Examining the utility of bulk otolith δ13C to describe diet in wild-caught black rockfish sebastes melanops

Abstract

Otolith carbon isotope δ13C values may provide temporally resolved diet proxies in fish. If otolith δ13C values reflect diet, isotope values from recent otolith and muscle tissue should correlate and known ontogenetic diet shifts should be reflected in comparisons between otolith material deposited during different life history stages. We analyzed paired otolith and muscle samples for δ13C from black rockfish Sebastes melanops to examine the potential of otoliths to reflect diet in small (200−299 mm fork length) and large (≥300 mm) fish. We found a significant positive regression between δ13C values from recent (~12 mo) otolith material and muscle in large fish, but not in small fish. Within individual otoliths, δ13C values were enriched by ~3‰ in recent otolith edge material compared to age-0 otolith core material and were consistent with known nearshore−offshore gradients in δ13C values at the base of the food web. Bulk otolith δ13C appeared to provide a broad indicator of dietary carbon sources, but variation in metabolism and dissolved inorganic carbon δ13C among and within individuals likely influences otolith δ13C as well and limits precision. Nevertheless, the results are promising and bulk otolith δ13C may be an appropriate tool to examine large trophic and ecosystem level shifts that have occurred concurrently with changes in habitat, commercial fishing, invasive species, climate change, and other direct or indirect human impacts using historic or ancient otoliths. Future studies should continue to consider the utility of bulk otolith δ13C to describe diet in other marine fish using this simple approach.