Estimating the spawning ground of Pacific saury Cololabis saira by using the distribution and geographical variation in maturation status of adult fish during the main spawning season

Abstract

We analyzed distribution and differences in body length, age, and maturation rate of Pacific saury Cololabis saira in the western North Pacific Ocean, during the main spawning season (winter), to locate its spawning ground. Sampling was conducted in the coastal area north of the Kuroshio axis (CA), the offshore area of the southern Japanese archipelago (SA), and the area east of the Honshu Island (EA) from 1996 to 2017. Higher densities of Pacific saury were widely observed on or just north (within 100–200 km) of the axis of the Kuroshio and the Kuroshio Extension currents. Body length, age composition, and sea‐surface temperature (SST) differed across the studied areas, with larger fish (>250 mm knob length, mainly age‐1) dominating at higher SST (16–21°C) in the SA and smaller fish (<250 mm knob length, age‐0) dominating at lower SST (14–16°C) in the EA. In the CA, both SST and body length values were in between those in the SA and EA. The area where matured Pacific saury were collected was broadly distributed in the study area, except for the CA and a part of the Transition zone where the SST was low and mean body length was short. This study suggests that the main spawning ground during winter, where the area of high density and high maturation rate overlap, is formed on or just north of the Kuroshio and the Kuroshio Extension axis.

Biological interactions potentially alter the large-scale distribution pattern of the small pelagic fish, Pacific saury Cololabis saira

“…The general seasonal migration pattern among the species is similar (Yatsu 2019). They generally spawn in subtropical waters from winter to spring (Takasuka et al 2008, Kanamori et al 2019, Fuji et al 2021. The Kuroshio−Oyashio transition zone and some subarctic waters are important feeding grounds for the adults and juveniles during summer (Okazaki et al 2019, Miyamoto et al 2022.…”

“…The main spawning grounds of the species other than Pacific saury are the subtropical waters around the Kuroshio current along the southern coast of Japan in winter and spring (Takasuka et al 2008, Kanamori et al 2019. The spawning ground of Pacific saury extends farther eastward than that of the other 3 species (Fuji et al 2021). Because of geographical differences in their spawning grounds, Pacific saury have a different distribution pattern than the other 3 species (see Table 1).…”

Effects of oceanographic environment on the distribution and migration of Pacific saury (Cololabis saira) during main fishing season

“…The population dynamics of Pacific saury are more affected by oceanic and climatic factors than by fishing, and annual variations in Pacific saury abundance are strongly associated with changes in winter SST in the Kuroshio Current and the mixed layer depth (MLD) in the Kuroshio Extension 3 , 11 – 14 . The mechanisms controlling the regime shifts in the abundance of Pacific saury have been previously well established: changes in the sizes and positions of the spawning ground alter the conditions of spawning and fish survival, which then affect the recruitment and abundance of this species 10 .…”

Reduced body weight of Pacific saury (Cololabis saira) causes delayed initiation of spawning migration

“…During their short life cycle, Pacific saury migrate through a large area (Figure 1). Their primary spawning ground is in the Kuroshio and Kuroshio Extension region during winter (Fuji et al, 2021; Watanabe & Lo, 1989). Hatched larvae and juveniles are transported east along the Kuroshio Extension (Oozeki et al, 2015), after which they migrate north from subtropical regions into subarctic Oyashio waters from May to July (Fukushima, 1979).…”