Biological and Ecological Responses of Swordtip Squid <scp> <i>Uroteuthis edulis</i> </scp> to the 2023–2024 Marine Heatwave Off Sendai Bay, Northeastern Honshu, Japan

ABSTRACT Age, growth, and maturation of the swordtip squid Uroteuthis edulis in Sendai Bay during warm‐water (WW) periods (2019–2022) and marine heatwave (MHW) periods (2023–2024) were studied. This species migrates to Sendai Bay mainly in summer and autumn during WW periods, and from spring to autumn during MHW periods: earlier and longer periods of migration with consequently increased catches. During MHW periods, this species spawned in Sendai Bay, the hatched juveniles grew and reproduced there, showing a significant increase in growth, and males were larger than during WW periods. In contrast, the growth of females was not significantly different on comparing growth equations for WW and MHW periods. The lifespan of U. edulis caught in Sendai Bay was 8 months for both males and females in WW periods, and 11 months for males and 9 months for females during MHW periods, indicating longer lifespans during periods of higher seawater temperatures. Moreover, when spring water temperatures were high, U. edulis born in summer and autumn could migrate to Sendai Bay in spring, but when spring water temperatures were low, only U. edulis born in winter migrated from summer onwards. These effects are thought to result from differences in the hatching dates and/or migration periods of U. edulis during these two periods. In summary, the higher temperatures due to changes in the Kuroshio Current during periods of MHW had a significant impact on U. edulis in Sendai Bay, extending its period of migration into the region, resulting in higher abundance, accelerated growth of males, and longer time to mature.