The effect of trophic level on carbon isotope values
The potential influence of trophic level on measured δ13C values was tested by comparison between carbon and nitrogen isotope compositions in scale tissue from salmon populations returning to the UK River Frome (RF) and North East Coast (NEC). The isotopic composition of carbon and nitrogen in fish tissues is controlled to different degrees by variations in isotopic composition at the base of the food web and by trophic discrimination. Nitrogen is more strongly fractionated than carbon during dietary assimilation, with a mean trophic increase of c.3‰ compared to c.1‰ in carbon262728. In our sampled scales, δ15N values correlate positively with size (RF salmon: n = 231, R2 = 0.41, p<0.001; NEC salmon: n = 162, R2 = 0.17, p<0.001), reflecting changes in trophic level (Fig. 1). Fish returning after more than one winter at sea clearly fed at higher trophic levels than the smaller 1SW returning fish. δ13C values also differ between 1SW and MSW returning fish, but show no positive relationship with either size (Fig. 1) or with δ15N values (Fig. 2), indicating that differences in the carbon isotope composition of scales cannot be explained by mass or trophic level. This strongly indicates that the carbon isotope composition of salmon collagen is dominated by variations in photosynthesis-related fractionation at the base of the food chain, and that salmon feeding in a common marine area are expected to have similar δ13C values45.
Despite the large variation in plankton δ13C values across the North Atlantic Ocean142348, salmon returning to the River Frome maintain consistently different δ13C values to those returning to the NE Coast throughout the 18 year sampling period (ANOVA, with geographic origin as sole co-factor; n = 523, df = 1, F = 116.9, p<0.001). In addition, year and sea age both significantly influence δ13C values (Year, n = 523, df = 17, F = 9.1, p<0.001; sea age df = 1, F = 16, p<0.001), with a significant interaction between all factors. These data strongly suggest that, in each sampled year, returning salmon from different natal origins fed in distinct locations. Stable isotope evidence for stock separation in feeding grounds is further explored in the supplementary information.