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

Abstract

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 noninvasive 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 (~150 g/ day) was replaced by greater diversity of fish in the diet as the breeding season progressed. We estimated that the colony extracted ~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.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Twilight foraging enables European shags to survive the winter across their latitudinal range

“…This absence of knowledge hinders future analyses that would ideally aim to predict the availability of predominant prey as well as the costs of foraging, which are closely related to temperature and therefore vary with location. If such data were available, more refined 'feasting' hypotheses could be tested that incorporate environmental conditions and associated costs as well as latitude (Cavallo et al 2020).…”

Trophic ontogeny of a generalist predator is conserved across space

“…Diet heterogeneity was expected to some degree among the seven locations, given the observed availability of different prey, both in composition and abundance, from the different seagrass systems. Indeed, such foraging plasticity in response to variation in prey availability has been observed in generalist predators (e.g., Cavallo et al 2020; Moorhouse-Gann et al 2020; Ng et al 2021). However, there was no apparent spatial structure in the stomach contents data such as a latitudinal trend, by distance, or between ecoregions (i.e., Northern Gulf of Mexico versus Floridian ecoregions; Spalding et al 2007).…”

Quantity over quality? Prey-field characteristics influence the foraging decisions of little penguins ( Eudyptula minor )

“…This may be due to the spatio-temporal constraints imposed on them during breeding. Outside the breeding season, little penguins have been recorded spending between 2 and 49 days at sea and foraging at maximum distances of 147 km from the colony [56]. In comparison, during early chick-rearing, foraging range and duration are highly restricted, with individuals typically alternating between guarding the chick in the nest and foraging each day.…”

“…Although little penguins are considered to be generalist foragers [56], it was predicted that individuals should still differentiate between high- and low-quality prey types in order to maintain the necessary time-energy balance. However, in the present study, patch numerical density was an important factor in predicting effort.…”