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

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.