Morphology of Arctic cod (Boreogadus saida) assessed according to habitat preference and age in the Beaufort Sea

Arctic cod (Boreogadus saida) represents the most widespread and abundant fish in the Arctic and is a critical trophic link in its ecosystems. Like many species endemic to the region, it has lost essential habitat as the extent and thickness of sea ice have declined substantially in recent decades. Extreme warming induced by climate change continues to deteriorate polar marine environments. Thus, understanding how Arctic cod uses and partitions its habitat/environment is central to its conservation. We assessed Arctic cod functional morphology using traditional (including gill rakers) and geometric morphometrics and explored whether these differed among habitats and age classes using multivariate techniques. While distinct ecotypes have been proposed, these were not detected in our analyses. Rather, results show similar patterns in the external morphology of Arctic cod across habitats and age classes in the Beaufort Sea. However, analysis of gill rakers revealed concurrent habitat- and age-specific changes likely associated with dietary preferences. Findings indicate that although Arctic cod does not specialise in external morphological features in any habitat, important aspects of its internal feeding morphology shift as it grows, likely underpinning important distributional changes and the species' critical role in transferring energy in Arctic marine ecosystems.

Methods
Arctic cod were collected along predefined transects in the Beaufort Sea as part of the Canadian Beaufort Sea Marine Ecosystem Assessment program run by Fisheries and Oceans Canada. Collected fish were frozen and sent to the Fish Population and Conservation Genetics laboratory at McGill University for processing. Here, fish were thawed and photographed in standardised manner. Because some fish were in poor conditions (after collection, freezing, and thawing) not all fish were usable in all analyses (as described in the text). Fish pictures were analysed for geometric morphometrics to assess shape and shape features. Traditional morphology was also assessed from standardised pictures. The gill arch on the right side of each usable fish was extracted under a dissecting microscope and the length of the gill arch and the number of gill rakers was recorded.

北极鳕鱼（Boreogadus saida）是北极海域分布最广、资源量最为丰富的鱼类，同时也是其所在生态系统中的关键营养级纽带。与该区域诸多特有物种一样，近几十年来随着海冰范围大幅缩减、厚度显著降低，北极鳕鱼已丧失了必需的栖息生境。气候变化引发的极端变暖持续恶化着极地海洋环境。因此，解析北极鳕鱼对栖息生境与环境的利用模式及资源分区策略，是该物种保护工作的核心议题。本研究采用传统形态测量方法（包含鳃耙（gill rakers）计数分析）与几何形态测量法（geometric morphometrics），对北极鳕鱼的功能形态特征开展评估，并借助多元分析技术（multivariate techniques）探究不同栖息生境与年龄组间的形态差异。尽管此前已有学者提出存在独特生态型的假说，但本研究的分析并未检测到此类生态型。研究结果显示，波弗特海（Beaufort Sea）区域的北极鳕鱼，其外部形态在不同生境与年龄组间均表现出一致的模式。然而，针对鳃耙的分析则揭示出与生境和年龄相关的协同变化，这可能与该物种的食性偏好存在关联。研究结果表明，尽管北极鳕鱼并未在外部形态特征上对特定生境产生特化，但其内部摄食形态的关键特征会随个体生长发生改变，这或许是其分布格局发生重要变化的基础，同时也支撑了该物种在北极海洋生态系统能量传递中发挥关键作用的论断。

方法
本研究依托加拿大渔业与海洋部（Fisheries and Oceans Canada）实施的加拿大波弗特海海洋生态系统评估项目，在波弗特海的预设样带中采集北极鳕鱼样本。采集得到的鱼类样本经冷冻保存后，被送往麦吉尔大学（McGill University）鱼类种群与保护遗传学实验室开展后续处理。实验人员首先对样本进行解冻，并采用标准化流程拍摄鱼类形态照片。由于部分鱼类在采集、冷冻与解冻过程中状态不佳，并非所有样本均可用于全部分析环节（详见正文描述）。研究人员通过鱼类形态照片开展几何形态测量分析，以评估其体形与形态特征；同时也通过标准化拍摄的照片开展传统形态测量分析。此外，实验人员在体视显微镜下提取每尾可用样本的右侧鳃弓，记录鳃弓长度与鳃耙数量。