Isotope-based inferences of skipjack tuna feeding ecology and movement in the southwestern Atlantic Ocean

Skipjack tuna (Katsuwonus pelamis) sustain a large-scale fishery in the southwest Atlantic Ocean (SWA), but information about its foraging ecology in this region is still limited. Here we use carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analysis of muscle from individuals collected in 2017-2018 (n = 383) to quantify diet composition and characterize movement patterns. We found a relatively small degree of variation in δ¹³C (range: -18.9 to -16.5‰) in comparison to δ¹⁵N values (6.7-14.7‰). At higher latitudes in the southern area (30-34°S), individuals had higher mean (±SD) δ¹⁵N values (12.2 ± 1.3‰) in comparison to those collected in the northern area (9.7 ± 1.5‰) between 20-26°S. At the northern area, isotope mixing models with informative priors showed that lanternfish (median: 50%) and krill (31%) were the primary foods. In the southern area, lanternfish (53%), krill (23%) and small pelagic fish (23%) were the primary food sources. Spatial shifts in diet composition were related to warming events that likely resulted in low abundance of sardines in the northern area. The latitudinal pattern in skipjack and krill δ¹⁵N values mirrored that of regional zooplankton isoscapes, suggesting residency at the timescale of isotopic turnover for muscle (~2-4 months), and that geographical variation in the baseline isotopic composition can be exploited to characterize seasonal movements of skipjack and other top marine consumers in this region.