Shifts in the composition and distribution of Pacific Arctic larval fish assemblages in response to rapid ecosystem change

The Pacific Arctic marine ecosystem has undergone rapid changes in recent years due to ocean warming, sea ice loss, and increased northward transport of Pacific‐origin waters into the Arctic. These climate‐mediated changes have been linked to range shifts of juvenile and adult subarctic (boreal) and Arctic fish populations, though it is unclear whether distributional changes are also occurring during the early life stages. We analyzed larval fish abundance and distribution data sampled in late summer from 2010 to 2019 in two interconnected Pacific Arctic ecosystems: the northern Bering Sea and Chukchi Sea, to determine whether recent warming and loss of sea ice have restricted habitat for Arctic species and altered larval fish assemblage composition from Arctic‐ to boreal‐associated taxa. Multivariate analyses revealed the presence of three distinct multi‐species assemblages across all years: (1) a boreal assemblage dominated by yellowfin sole (Limanda aspera), capelin (Mallotus catervarius), and walleye pollock (Gadus chalcogrammus); (2) an Arctic assemblage composed of Arctic cod (Boreogadus saida) and other common Arctic species; and (3) a mixed assemblage composed of the dominant species from the other two assemblages. We found that the wind‐ and current‐driven northward advection of warmer, subarctic waters and the unprecedented low‐ice conditions observed in the northern Bering and Chukchi seas beginning in 2017 and persisting into 2018 and 2019 have precipitated community‐wide shifts, with the boreal larval fish assemblage expanding northward and offshore and the Arctic assemblage retreating poleward. We conclude that Arctic warming is most significantly driving changes in abundance at the leading and trailing edges of the Chukchi Sea larval fish community as boreal species increase in abundance and Arctic species decline. Our analyses document how quickly larval fish assemblages respond to environmental change and reveal that the impacts of Arctic borealization on fish community composition spans multiple life stages over large spatial scales.近年来，受海洋增温、海冰消融以及北太平洋源水向北冰洋输运强度增强的影响，太平洋北极海域海洋生态系统经历了快速变化。这类由气候介导的变化已被证实与亚北极（boreal）及北极鱼类种群的幼体和成体分布范围偏移相关，但目前尚不清楚早期生活史阶段是否也存在此类分布变化。本研究分析了2010至2019年夏末，在两个相互连通的太平洋北极生态系统——北白令海与楚科奇海采集的仔鱼丰度与分布数据，旨在厘清近期海温升高与海冰消融是否限制了北极物种的栖息生境，并改变了仔鱼群落的组成，使其从北极关联类群向北方寒带关联类群转变。多元统计分析结果显示，研究期间共存在3个截然不同的多物种种群群落：(1) 以黄盖鲽（Limanda aspera）、毛鳞鱼（Mallotus catervarius）和狭鳕（Gadus chalcogrammus）为优势种的北方寒带群落；(2) 由北极鳕（Boreogadus saida）及其他常见北极物种组成的北极群落；(3) 由前两个群落的优势种共同构成的混合群落。研究发现，由风力与海流驱动的暖性亚北极水团北向平流，以及2017年起在北白令海与楚科奇海出现并持续至2018、2019年的史无前例的低海冰条件，引发了全群落范围的转变：北方寒带仔鱼群落向北及远海扩张，北极仔鱼群落则向极地退缩。本研究结论表明，随着北方寒带物种丰度上升、北极物种丰度下降，北极海域增温正对楚科奇海仔鱼群落的前沿与后缘区域的丰度变化产生最为显著的驱动作用。本研究的分析不仅阐明了仔鱼群落对环境变化的响应速度，同时揭示了北极北方化对鱼类群落组成的影响可在大空间尺度上覆盖多个生活史阶段。