Impacts of temperature and food availability on the condition of larval Arctic cod (Boreogadus saida) and walleye pollock (Gadus chalcogrammus) ICES Journal of Marine Science

The Arctic marine environment is rapidly changing with rising sea surface temperatures, declining sea ice habitat and projected increases in boreal species invasions. The success of resident Arctic fish will depend on both their thermal tolerance and their ability to cope with changing trophic interactions. Larval fish energetic condition is closely associated with mortality rates and therefore provides an indicator of overall well-being or fitness. In this study, we experimentally determined larval morphometric and lipid-based condition in an Arctic gadid (Arctic cod, Boreogadus saida) and a boreal gadid (walleye pollock, Gadus chalcogrammus) in response to different temperatures and food rations. Our results suggest that larval condition is highly sensitive to both factors but varies in a species- and ontogenetic-dependent manner. Results indicated that condition metrics based on length–weight relationships were not as sensitive as those based on lipid storage. Further, condition metrics changed with ontogeny and were best used within a developmental stage rather than across developmental stages. As expected, larval condition in first-feeding Arctic cod was higher at colder temperatures (2–5°C) than in the boreal gadid (5–12°C). However, at more developed larval stages the peak condition for Arctic cod was at warmer temperatures (7°C), while walleye pollock had the same thermal optimum as during earlier stages. Arctic cod were more sensitive to food ration at first feeding than walleye pollock, however; at later larval stages both species had a negative condition response to low food ration, especially at elevated temperatures (5 vs. 7°C). The lower thermal tolerance of Arctic cod, coupled with a higher sensitivity to food availability indicates that Arctic cod are particularly vulnerable to on-going environmental change. Arctic cod is a lipid-rich keystone species and therefore a reduction in their energetic condition during summer has the potential to affect the health of higher trophic levels throughout the Alaskan Arctic.

北极海洋环境正随海表温度升高、海冰栖息地缩减以及预测中的北方物种入侵加剧而快速发生变化。本地北极鱼类的存活成功与否，将同时取决于其热耐受能力与应对营养级互作变化的能力。幼鱼的能量状态与死亡率紧密相关，因此可作为评估其整体健康状况与适合度的指示指标。本研究通过实验，测定了北极鳕科（gadid）鱼类北极鳕（Arctic cod, *Boreogadus saida*）与北方鳕科（boreal gadid）鱼类黄线狭鳕（walleye pollock, *Gadus chalcogrammus*）的幼鱼形态学指标与基于脂质的能量状态，以探究其对不同温度与食物投喂量的响应。研究结果表明，幼鱼能量状态对这两个因素均高度敏感，但呈现出物种特异性与个体发育阶段依赖性。结果显示，基于体长-体重关系的状态指标，其敏感性不及基于脂质储存的状态指标。此外，能量状态指标随个体发育阶段发生变化，因此更适合在单一发育阶段内应用，而非跨发育阶段进行比较。正如预期，初次摄食阶段的北极鳕幼鱼在低温（2–5℃）下的能量状态，高于北方鳕科的黄线狭鳕（5–12℃）。但在更发育的幼鱼阶段，北极鳕的能量状态峰值出现在更高温度（7℃），而黄线狭鳕的热最适温度与早期幼鱼阶段一致。不过，初次摄食阶段的北极鳕对食物投喂量的敏感性高于黄线狭鳕；而在后期幼鱼阶段，两个物种的能量状态均对低食物投喂量呈现负响应，在高温条件下（5℃ vs 7℃）尤为显著。北极鳕较低的热耐受能力，加上对食物可获得性更高的敏感性，表明其极易受到当前持续进行的环境变化的影响。北极鳕是富含脂质的关键物种，因此夏季其能量状态的降低，有可能影响整个阿拉斯加北极地区更高营养级生物的健康。