Variable uptake and elimination of stable nitrogen isotopes between tissues in fish

Abstract

We conducted a diet-switching experiment using freshwater ocellate river stingrays (Potamotrygon motoro) fed a novel earthworm (Eisenia foetida) diet to establish the relative contributions of growth and metabolism to δ15N values in an elasmobranch species. We specifically controlled for the potential effects of protein composition of experimental diets on δ15N turnover to determine whether δ15N turnover after a low to high δ15N diet switch (uptake) and a high to low δ15N diet switch (elimination) will occur at the same rate within each consumer tissue. Our results showed that the turnover of δ15N from metabolism and growth differed between uptake and elimination phases in the liver, blood, cartilage, and muscle of freshwater stingrays. During uptake, liver was found to track dietary δ15N more closely than the other tissues, with the highest metabolic turnover rate of δ15N (0.015 day–1), whereas cartilage had the slowest rate of metabolic δ15N turnover (0.0022 day–1) relative to a constant rate of growth among tissues (0.003 day–1). We propose that estimates of trophic position from muscle sampling alone have considerable uncertainty, particularly for scavenging or omnivorous species. We suggest that multitissue sampling can identify this problem and lead to a more robust evaluation of trophic dynamics for individual species.