Reproductive success can be influenced by the match or mismatch between the spawning season and optimal environmental conditions for larval survival. However, it is challenging to understand how temporal factors affect population dynamics. In the central Seto Inland Sea, Hiuchi-nada, western Japan, the recruitment of Japanese anchovy (Engraulis japonicus) has markedly decreased in the last decade. To explore the causes of this decline, egg survey and commercial catch data from 1994 to 2019 (26 years) between April and August were used to examine egg production, hatch date, and early survival in relation to environmental factors. Otolith microstructure analysis indicated that most landed larval and juvenile fish (recruits) hatched in May and June. Egg and recruit abundances were negatively correlated over the study period. The survival indices from hatching to recruitment were higher in waters at 16–24°C in a given season until the mid-2000s. Thereafter, they decreased over the studied temperature range and remained relatively low over the last five years. The copepod nauplii densities from 2015 to 2019 were significantly higher than those from 2002 to 2005 due possibly to the effect of increasing temperature, but the ratio of copepod nauplii density to larval density in May and June from 2015 to 2019 was approximately 34% and 66% of those from 2002 to 2005, respectively. These findings and the hydrodynamic features of these waters suggest that the recent decline in recruit abundance is partly due to a decrease in larval survival in response to increasing intracohort competition for prey in early-life stages.

中文翻译：

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日本濑户内海中部环境因素对日本鳀鱼（Engraulis japonicus）孵化日期和早期存活的时间变化的响应

繁殖成功会受到产卵季节与幼虫存活的最佳环境条件的匹配或不匹配的影响。然而，了解时间因素如何影响人口动态具有挑战性。在日本西部桧内滩濑户内海中部，招募日本鳀鱼（Engraulis japonicus) 在过去十年中显着下降。为了探索这种下降的原因，使用 1994 年至 2019 年（26 年）4 月至 8 月间的鸡蛋调查和商业捕捞数据来检查与环境因素相关的鸡蛋产量、孵化日期和早期存活率。耳石微观结构分析表明，大多数上岸的幼鱼和幼鱼（新鱼）在 5 月和 6 月孵化。在研究期间，鸡蛋和新兵丰度呈负相关。直到 2000 年代中期，在特定季节，从孵化到招募的存活指数在 16-24°C 的水域中较高。此后，它们在研究的温度范围内下降，并在过去五年中保持相对较低。2015-2019年桡足类无节幼体密度显着高于2002-2005年，可能受气温升高影响，但2015-2019年5-6月桡足类无节幼体密度与幼体密度之比约为34%和66分别为 2002 年至 2005 年的百分比。这些发现和这些水域的流体动力学特征表明，最近招募丰度的下降部分是由于幼虫存活率下降，这是对早期生命阶段猎物的群内竞争加剧的反应。