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<p>Changes in biological traits such as growth, body condition, and maturity rate have been observed in relation to biomass fluctuations in fish species. Such changes among spawners could influence population productivity. The biomass of chub mackerel <i>Scomber japonicus</i> was low in the early 2000s, increased dramatically with the strong 2013&#xa0;year class, and declined from 2019. In this study, we examined how biological traits (age/size structure, fork length [FL], body condition [CF], and gonadosomatic index [GSI]) of chub mackerel spawners changed from 2006 to 2023. The strong 2013&#xa0;year class began appearing in spawning grounds at age 2 in 2015. FLs of the 2013–2019&#xa0;year classes were smaller than those of the 2004–2012&#xa0;year classes. Since 2017, the proportions of age-2 and age-3 fish among spawners have decreased, increasing the proportion of older spawners. Both CF and GSI have decreased since around 2015. The decline in GSI suggests reduced energy allocation to reproduction. Although both CF and GSI were negatively affected by conspecific biomass, they remained low even following the biomass decline in 2019. This implies that other factors, such as increased sardine biomass and changes in the marine environment, may also have suppressed CF and GSI.</p>

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The strong 2013 year class triggered an increased proportion of older individuals and a decreased gonadosomatic index in spawning chub mackerel Scomber japonicus

  • 期刊论文

  • 发表日期：

    2026年1月21日

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  • Sayoko Isu,
  • Yasuhiro Kamimura,
  • Sho Furuichi,
  • Takahiro Harada,
  • Kyu Okabe,
  • Takamasa Ichikawa,
  • Ryuji Yukami

摘要

Changes in biological traits such as growth, body condition, and maturity rate have been observed in relation to biomass fluctuations in fish species. Such changes among spawners could influence population productivity. The biomass of chub mackerel Scomber japonicus was low in the early 2000s, increased dramatically with the strong 2013 year class, and declined from 2019. In this study, we examined how biological traits (age/size structure, fork length [FL], body condition [CF], and gonadosomatic index [GSI]) of chub mackerel spawners changed from 2006 to 2023. The strong 2013 year class began appearing in spawning grounds at age 2 in 2015. FLs of the 2013–2019 year classes were smaller than those of the 2004–2012 year classes. Since 2017, the proportions of age-2 and age-3 fish among spawners have decreased, increasing the proportion of older spawners. Both CF and GSI have decreased since around 2015. The decline in GSI suggests reduced energy allocation to reproduction. Although both CF and GSI were negatively affected by conspecific biomass, they remained low even following the biomass decline in 2019. This implies that other factors, such as increased sardine biomass and changes in the marine environment, may also have suppressed CF and GSI.