Interannual variation of the diet shifts and their effects on the fatness and growth of age‐0 Pacific bluefin tuna (Thunnus orientalis) off the southwestern Pacific coast of Japan

Abstract

We investigated the effect of feeding habits on the fatness (length–weight relationship (LW)) and otolith growth of age‐0 Pacific bluefin tuna (PBF) (fork length (FL) of 15.0–29.9 cm) collected off the southwestern Pacific coast of Japan from July to August during 2010–2015. The feeding rate on Clupeiformes (FRc) was substantially influenced by the path of the Kuroshio Current during early summer. We suggest that the following patterns occurred in the feeding habits of PBF. In 2012 and 2014, the short distance from the coast to the Kuroshio provided high overlap between PBF and Clupeiformes and increased the opportunity for feeding, and this pattern likely caused the observed diet shift from Crustacea to Clupeiformes by smaller PBF in the studied years than those in other years. In 2011 and 2015, the encounter probability for Clupeiformes might have been low due to the long distance to the Kuroshio Current, which led PBF to actively consume non‐Clupeiformes Teleostei. In contrast, in August 2010 and 2015, PBF did not encounter Clupeiformes or other Teleostei and may have been forced to feed on a large number of Crustacea; this effect was likely due to high water temperatures. Despite the interannual fluctuation in the timing of the diet shift and the feeding intensity (FI), these factors did not contribute to growth in length; rather, they contributed to fatness. Considering size‐dependent mortality in the larval stage, these results suggest that only fast‐growing individuals survive, even if the feeding conditions drastically change.

Spatial and Seasonal Variations of Stable Isotope Ratios of Particulate Organic Carbon and Nitrogen in the Surface Water of the Kuroshio

“…As the Kuroshio path has a large impact on the ecosystems in the Kuroshio (Hiraoka et al, 2019;Nakata et al, 2000), further studies are necessary to understand the annual variations and anthropogenic impacts on primary production in the Kuroshio area.…”

“…The δ 15 N increases with depth (Casciotti et al, 2008), and the depth is the proxy for the light conditions, which have an impact on the δ 13 C of phytoplankton such as coccolithophores (Rost et al, 2002). The latitude of the Kuroshio axis is the index of the Kuroshio path, which changes the ecosystem type and productivity in the vicinity of the Kuroshio (Hiraoka et al, 2019;Nakata et al, 2000), but the effects to the primary production have not been sufficiently evaluated. Salinity is the index of mixing between riverine and/or estuary POM and oceanic POM.…”

Natal origin and age-specific egress of Pacific bluefin tuna from coastal nurseries revealed with geochemical markers

“…The path and coastal intrusion of the Kuroshio Current varies seasonally and annually in the Ryukyu Archipelago and off the east coast of Japan 34 . Inshore-offshore fluctuations in the Kuroshio Current and associated mesoscale features (e.g., large meander) commonly occur and are known to influence the spatial distribution of prey 58 , 59 . Moreover, the position of the Kuroshio Current relative the coastline also influences the habitat use and movement of age-0 PBT between inshore and offshore waters 17 , 56 , 60 .…”

Natal Origin and Age-Specific Egress of Pacific Bluefin Tuna From Coastal Nurseries Revealed With Geochemical Markers

“…The path and coastal intrusion of the Kuroshio Current varies seasonally and annually in the Ryukyu Archipelago and off the east coast of Japan 34 . Inshore-offshore uctuations in the Kuroshio Current and associated mesoscale features (e.g., large meander) commonly occur and are known to in uence the spatial distribution of prey [57][58] . Moreover, the position of the Kuroshio Current relative the coastline also in uences the habitat use and movement of age-0 PBT between inshore and offshore waters 17,55,59 .…”