Temporal and Age‐Based Variation in Juvenile Sablefish Diet Composition and Quality: Inferences from Stomach Contents and Stable Isotopes

Abstract Prey communities in the North Pacific Ocean have been disrupted by marine heatwaves, and reductions forage fishes had notable impacts on upper‐trophic‐level consumers. Little is known about potential effects of a changing prey base for some commercially valuable fishes, such as Sablefish Anoplopoma fimbria . The objectives this study were to evaluate temporal age‐based shifts diets juvenile Sablefish, with focus understanding their reliance high‐quality fishes. We collected from bay Southeast Alaska over 2 years (2017–2019) during first autumn (September–October; age 0), late winter (March; 1), second summer (July; 1). Herring Clupea pallasii constituted majority diet weight (82.1%) frequency occurrence (40.7%), variation among months, years, age‐classes. Stable isotopes corroborated our interpretation composition stomach contents indicated that age‐0 sampled October 2017 relied more depleted carbon sources than other groups, potentially explained consumption adult salmon carcasses. Significant relationships between stable isotope ratios length size‐based differences most prevalent March July. exploited taxa variable quality (0.02–5.3 kJ/g), but mean energy density consumed differed little or months (3.62–4.48 kJ/g). Overall, 21% stomachs empty, percentage empty peaking (46%). Stomach content weights expressed body highest 2018, when comprised 80% weight. Consumption high‐energy prey, Herring, may contribute rapid growth critical prewinter period. If strong year‐classes become frequent warming ocean, they will require substantial resources support adulthood.