Juvenile steelhead Oncorhynchus mykiss enter the northern California Current ecosystem from the Columbia River and other Pacific Northwest coastal rivers and include several populations that are listed under the U.S. Endangered Species Act. However, relatively little is known about the response of these populations to interannual variability in ocean conditions. In 2015 and 2016, anomalous ocean conditions, called the warm “Blob,” persisted in the northern California Current, increasing ocean temperatures by over 2.5°C. To determine how steelhead respond to such major shifts in temperature, we compared diet composition, stomach fullness, size, and body condition of juvenile steelhead collected in 2015 and 2016 to those of a subset of steelhead collected in previous surveys (2001, 2002, 2004, and 2006–2011) that included warm, cool, and neutral ocean years. In addition, we used bioenergetics models to assess the importance of the marine environment in contributing to changes in steelhead condition under different ocean temperature scenarios. Steelhead from 2015 and 2016 exhibited some of the poorest body condition values but were the largest sizes across all years evaluated. Steelhead diet composition varied between warm and cold years and between warm and average years, with steelhead consuming more insects, juvenile rockfishes, and rare and unidentified fish in warm years. Unusual taxa, including gelatinous salps (2015 and 2016) and juvenile smelts (2016), were consumed during Blob‐influenced years. Bioenergetics models indicated that interannual variability in growth was influenced by changes in temperature and feeding conditions in the marine environment, with significant differences in growth between warm and cold ocean years apparent as early as day 2 of the simulated marine residence. These findings highlight the potential for warm ocean years to influence the diet composition and morphology of Columbia River steelhead populations and may lead to a better understanding of factors influencing survival of juvenile steelhead during early marine residence.

中文翻译：

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北加州当前的两个异常温暖的年份：对早期海洋硬头head饮食组成，形态和潜在生存的影响

少年虹鳟虹鳟从哥伦比亚河和其他西北太平洋沿海河流进入加利福尼亚北部的当前生态系统，其中包括《美国濒危物种法》中列出的几种种群。但是，关于这些种群对海洋条件年际变化的反应知之甚少。在2015年和2016年，北加利福尼亚洋流持续存在异常的海洋状况，称为温暖的“斑点”，使海洋温度升高了2.5°C以上。为了确定硬头对这种主要温度变化的反应，我们将2015年和2016年收集的幼年硬头的饮食组成，胃部饱满度，大小和身体状况与先前调查（2001年，2002年和2004年）收集的部分硬头进行了比较，以及2006-2011年），其中包括温暖，凉爽和中性的海洋年。此外，我们使用生物能学模型评估了海洋环境在不同海洋温度情况下对改变海豹头状况的重要性。2015年和2016年的Steelhead表现出一些最差的身体状况值，但在所有评估的年份中都是最大的。硬头鱼的饮食组成在温暖和寒冷的年份之间以及温暖和平均的年份之间有所不同，在温暖的年份，硬鳞鱼的昆虫，幼鱼和幼鱼的数量较多。在受到Blob影响的年份中，食用了不寻常的类群，包括胶状小块（2015年和2016年）和幼稚的熔炼物（2016年）。生物能学模型表明，海洋环境中温度和摄食条件的变化会影响生长的年际变化，早在模拟海洋居住的第2天，暖海和冷海年之间的生长差异就很明显。这些发现凸显了温暖的海洋岁月可能影响哥伦比亚河硬头鱼种群的饮食组成和形态，并可能导致人们更好地理解影响早期海底栖息的少年硬头鱼生存的因素。