Stable isotopes in eye lenses reveal migration and mixing patterns of diamond squid in the western North Pacific and its marginal seas

Abstract

Knowledge of the movements of marine organisms is essential for effective conservation schemes. Here, we investigated the lifetime habitat use of diamond squid, Thysanoteuthis rhombus, collected in the western North Pacific and its marginal seas (the Sea of Japan and the East China Sea) during 2021–2022, whose migratory ecology is poorly known, using bulk stable nitrogen and carbon isotope ratios in eye lenses. From the eye lens isotope profiles, the chronology of the isotopic baseline of squid habitat was estimated by removing the effect of size-dependent changes of trophic position. Then, the baseline estimates were compared to the isoscapes of particulate organic matter. The baseline chronologies showed fluctuations during the paralarval and juvenile stages, becoming stable during the adult stage, suggesting that significant movements mainly occur during the early life stages due to current transport, with adults potentially not undertaking long-distance migrations. The squids in the marginal seas mostly originated from outside the subtropical gyre, while the squids in the subtropical gyre had various sources, including outside the gyre and southern and northern parts within the gyre, revealing a complex mixing pattern of the species. These results show that isotope chronology combined with baseline isoscapes are effective tools to understand animal migrations, which can help managing various cephalopods and fish.

Investigation of inter-annual variation in the feeding habits of Japanese sardine (Sardinops melanostictus) and mackerels (Scomber spp.) in the Western North Pacific based on bulk and amino acid stable isotopes

“…are abundant, than in the transition zone (Ishikawa et al, 2023). Furthermore, the significantly lower bulk d 13 C values in age-1+ mackerel compared to Japanese sardine (Figure 2C) are consistent with the fact that mackerel is partially dependent on the food web of the Oyashio system, because d 13 C values of particulate organic matter (POM) and fishes in the cold Oyashio region are significantly lower compared with warm Kuroshio region (Ohshimo et al, 2019;Sakamoto et al, 2023). Although detailed data on migration history is not available for the samples in this study, the results indicate that changes in migration area or depth with growth may lead to inter-annual changes in food resources.…”