Effects of temperature on tissue–diet isotopic spacing of nitrogen and carbon in otolith organic matter

Abstract

Reconstruction of the trophic position of a fish can be performed by analysing stable nitrogen and carbon isotopes in otolith protein. However, ambient temperature may affect the tissue–diet isotopic spacing of stable isotopes from diet to predator tissue and bias estimates of trophic position. To test this, otolith protein, heart and muscle tissue from a rearing experiment with juvenile cod held at different temperatures (4, 7, 10 and 14°C) were analysed. There was no significant effect of temperature on otolith δ15N, whereas muscle and heart exhibited a slight decrease in δ15N values with increasing temperature corresponding to maximum of 0.6‰ over the 10°C range. By contrast, the otolith protein δ13C values at 4 and 7°C were significantly higher than for 10 and 14°C, suggesting an approximate 1‰ increased tissue–diet enrichment at the lower temperatures. Temperature had no significant effect on muscle and heart δ13C values. Considering the annual mean variation in ocean temperatures, our results indicate that the trophic signals recorded in the otoliths will reflect changes in diet isotope values with little bias from the ambient temperature experienced by the fish.

Reading the biomineralized book of life: expanding otolith biogeochemical research and applications for fisheries and ecosystem-based management

“…Nitrogen in otoliths is in the form of proteins, comprising only 0.1-3% by mass (Campana 1999;Dauphin & Dufour 2003;Thomas & Swearer 2019). Until recently, the majority of δ 15 N studies combined multiple otoliths to obtain enough N for elemental-analysis coupled to isotope ratio mass spectrometry (EA-IRMS) (Godiksen et al 2019;Grønkjaer et al 2013;Sirot et al 2017;Vandermyde & Whitledge 2008). However, advances using a persulfate-denitrifier method, allow N isotope measurements from small, individual otoliths (Cheng et al 2018;Lueders-Dumont et al 2018, 2020.…”

“…However, advances using a persulfate-denitrifier method, allow N isotope measurements from small, individual otoliths (Cheng et al 2018;Lueders-Dumont et al 2018, 2020. For δ 13 C measurements of organics, otoliths are demineralized prior to isotopic analysis, facilitating measurements of organic C without dilution from the inorganic carbonate C that comprises most of the otolith mass (Godiksen et al 2019;Grønkjaer et al 2013;McMahon et al 2011aMcMahon et al , 2011bSirot et al 2017;Vane et al 2018a). Otoliths must be demineralized as the mineral fraction is a tracer of other, non-diet related variables (see Ecophysiology section).…”

“…Studies have only recently begun to measure the isotopic composition of otolith organics, including studies comparing muscle to otolith organic matrix isotope composition. In the few studies to compare signatures, otolith matrix and muscle protein δ 13 C values are similar and highly correlated with one another, including using CSIA-AA (McMahon et al 2011a(McMahon et al , 2011b and bulk analyses (Godiksen et al 2019;Grønkjaer et al 2013;Sirot et al 2017). The same is true for CSIA-AA δ 15 N values (Vane et al 2018b) and bulk analyses (Cheng et al 2018;Lueders-Dumont et al 2020;Vandermyde & Whitledge 2008).…”

“…Controlled laboratory experiments demonstrate stable isotopes of otolith organics are effective tracers of diet (e.g., Grønkjaer et al 2013;Lueders-Dumont et al 2020, with dietary tracing ability of otoliths immune to temperature effects (Godiksen et al 2019). Overall, otolith proteins rapidly incorporate N from diet (Shiao et al 2018) and in timescales similar to liver, the tissue with the shortest turnover time (Lueders-Dumont et al 2022).…”

Introduction to the 6th International Otolith Symposium

“…Chung et al (2019) comprehensively reviewed this technique to illustrate the potential of otolith d 13 C as a metabolic proxy that allows otoliths to record individual fish physiology. Godiksen et al (2019) described the temperature effects on tissue-diet isotopic trophic discrimination factors of nitrogen and carbon in otolith organic matter. Otolith isotopes were then used to reconstruct the trophic position of juvenile cod (Gadua morhua) and found temperature may influence the discrimination factors that could bias trophic-position estimates for fishes living in environments with seasonal temperature variation.…”