Data for: Fisheries shocks provide an opportunity to reveal multiple recruitment sources of sardine in the Sea of Japan

Understanding recruitment sources is essential for stock assessments of
marine fish populations. In 2014 and 2019, schools of sardine in the Sea
of Japan and East China Sea (SJ-ECS), which come to spawn in Japanese
coastal areas every spring, were shockingly sparse. Abundances of eggs and
juveniles also declined abruptly, indicating a sharp decline in
reproduction in the SJ-ECS. However, in spring of 2015 and 2020, age-1
fish appeared as usual in the coastal areas, challenging the current
assumption that sardine in the system is a self-recruiting subpopulation.
To test the self-recruiting hypothesis, we analysed the stable oxygen and
carbon isotopes (δ18O, δ13C) for otolith areas formed during the first
spring and summer in otoliths of age-0 and age-1 sardines in 2010 and
2013–2015 year-classes captured in the SJ-ECS, as indices of temperature
and metabolic trajectories. Age-0 sardines generally showed a significant
decrease in otolith δ18O from spring to summer, reflecting seasonal
warming in the SJ-ECS. However, the majority of age-1 captured in spring
2011, 2015, and 2016 showed non-decreasing profiles. The δ18O for summer
thus revealed different migration groups: the “locals" growing up off
the Japanese coast and the migrating “nonlocals", the former not
being the main source of recruitment, contrary to previous assumptions.
The isotope values of the “nonlocals” overlapped with those of age-0
captured in the subarctic Pacific, suggesting that the “nonlocals” may be
migrants from the Pacific, or an unobserved northward migration group in
the SJ-ECS. Only in 2014 did the majority of age-1 consist of the
“locals”, suggesting that the abrupt decline in catches was caused by the
absence of the “nonlocals” and accompanying adults. Our results highlight
the considerable uncertainty in the population structure assumed in
current stock assessment models for Japanese sardine, thereby requiring
focused investigations on their migrations for sustainable fisheries.

明确鱼类种群的补充来源对于海洋鱼类种群资源评估至关重要。2014年和2019年，每年春季前往日本沿岸产卵的日本海与东海（SJ-ECS）沙丁鱼鱼群数量异常稀少。鱼卵和幼鱼的数量亦急剧下降，表明该海域（SJ-ECS）沙丁鱼的繁殖量显著减少。然而，2015年和2020年春季，1龄鱼仍如期出现在沿岸区域，这对当前认为该系统内沙丁鱼为自我补充亚种群（self-recruiting subpopulation）的假设提出了挑战。

为验证自我补充假设（self-recruiting hypothesis），我们分析了2010年及2013–2015年世代在SJ-ECS捕获的0龄和1龄沙丁鱼耳石（otolith）中首个春夏季节形成区域的稳定氧、碳同位素（δ18O、δ13C），将其作为温度和代谢轨迹的指标。0龄沙丁鱼耳石的δ18O值通常从春季到夏季显著下降，反映了SJ-ECS的季节性升温。然而，2011、2015和2016年春季捕获的大多数1龄鱼呈现非下降型δ18O剖面。夏季δ18O值因此揭示了不同的迁移群体：在日本沿岸生长的‘本地群’（locals）和迁移的‘外地群’（nonlocals），与先前假设相反，前者并非补充的主要来源。

‘外地群’（nonlocals）的同位素值与亚北极太平洋捕获的0龄鱼重叠，表明‘外地群’可能是来自太平洋的迁移者，或是SJ-ECS中未被观测到的北向迁移群体。仅2014年，大多数1龄鱼由‘本地群’（locals）组成，这表明渔获量的急剧下降是由于‘外地群’及伴随成鱼的缺失所致。我们的研究结果凸显了当前日本沙丁鱼资源评估模型中所假设的种群结构存在显著不确定性，因此需针对其迁移行为开展重点研究，以实现渔业可持续发展。