This study aimed to reconstruct temperatures experienced during the larval period by adult Pacific bluefin tuna Thunnus orientalis using high-resolution otolith stable oxygen isotope (δ18O) analysis. A novel otolith sample preparation protocol for secondary ion mass spectrometry (SIMS) analysis developed in this study reduced the background noise of SIMS measurements, enabling analyses of >10 times higher resolution around the otolith core compared to previous studies using conventional isotope ratio mass spectrometry (IRMS). The values obtained from SIMS were compared to those obtained by microvolume δ18Ootolith analysis using micromilling and conventional continuous-flow IRMS (CF-IRMS). There was a systematic offset (average 0.41‰ with SIMS resulting in lower values) most likely caused by matrix effects on SIMS δ18Ootolith values that can be calibrated using a strong linear relationship between SIMS and CF-IRMS measurements (r2 = 0.78, p < 0.001). The core-to-edge δ18Ootolith of 5 Pacific bluefin tuna revealed fine-scale seasonal variations in water temperature agreeing with known migration patterns. In addition, the ambient water temperature experienced during larval stages (about 10-20 d post hatch) estimated from otolith core δ18O ranged from 26.7 to 30.7°C, overlapping with temperatures associated with the occurrence of larval Pacific bluefin tuna. Combining SIMS and microvolume CF-IRMS δ18O otolith analyses offers a microscale examination of fish ecology that is not possible with conventional IRMS techniques. This novel method is particularly useful for understanding the early life history of fish that may be affected by climate change and reconstructing a well-resolved migration history for fish species that have small otoliths and/or narrow growth increments.
Species-specific ecological traits in fishes are likely to vary between populations or stocks due to differences in regional oceanic conditions, such as latitudinal temperature. We examined potential intraspecific differences in the swimming performance and metabolism of Pacific chub mackerel (Scomber japonicus) from the Northwest and Northeast Pacific stocks, which are distributed on opposite sides of the North Pacific at similar latitudes, but where the temperature contrast is large. Swimming bioenergetics and metabolic data of Northwest stock mackerel were measured at 14, 18, and 24°C using variable-speed swim-tunnel respirometers, and then the resulting bioenergetic parameters were compared with previous findings from the Northeast stock. At a given size, the maximum sustainable swimming speed (Umax) of the Northwest stock showed no significant difference compared to the Northeast stock at 18 and 24°C, but was lower at 14°C. In addition, the oxygen consumption rate (MO2) of the Northwest stock showed lower mass dependence and different temperature dependence at a given swimming speed than in the Northeast stock. Combined with stock-specific data on growth and experienced temperatures in the wild, these bioenergetic differences indicate that the swimming performance and metabolism of the two stocks are specific to their local environment to maximize bioenergetic efficiency.
This study examined starch granules from the dental calculus of specimens from the Epi-Jomon (Zoku-Jomon in Japanese, ca. 350 BCE–350 CE) period in Japan for taxonomic identification of plant food items and the reconstruction of human socioeconomic practices. Dental calculus was extracted from 21 individuals across six Epi-Jomon sites in Hokkaido. Moreover, 12 starch granules and starch clusters were recovered from nine individuals. The morphologies of the extracted starch granules were then classified into five types: elliptical, angular circular, polygonal, pentagonal, and damaged. Morphometric analysis indicated that a small portion of these starch granules may have derived from acorns, nuts, and bulb or tuber plants, with one starch granule supposedly from rice. Although extracted starch granules are poor predictors of food diversity at the individual level, the results can identify potential food sources of the surveyed population. This is the first study to determine how well plant microremains in dental calculus reflect a plant diet in the Epi-Jomon population. The starch granules discovered at the surveyed sites provide essential information about the utilization of plant species and cultural contacts in Hokkaido during this period. This is of great significance in reconstructing the Epi-Jomon subsistence patterns in Hokkaido and exploring cultural interactions between hunting-gathering-fishing and agrarian societies.
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