Intrinsic tracers reveal recent foraging ecology of giant Pacific bluefin tuna at their primary spawning grounds

Abstract

Pacific bluefin tuna Thunnus orientalis (PBFT) play important economic and ecological roles in the western Pacific Ocean. We currently lack basic information on PBFT foraging that would facilitate ecologically informed recovery strategies for this species. We used stable isotope analysis to investigate recent (previous ~1.5 yr based on isotopic turnover rate) trophic ecology of 261 giant (>180 cm), sexually mature PBFT entering their major spawning grounds off Taiwan. We performed amino acid-compound specific isotope analysis (AA-CSIA) on a subset of PBFT and select prey to assess the trophic position of PBFT in the western Pacific and to validate putative recent trans-Pacific migration from the eastern Pacific Ocean. Bayesian isotopic mixing model results suggested recent PBFT foraging off eastern Japan and in the Kuroshio-Oyashio transition region, with minimal inputs from the Sea of Japan and Taiwan waters. PBFT did not appear to feed primarily on zooplanktivorous forage fish (e.g. sardine, anchovy) but on higher trophic-level prey including mackerels, squids, and pomfrets. AA-CSIA confirmed a high trophic position (> 5) of PBFT in this region and identified putative recent trans-Pacific migration of 2 individuals. This study identifies the prey base that sustains giant PBFT before migrating to spawning grounds off Taiwan and sets the stage for future studies comparing the movements and ecology of PBFT in the western Pacific Ocean.

Stable isotope analysis reveals ontogenetic feeding shifts in Pacific blue marlin (Makaira nigricans) off eastern Taiwan

“…higher δ 15 N values presumably due to the ability of larger fish to capture larger and more diverse prey at higher trophic levels (Davenport & Bax, 2002;Graham et al, 2007;Tsai et al, 2014). Different body sizes of predators have also been linked to differences in resource utilisation and shifts in preferred habitat (Werner & Gilliam, 1984 Isotopic studies on T. orientalis in western and eastern Pacific showed similar patterns to our study in that δ 15 N values of T. orientalis in the western Pacific Ocean increased with size while those in eastern Pacific Ocean did not (Madigan et al, , 2016. This discrepancy may be related to the differences of primary production and foodweb structure in different regions (Sommer et al, 2002).…”

“…In contrast to M. nigricans in Taiwan, those in Palau showed no isotopic increase with body size. Isotopic studies on T. orientalis in western and eastern Pacific showed similar patterns to our study in that δ 15 N values of T. orientalis in the western Pacific Ocean increased with size while those in eastern Pacific Ocean did not (Madigan et al ., , ). This discrepancy may be related to the differences of primary production and food‐web structure in different regions (Sommer et al ., ).…”

“…Trophic position of M. nigricans ranged from 3.97 for smaller fish (< 200 cm) to 4.97 for larger fish (> 200 cm). The increase of δ 15 N values and increased trophic position has been correlated to increases in body mass in large fish (Lindsay et al ., ; Jennings et al ., ) and many pelagic predators display ontogenetic shifts in diet and resultant higher trophic position changes with size (Estrada et al ., ; Madigan et al ., ). Similar patterns were found in yellowfin tuna Thunnus albacares (Bonnaterre 1788), with higher δ 15 N values observed at sizes >45 cm in Ecuador (Varela et al ., ) and changing diet compositions at different growth stages (Popp et al ., ).…”

“…Madigan et al . () showed a similar pattern in T. orientalis , in that δ 15 N values in east of Taiwan were lower than those in the more productive waters off Japan and in the Kuroshio–Oyashio transition region.…”

Environmental associations of Pacific bluefin tuna (Thunnus orientalis) catch in the California Current system

“…The lack of correspondence with surface primary productivity is likely due to the tendency of PBF, and other tunas, to feed opportunistically and prey on a diverse forage base, including fish, cephalopods, and crustaceans (Madigan et al., ; Pinkas, Oliphant, & Iverson, ; Shimose, Watanabe, Tanabe, & Kubodera, ). Early studies of PBF diets showed a predominance of anchovy in stomach contents, with this species comprising up to 86% of the prey assemblage (Pinkas et al., ).…”