Stable isotope (δ13C and δ15N) and trophic position of Patagonian sprat (Sprattus fuegensis) from the Northern Chilean Patagonia

Patagonian sprat (Sprattus fuegensis) is an important forage fish species along the southern tip of South America. We determined the isotopic composition (δ13C and δ15N) and trophic position of Patagonian sprat for two zones in the North Chilean Patagonia: the Inner Sea of Chiloé (42-45°S) and the Southern Channels (channels north of the Taitao Peninsula, 45-47°S). Samples of muscle tissue were obtained from adult sprat in fall 2013, along with particulate organic carbon (POC) and the mesozooplankton (copepods, euphausiids, amphipods) that make up the sprat diet. The δ13C values we found for POC in the North Chilean Patagonia ranged from −22‰ (ISCh) to −20.3‰ (SC), suggesting little influence of terrigenous organic carbon. The mean δ13C values determined from copepods in our study were −20.7 ± 0.7‰ (ISCh) and −20.5 ± 08‰ (SC); the values found for euphausiids (−18.4 ± 10 to −18.8 ± 0.8) and amphipods (−19.6 ± 0.8 to −18.9 ± 1.0) were even higher, revealing that they were composed mostly of marine organic carbon. Average lipid corrected δ13C (δ13C′) values for Patagonian sprat muscle tissues in the North Chilean Patagonia ranged between −16.8 ± 0.5‰ (ISCh) and −15.8 ± 0.4‰ (SC), suggesting this species' diet was based primarily on organisms with a marine organic carbon signal. Differences in δ13C were observed in small (<12 cm) Patagonian sprat between SC (higher values) and ISCh, suggesting local variations in the diet. A positive relationship was found between δ15N values and fish length for Patagonian sprat in ISCh, where higher productivity has been reported, indicating an increase in the trophic position with body size. Smaller Patagonian sprat in SC, where productivity is lower, consumed organisms from a higher trophic level than those in ISCh. This versatile feeding behavior (local differences between regions, and changes in diet with fish length) may be part of a feeding strategy that is beneficial in areas with very strong seasonal variations in food webs and overall food availability.