Multi-proxy approach for studying the foraging habitat and trophic position of a migratory marine consumer in the southwestern Atlantic Ocean

Skipjack tuna Katsuwonus pelamis sustains the largest catches among tuna species. Despite its relevance for global food security and the tuna canning industry, information about its foraging ecology in the southwestern Atlantic Ocean (SWAO) is limited. We combined amino acid (AA) and bulk tissue nitrogen isotope (d15N) analysis with stomach content analysis (SCA) to study skipjack foraging habitat and trophic position (TP) in the SWAO. Mean (+/- SD) delta N-15 values of source AAs lysine (Lys: 6.5 +/- 1.3%) and phenylalanine (Phe: 10.6 +/- 1.3%) were higher at higher latitudes (30-34 degrees S) of the southern area relative to the northern area between 20-26 degrees S (Lys: 3.4 +/- 1.1%, Phe: 8.2 +/- 1.2%). Correlations between bulk tissue delta N-15 (delta(15)Nbulk) and source AA delta N-15 showed that Lys is a more robust indicator of delta N-15 baseline variation in this region than Phe. Regional mixing models based on AA delta N-15 indicated spatial structure in foraging habitat: smaller individuals use the southern area and larger individuals forage at lower latitudes or in offshore areas. TP estimates using the Glx-Lys trophic-source AA pair were in much better agreement with TP estimates based on SCA (TPSCA) and on bulk tissue analysis (TPbulk) than those using Glx-Phe. Skipjack forages across multiple food webs throughout its lifecycle in the SWAO and increases its TP with ontogeny (TPGlx-Lys: 3.5-4.0). Stock management strategies should consider that the southern region supports dense aggregations of juvenile skipjack, which may be more vulnerable to non-selective fisheries.