Growth, energy storage, and feeding patterns reveal winter mortality risks for juvenile Pacific herring in Prince William Sound, Alaska, USA

First winter survival of juvenile cold temperate fish can be an important recruitment driver. Winter survival may be influenced by size and energy reserves, with larger, fatter individuals less vulnerable to predation and starvation. However, limited information regarding relationships among size, growth, and energy reserves often hampers understanding recruitment processes for economically and ecologically important marine species. To better understand winter mortality risks, we examined growth and lipid storage patterns in young-of-the-year (YOY) Pacific herring Clupea pallasii in Prince William Sound, Alaska, USA, near the onset (November) and end (March) of 7 winters during 2009-2016 that occurred before and during the North Pacific marine heat wave. Herring length in November determined energy allocation, with a shift from protein-based growth to lipid storage occurring at similar to 76 mm fork length. We suggest that size-selective predation pressure causes small herring below this size to favor growth over storing fat. Low March lipid stores apparently compelled herring to avoid starvation by foraging, behavior that could increase predation risk especially for small herring. Larger herring ate more high-quality euphauslid prey than did small herring during November, reinforcing the advantages of large size. Herring lipid stores were highest in the coldest study year, rather than the year with the best diets, presumably due to low temperature slowing metabolic rates. Our findings suggest overwinter survival models could be improved with unbiased estimates of late autumn YOY herring size and energy distributions, seasonal temperature measurements, estimates of food consumption, and knowledge of local predator densities.