Uncertainties associated with trophic discrimination factor and body size complicate calculation of delta N-15-derived trophic positions in Arapaima sp.

Abstract

Stable isotope ratios of nitrogen (δ15N) have been used to estimate trophic position (TP) of organisms due to the predictable enrichment of nitrogen‐15 in consumer tissues relative to their diet. We explored estimates of trophic position using liver and muscle δ15N and stomach content analysis in a broad size range of Arapaima sp. from Amazonian floodplain lakes. Estimates of TP based on liver δ15N were more closely related to the stomach content data than estimates based on muscle δ15N, possibly because of the higher turnover of nitrogen in liver. Total length and season explained most of the variation in TP values estimated from δ15N, showing that they have more effect than prey trophic position on δ15N values. The TP estimated by identification of stomach content was 3.6 and was unrelated to the size of the arapaima. This highlights the need for a better understanding of the factors that affect values of δ15N and stomach content analysis is still needed to unravel the trophic ecology of predatory fishes.