Predators must consume enough prey to support costly events, such as reproduction. Meeting high energetic requirements is particularly challenging for migrating baleen whales as their feeding seasons are typically restricted to a limited temporal window and marine prey are notoriously patchy. We assessed the energetic value of the six most common nearshore zooplankton species collected within the Oregon, USA range of the Pacific Coast Feeding Group (PCFG) gray whale (Eschrichtius robustus) feeding grounds, and compared these results to the energetic value of the predominant amphipod species fed on by Eastern North Pacific (ENP) gray whales in the Arctic. Energetic values of Oregon zooplankton differed significantly between species (Kruskal-Wallis 2 = 123.38, df = 5, p<0.0001), with Dungeness crab (Cancer magister) megalopae displaying the highest mean caloric content of all tested species (4.21  1.27 kJ g-1). This value, as well as the mean energetic value of the mysid Neomysis rayii (2.42  1.06 kJ g-1), are higher than the mean caloric content of Ampelisca macrocephala, the predominant Arctic amphipod. Extrapolations of these results to daily energetic requirements of gray whales indicate that lactating and pregnant gray whales feeding in the PCFG range would require between 0.7-1.03 and 0.22-0.33 metric tons of prey less per day if they fed on Dungeness crab megalopae or N. rayii, respectively, than a whale feeding on A. macrocephala in the Arctic. Yet, these results do not account for differences in availability of these prey species to foraging gray whales. We therefore suggest that other factors, such as prey density, energetic costs of feeding, or natal philopatry and foraging site fidelity play a role in the differences in population sizes between the PCFG and ENP gray whales. Climate change is implicated in causing reduced body condition and increased mortality of both PCFG and ENP gray whales due to decreased prey availability and abundance. Therefore, improved understanding of prey dynamics in response to environmental variability in both regions is critical.

中文翻译：

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灰鲸会计算卡路里吗？比较北太平洋东部两个不同觅食地灰鲸猎物的能量值

捕食者必须消耗足够的猎物来支持代价高昂的事件，例如繁殖。满足高能量需求对于迁徙的须鲸来说尤其具有挑战性，因为它们的觅食季节通常仅限于有限的时间窗口，而且海洋猎物的斑驳也是众所周知的。我们评估了在美国俄勒冈州太平洋海岸饲养组 (PCFG) 灰鲸 (Eschrichtiusrobustus) 觅食地范围内收集的六种最常见的近岸浮游动物物种的能量值，并将这些结果与主要片脚类动物的能量值进行比较由北太平洋东部 (ENP) 灰鲸在北极捕食的物种。俄勒冈州浮游动物的能量值在物种之间存在显着差异（Kruskal-Wallis 2 = 123.38，df = 5，p<0.0001），巨蟹 (Cancer magister) megalopae 显示出所有测试物种的最高平均热量含量 (4.21  1.27 kJ g-1)。该值以及 Mysid Neomysis rayii 的平均能量值 (2.42  1.06 kJ g-1) 高于主要的北极片脚类动物 Ampelisca macrocephala 的平均热量含量。将这些结果外推到灰鲸的日常能量需求表明，在 PCFG 范围内喂养的哺乳期和怀孕的灰鲸，如果以巨蟹蟹或 N 为食，每天需要减少 0.7-1.03 到 0.22-0.33 公吨的猎物。 rayii，分别比在北极以 A. macrocephala 为食的鲸鱼。然而，这些结果并没有解释这些猎物在觅食灰鲸时的可用性差异。因此，我们建议其他因素，PCFG 和 ENP 灰鲸之间的种群规模差异，例如猎物密度、摄食的能量成本或出生的亲和和觅食地点的保真度等因素。由于猎物可用性和丰度减少，气候变化与 PCFG 和 ENP 灰鲸的身体状况下降和死亡率增加有关。因此，更好地了解猎物动态以响应这两个地区的环境变化至关重要。