Using otolith organic matter to detect diet shifts in Bardiella chrysoura, during a period of environmental changes

Accurate knowledge on fish trophic ecology and its modifications is crucial for understanding the impact of global change on ecosystems. In this context, we investigated the value of the δ¹³C and δ¹⁵N of otolith soluble organic matter (SOM) for identifying temporal diet shifts in American silver perch Bairdiella chrysoura over a 30-yr period characterized by strong changes in its population size and habitats within the Terminos Lagoon (Mexico). We first compared the otolith SOM isotopic signatures from present-day adults to those of muscle and the main local prey. Our results suggest that otolith SOM can be confidently extracted and analyzed for both present and past otoliths of this species. The mean otolith SOM signatures obtained (-15.92 ± 1.35‰ for δ¹³C and 9.38 ± 0.93‰ for δ¹⁵N) were consistent with those of the diet as 85% of the individual signatures were included within the prey isotopic niche area. Moreover, this study supports a trophic enrichment factor between diet and otolith (TEF_diet-otolith) close to 0 for δ¹⁵N, while for δ¹³C, the TEF_{otolith-muscle} of +0.02‰ warrants further investigation. Then, we compared past and contemporary otolith SOM signatures to investigate temporal diet shifts in B. chrysoura. This showed that δ¹³C and δ¹⁵N differed significantly between the past and present period even if the temporal shift remained relatively small (respectively +1.17‰ and -0.55‰). The present study substantiates the use of otolith SOM δ¹³C and δ¹⁵N as a proxy of fish present and past trophic position, opening the possibility for major progress in studies of temporal changes in food web ecology.