Stable Isotopes in Eye Lenses Record Patterns and Variation in Resource-Use Ontogeny of Three New Zealand Kelp Forest...

Fishes can undergo dramatic social and morphological changes throughout development that drive ontogenetic shifts in diet and habitat association. Measurements of trophic ontogeny at the individual level often complement population-level assessments, detailing foraging strategies that have underpinned long-term growth and survival. Using stable isotope measurements from muscle and eye lenses, we modeled size-based patterns in basal resource use and trophic position across multiple levels of organization for three New Zealand reef fishes (Notolabrus fucicola, Odax pullus, and Parapercis colias). From lens-derived data series, we were able to estimate trends and variability in lifetime trophic ontogeny of each species, as well as size-structured changes in breadth and interspecific overlap of resource use. For adults, broadly similar trophic shifts were reflected in isotopic composition of both muscle tissue and lens layers, with subtle differences between tissues for some combinations of species and ecological metric. Critically, only samples from eye lenses yielded estimates of resource use that supported early growth. Specifically, our measurements suggested heightened reliance on macroalgal food webs during post-settlement dispersal of two carnivores (N. fucicola, P. colias), as well as variable peaks in omnivory at small sizes for O. pullus, a primary herbivore. Trophic shifts modeled from eye lenses of carnivorous species were generally similar throughout early development, but highly inconsistent among juvenile O. pullus. Analyses of eye lenses also yielded evidence of trophic breadth contraction around size-at-maturity of all three species, coincident with apparent differentiation of adult resource use between sampled carnivores. Finally, in addition to high-resolution assessments of trophic ontogeny, we provide analytic considerations that may strengthen use of eye lenses for investigation of fish life history, particularly through examination of calibration assumptions in a novel system.