Hatch Timing, Growth, and Condition of Juvenile Arctic Cod (Boreogadus saida) in a Warming Arctic: Insights Into Overwinter Survival Strategies

Abstract

Arctic cod (Boreogadus saida) is a highly abundant fish species with a circumpolar distribution and is generally considered to be an energetically key component of the Arctic food web. Warming conditions and corresponding shifts in sea-ice extent and timing, prey composition, and predator/competitor interactions pose a risk to this cold-adapted species. Laboratory studies have demonstrated that critical early life stages have a narrow thermal tolerance and are particularly vulnerable to warming conditions. Here I examine how growth and hatching dynamics differed across and within years, with a particular emphasis on the role of temperature on these processes. I compared the pre-winter size and lipid storage of juvenile Arctic cod, which impact overwinter survival and recruitment dynamics, during a period of dramatic warming. Fish were captured during 2013, 2017, and 2019 summer surveys in the northern Bering and Chukchi Seas. Otoliths were processed to provide age and growth information and total fatty acids were assessed separately as a metric of lipid storage. Water temperatures during the summers of 2017 and 2019 were warmer than in 2013, and fish from 2017 and 2019 were larger and older on average. The summer growth and lipid storage of juvenile Arctic cod were similar among years, with the exception of 2017 fish having markedly lower concentrations of total fatty acids. We fit a series linear models to evaluate the effect of capture date, temperature, and sample year on both growth and lipid storage, and judged the weight of evidence using likelihood ratios. There was some evidence that earlier in the sampling season, temperature affected both the growth (χ²₍₂₎ = 7.99, p = 0.018) and lipid storage (χ² ₍₂₎ =8.61, p = 0.014) of juvenile Arctic cod, but exerted little to no influence at later stages. The results of this study suggest that sampled fish were able to attain larger pre-winter sizes in 2017 and 2019 because they hatched earlier and thus had longer growth periods, not because they grew faster. Temperature explained little of the variation in the growth or lipid storage of juvenile Arctic cod, despite their exposure to thermally distinct conditions during the weeks preceding capture. This suggests that other ecological factors, such as diet and water mass dynamics, that tend to covary with temperature, are currently exerting a greater influence on pre-winter survival than temperature alone.