Temporal and size-based patterns in juvenile sablefish energy allocation and diet

Abstract

A recent marine heatwave in the Gulf of Alaska caused depressed growth, poor body condition, and low survival in many fish species, but juvenile sablefish (Anoplopoma fimbria) thrived. These fast-growing piscivores are the target of a valuable commercial fishery in Alaska as adults and have historically shown high variability in recruitment. The first winter is a period of nutritional stress and high mortality for many fish species and first winter survival may dictate year class strength, but the importance of the first winter for juvenile sablefish is understudied. We examined juvenile sablefish energy storage, growth, and diet during their first two years of life, specifically as newly settled juveniles in their first autumn, in late winter, and during their second summer and autumn. Sablefish grew rapidly in autumn and growth slowed but continued through winter. Mean energy density (kJ g⁻¹) declined over the winter but total energy (kJ individual⁻¹) increased significantly between October and March. Slopes of energy density and total energy versus length regressions were atypical for high latitude marine fish in that they were steepest in March. This indicates that large fish grew during winter with minimal energy depletion while small fish grew but depleted their energy stores. Stable isotope results revealed that larger fish were enriched in [delta]¹³C and [delta]⁻¹⁵N in March relative to smaller fish, suggesting diet differences may contribute to size-specific energy storage patterns during winter. Pacific herring (Clupea pallasii) dominated diets but consumption of herring and other prey varied seasonally and annually. Relative stomach content weights were highest in autumn 2018, which was a period of rapid growth. Results of this study show advantages for sablefish achieving large size prior to winter and broadly support the hypothesis that first winter is a life history bottleneck for juvenile sablefish. The generalist feeding strategy of sablefish and rapid growth early in life may provide the ability to adapt to a wide range of environmental conditions.