Two Anomalously Warm Years in the Northern California Current: Impacts on Early Marine Steelhead Diet Composition, Morphology, and Potential Survival

Abstract

AbstractJuvenile 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.