Stable isotope ratios of a tropical marine predator: confounding effects of nutritional status during growth

Abstract

Stable isotope analysis of carbon and nitrogen is frequently used to study the diets and foraging ecology of marine predators. However, isotopic values may also be affected by an individual's nutritional status and associated physiological processes. Here, we use C and N stable isotopes in blood and feathers of blue-footed booby chicks at the Galápagos Islands to examine how isotopic values are related to body condition and growth rate, and to assess the consistency in the isotope ratios of individuals during growth. Size dimorphism in blue-footed boobies provided an additional opportunity to examine how isotope ratios differ between sexes in relation to body size and growth rate. There was no significant difference between sexes but both C and N stable isotopes were significantly negatively related to the body condition of chicks. These data were consistent with individual variation in physiological processes affecting fractionation, although we cannot rule out the possibility that they were also influenced to some extent by population-level variation in the stable isotope ratios of prey fed to chicks, for instance related to prey size, depth or lipid content. Our results highlight the need for methods that take proper account of confounding physiological factors in isotopic studies of foraging ecology and diet.

Higher trophic level prey does not represent a higher quality diet in a threatened seabird: implications for relating population dynamics to diet shifts inferred from stable isotopes

“…In three Arctic-breeding seabirds, two showed a positive effect of δ 15 N values on body mass, but only in one of two tissue types sampled (Moody et al 2012). Similarly, to our findings, Cruz et al (2012) found that blue-footed booby Sula nebouxii chicks with higher δ 15 N values had poorer body condition. Likewise, a higher trophic level diet was associated with lower body condition in adult female marbled murrelets Brachyramphus marmoratus (Janssen et al 2009), and a slower growth rate in South American sea lion Otaria flavescens pups (Drago et al 2010).…”

Sexual segregation in a wide-ranging marine predator is a consequence of habitat selection

“…Higher δ 15 N in males from Grassholm may occur if males consume a higher proportion of whitefish fishery discards than females (Stauss et al 2012). However, at Bass Rock, the between-sex differences in δ 15 N each year were small and could have arisen from the observed habitat segregation between males and females (as a consequence of variation in isotopic baselines in the areas where individuals foraged; Woodcock et al 2012) or from lower body condition among males (as a consequence of variation in physiological processes affecting fractionation; Lee Cruz et al 2012) or both.…”

Seasonal consistency and individual variation in foraging strategies differ among and within Pygoscelis penguin species in the Antarctic Peninsula region

“…Several studies have examined the isotopic values of feathers and blood synthesized under the same diet (Sanpera et al 2007;Bugoni et al 2008;Quillfeldt et al 2008;Kohler et al 2011;Cruz et al 2012;Cherel et al 2014). Based on a metaanalysis of these studies, we used the following regressions from Cherel et al (2014) to standardize values to directly compare between δ 15 N and δ 13 C values obtained from the feather and blood tissues of the same individual penguin:…”