Loss of connectivity caused by anthropogenic barriers is a key threat for migratory freshwater species. The anadromous life history of salmonids means that barriers on streams can decrease the amount of habitat available for spawning and rearing. To set appropriate targets for restoration, it is important to know how different populations have been impacted in terms of the location and extent of historically available habitat that has been lost or has become inaccessible. Using mapped and predicted barriers to fish passage in streams and diking infrastructure, the amount of both floodplain and linear stream habitat that remains accessible today was estimated for 14 populations of salmon in the Lower Fraser River, British Columbia, Canada’s most productive salmon river. To place these estimates within a historical context, the floodplain area was estimated using vegetation records from the 1850s, and lost streams were estimated using a digital elevation model-derived stream network. To bolster areas where little mapping has been done, current barrier data were used to predict locations likely to have barriers. Accessibility to floodplain was poor across the entire region with only 15% of the historical floodplain remaining accessible. Linear stream habitat ranged in accessibility from 28% to 99% across populations based on mapped barriers. Inclusion of predicted barriers revealed an additional 33 km of potentially inaccessible stream habitat and the modeled stream network located approximately 1700 km of stream length that has been completely lost. Comparing habitat accessibility and barrier density against the assessed status of populations revealed insights useful for understanding the impact of barriers on spawning and rearing and guiding the allocation of restoration effort. Applying methods for addressing missing data, such as lost streams and unmapped barriers, was essential for estimating the accessibility of habitat within a historical context. While much emphasis has been placed on the role of marine conditions in wild Pacific salmon recovery, the magnitude of habitat loss in the Fraser cannot be ignored and suggests it is a major driver of observed salmon declines.

中文翻译：

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量化加拿大下弗雷泽河太平洋鲑鱼失去的和难以接近的栖息地

人为障碍造成的连通性丧失是对洄游淡水物种的主要威胁。鲑鱼的溯河生活史意味着溪流上的障碍会减少可用于产卵和饲养的栖息地数量。为了设定适当的恢复目标，重要的是要了解不同种群在历史上可用栖息地的位置和范围方面如何受到影响，这些栖息地已经消失或变得无法进入。使用地图和预测的河流和堤防基础设施中鱼类通过的障碍，估计今天仍然可以进入的洪泛区和线性河流栖息地的数量，估计了加拿大产量最高的鲑鱼河流不列颠哥伦比亚省下弗雷泽河的 14 个鲑鱼种群。将这些估计置于历史背景下，洪泛区面积是使用 1850 年代的植被记录估算的，流失的河流是使用数字高程模型衍生的河流网络估算的。为了支持几乎没有进行地图绘制的区域，使用当前的障碍数据来预测可能有障碍的位置。整个地区对洪泛区的可达性很差，只有 15% 的历史洪泛区仍然可以到达。根据绘制的障碍，线性河流栖息地的可及性范围从 28% 到 99% 不等。包括预测的障碍物，揭示了另外 33 公里的可能无法进入的河流栖息地，模拟的河流网络位于大约 1700 公里的河流长度已完全消失。将栖息地可达性和屏障密度与评估的种群状况进行比较，揭示了有助于理解屏障对产卵和饲养的影响以及指导恢复工作分配的见解。应用解决缺失数据的方法，例如丢失的河流和未绘制的障碍，对于估计历史背景下栖息地的可达性至关重要。虽然海洋条件在太平洋野生鲑鱼恢复中的作用受到了很大重视，但弗雷泽栖息地丧失的严重程度不容忽视，这表明它是观察到的鲑鱼数量减少的主要驱动因素。对于在历史背景下估计栖息地的可达性至关重要。虽然海洋条件在太平洋野生鲑鱼恢复中的作用受到了很大重视，但弗雷泽栖息地丧失的严重程度不容忽视，这表明它是观察到的鲑鱼数量减少的主要驱动因素。对于在历史背景下估计栖息地的可达性至关重要。虽然海洋条件在太平洋野生鲑鱼恢复中的作用受到了很大重视，但弗雷泽栖息地丧失的严重程度不容忽视，这表明它是观察到的鲑鱼数量减少的主要驱动因素。