Interannual variations in diet of Japanese jack mackerel (Trachurus japonicus) juveniles in the southwestern Sea of Japan in relation to recent growth rate

Abstract

We examined interannual variations in the diet of Trachurus japonicus juveniles in relation to prey density and recent 5‐day instantaneous growth rate (IGR) in the southwestern part of the Sea of Japan (SJ) in late spring during 2011–2013. Habitat temperature of juveniles in 2011 was lower than the other 2 years. In all 3 years, more than 97% of juveniles had food in the stomach. Small juveniles <35‐mm standard length (SL) fed mainly on adult female Paracalanus parvus s.l., the most dominant calanoid copepod in the water column, without a significant interannual difference. In 2011 and 2012, large juveniles ≥35‐mm SL predated heavily upon adult female Calanus sinicus which form a high‐energy food for the juveniles. In contrast, in 2013, the large juveniles preyed on a large number of a variety of small‐sized prey items, such as P. parvus s.l., euchaetid copepods, and ostracods. In 2011 when the feeding condition was considered to be favorable, but the habitat temperature was lowest, the IGR of large juveniles was markedly lower than in the other 2 years. Meanwhile, there was no significant difference in the IGR between 2012 and 2013 when the diet composition was markedly different, but habitat temperature was comparable. The present observations indicate that the between‐year difference in juvenile IGR was not explained well by variations in the diet composition. Instead, habitat temperature was concluded to be the more dominant factor causing the observed between‐year differences in the IGR in the southwestern SJ.

Linking environmental drivers, juvenile growth, and recruitment for Japanese jack mackerel Trachurus japonicus in the Sea of Japan

“…Major prey organisms for larvae and juveniles of T. japonicus have been reported as Paracalanus parvus s. l. (hereafter referred to as P. parvus) and Calanus sinicus in the ECS (Sassa et al, 2019;. Our recent study demonstrated that prey organisms for small (<35 mm standard length, SL) juveniles of T. japonicus used in this study were mainly P. parvus, while those for large (≥35 mm SL) juveniles were C. sinicus during 2011-2013 (Sassa et al, 2021). We therefore assumed that mean density of P. parvus and C. sinicus in waters with occurrence of T. japonicus juveniles would be indicative of the prey abundance for small and large T. japonicus juveniles, respectively.…”