The walleye pollock Gadus chalcogrammus is an important commercial species in Japan whose larvae and eggs may be negatively affected by the cold water mass from the coastal Oyashio current that is present in the spawning ground of the Japanese Pacific stock of this species. Therefore, we investigated egg and larval specific density, larval mortality, and behavioral response to temperature change during the ontogenetic development of the walleye pollock to understand the effect of this cold surface water mass (<1.5 °C). Egg and larval specific density varied during development but were lower than the corresponding values from the coastal Oyashio waters. Within our study temperature range (0.3 °C–10.0 °C), the number of days to 50% mortality (D₅₀) was high at 3.1 °C. Below and above this temperature, the D₅₀ showed a decreasing trend. Regarding larval response, at 1.5°C and 5.0°C, newly hatched larvae occurred abundantly in the surface layer, irrespective of the surface and rearing temperatures. When these larvae were released into a thermally stratified water column (surface: 1.5 °C, bottom: 5.0 °C), larvae reared at 5.0 °C with the mouth open and yolk sac completely absorbed moved to the lower layers. However, larvae reared at 1.5 °C remained in the surface layer. These results suggest that the cold water mass could negatively affect larval survival and may limit the escape ability of larvae from unfavorable cold conditions.

中文翻译：

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低温对大眼鳕鱼早期生命阶段的影响，Gadus chalcogrammus——一项实验室研究

白斑狭鳕Gadus chalcogrammus是日本重要的商业物种，其幼虫和卵可能会受到来自该物种日本太平洋种群产卵场的沿海亲潮流冷水团的负面影响。因此，我们调查了白斑鳕个体发育过程中卵和幼虫的比密度、幼虫死亡率和对温度变化的行为反应，以了解这种冷表面水团 (<1.5 °C) 的影响。卵和幼虫的比重在发育过程中发生变化，但低于沿海亲潮水域的相应值。在我们的研究温度范围内（0.3 °C–10.0 °C），达到 50% 死亡率的天数（D ₅₀) 高达 3.1 °C。低于和高于此温度，D ₅₀显示出下降趋势。关于幼虫反应，在 1.5°C 和 5.0°C 时，新孵化的幼虫大量出现在表层，而与表层和饲养温度无关。当这些幼虫被释放到热分层水柱（表面：1.5°C，底部：5.0°C）时，幼虫在 5.0°C 下张开嘴长大，卵黄囊完全被吸收移至下层。然而，在 1.5 °C 饲养的幼虫仍留在表层。这些结果表明冷水团可能对幼虫的存活产生负面影响，并可能限制幼虫逃离不利寒冷条件的能力。