this method, we inferred species-specific patterns in prey availability and estimated taxon-specific biomass consumption. 3. We recorded foraging trip duration and body mass change of breeding little penguins Eudyptula minor and combined these with diet composition identified via non-invasive faecal DNA metabarcoding to derive CPUE indices for individual prey taxa. 4. We captured weekly patterns of availability of key fish prey in the penguins' diet and identified a major prey shift from sardine Sardinops sagax to red cod Pseudophycis bachus between years. In each year, predation on a dominant fish species (similar to 150 g/day) was replaced by greater diversity of fish in the diet as the breeding season progressed. We estimated that the colony extracted similar to 1,300 tonnes of biomass from their coastal ecosystem over two breeding seasons, including 219 tonnes of the commercially important sardine and 215 tonnes of red cod. 5. This enhanced pCPUE is applicable to most central-placed foragers and offers a valuable alternative to existing metrics. Informed prey-species biomass estimates extracted by apex and meso predators will be a useful input for mass-balance ecosystem models and for informing ecosystem-based management.