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

Abstract

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 proteinbased growth to lipid storage occurring at ~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 euphausiid 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.

Ecosystem response persists after a prolonged marine heatwave

“…The sustained warm water anomalies over winter that were characteristic of the PMH were of particular concern for the GOA subarctic ecosystem. Cool winter temperatures can be important to slow metabolic demands for food and maintain lipid reserves for fish 55 . Bioenergetic winter stress was also hypothesized to contribute to increased mortality of adult Pacific cod in the GOA 37 .…”

First winter energy allocation in juvenile sablefish Anoplopoma fimbria, a fast growing marine piscivore

“…In highlatitude marine environments, food availability is generally lowest in winter, and water temperatures may approach the lower limit of thermal tolerances for some species (Hurst 2007). Pre-winter energy stores are critical to surviving the first winter; therefore, juveniles of many fish species increase fat deposition in late summer and autumn, and survivors of winter often emerge in poorer condition than in the previous fall (Madenjian et al 2000, Hurst & Conover 2003, Heintz & Vollenweider 2010, Sewall et al 2019. Within a cohort, smaller fish often have reduced survival over winter compared to larger fish (Hunt 1969, Hurst & Conover 1998, Beamish & Mahnken 2001, Sewall et al 2019 due to energy depletion allometries (Schultz & Conover 1999) and their limited ability to store energy.…”

“…Pre-winter energy stores are critical to surviving the first winter; therefore, juveniles of many fish species increase fat deposition in late summer and autumn, and survivors of winter often emerge in poorer condition than in the previous fall (Madenjian et al 2000, Hurst & Conover 2003, Heintz & Vollenweider 2010, Sewall et al 2019. Within a cohort, smaller fish often have reduced survival over winter compared to larger fish (Hunt 1969, Hurst & Conover 1998, Beamish & Mahnken 2001, Sewall et al 2019 due to energy depletion allometries (Schultz & Conover 1999) and their limited ability to store energy. Consequently, size-selective overwinter mortality is common, with improved survival in larger individuals (Hunt 1969, Hurst & Conover 1998, Beamish & Mahnken 2001, Sewall et al 2019.…”

“…Within a cohort, smaller fish often have reduced survival over winter compared to larger fish (Hunt 1969, Hurst & Conover 1998, Beamish & Mahnken 2001, Sewall et al 2019 due to energy depletion allometries (Schultz & Conover 1999) and their limited ability to store energy. Consequently, size-selective overwinter mortality is common, with improved survival in larger individuals (Hunt 1969, Hurst & Conover 1998, Beamish & Mahnken 2001, Sewall et al 2019.…”

“…Understanding how energy allocation strategy changes with ontogeny can provide insights about key selection pressures and survival bottlenecks during early life stages. Small fish benefit from somatic growth because larger size reduces predation risk from size-selective predators (Juanes & Conover 1994, Miranda & Hubbard 1994, Sogard 1997, in creases access to higher-energy prey resources (Olson 1996, Scharf et al 2000, Sewall et al 2019, and improves capacity to survive starvation (Johnson & Evans 1990, Schultz & Conover 1999, Sogard & Olla 2000. Larger fish within a given year class can accumulate greater energy reserves than smaller individuals in the same cohort, and energy stores are often positively correlated with length prior to winter (Schultz & Conover 1999, Sogard & Olla 2000, Heintz & Vollenweider 2010.…”