Annual reproductive cycle and spawning ground formation in Japanese jack mackerel Trachurus japonicus in the Uwa Sea, Japan

Spatiotemporal variability in the occurrence of juvenile Japanese jack mackerel Trachurus japonicus along coastal areas of the Kuroshio Current

“…As discussed below, landings from set nets depend on small‐scale currents and the migration of schools, and juveniles from w‐CAK could occasionally dominate at the local scale, even if the abundance at the area scale (e.g., w‐CAK and e‐CAK) was lower than that from the s‐ECS. A similar inference was also made for the EH subarea by Hashida et al, (2017), Hashida et al, (2019), who reported a difference between the main hatching period for juveniles (March–April) and the local spawning period (late April–early July). Because spawning in the e‐CAK occurs considerably later (May–July), no contribution to the spring onset was expected.…”

“…japonicus that contribute to its occurrence in these areas have been classified into the local spawning grounds in the coastal areas along the Kuroshio (collectively referred to as “CAK”) and the remote spawning ground in the southern East China Sea (s‐ECS) (Figure 1). The local spawning grounds at the scale of bays and channels along the CAK, identified by sampling mature adults, have been documented since at least the 1950 s (Hashida et al, 2019; Hattori, 1964; Sakaji, 2001; Sakamoto et al, 1986; Sawada, 1974; Yakushiji, 2001; Yokota & Mita, 1958). Using maturation indices (mainly the gonadosomatic index) for adult T .…”

“…Using maturation indices (mainly the gonadosomatic index) for adult T . japonicus , the spawning seasons in each of the areas within the CAK were estimated to be May–July in Sagami Bay (Kobata, 1972; Sawada, 1974), February–June in Kii Channel (Sakamoto et al, 1986), January–June in Sukumo Bay (Sakaji, 2001), and February–June (Yakushiji, 2001) and January–July (Hashida et al, 2019) in Bungo Channel (Figure 1).…”

Hatch date and early growth of juvenile Japanese jack mackerel Trachurus japonicus caught in the Uwa Sea

“…In the Uwa Sea, which is located in the Pacific Ocean (Figure 1), the Japanese jack mackerel is an important target species, especially for purse seine fisheries. Young-of-the-year Japanese jack mackerel with a fork length (L F ) of 20-140 mm (hereafter referred to as juveniles), centring on 30-80 mm, are recruited from May to June in the Uwa Sea (Hashida et al, 2017). Based on the linear correlation between the number of daily rings on the otolith and L F , juveniles were inferred to have hatched during March to April (Hashida et al, 2017).…”

“…Young-of-the-year Japanese jack mackerel with a fork length (L F ) of 20-140 mm (hereafter referred to as juveniles), centring on 30-80 mm, are recruited from May to June in the Uwa Sea (Hashida et al, 2017). Based on the linear correlation between the number of daily rings on the otolith and L F , juveniles were inferred to have hatched during March to April (Hashida et al, 2017). Hashida et al (2019) This study aimed to elucidate the age at the onset of metamorphosis (d m ) as a proxy for the development rate and early growth rate to assess the abundance variation and origin of juveniles.…”