Consideration of habitat quality in a river connectivity index for anadromous fishes

River fragmentation is one of the main threats to diadromous fish species. We aimed to create a new and simple connectivity index to calculate habitat accessibility that considers habitat suitability for fish species, using the Bidasoa River basin in the north of Spain and Atlantic salmon (Salmo salar) as an example. A habitat connectivity index (HCI) was calculated for the river basin using upstream passability and segment length as variables. We then calculated a new habitat quality index for each river segment and multiplied it by river segment length to create the Breeding Habitat Connectivity Index (HCIb). These 2 indices were first calculated using only upstream barrier passability and then by adding downstream passability. In each case, the indices show different outcomes but a similar pattern: in all cases, main-stem obstacles closest to the river mouth most affected the connectivity index, even when habitat quality was considered. Although we cannot compare the indices to the real area used by salmon because spatial tracking was not performed during the study years, we consider that including habitat quality in a river connectivity index adds useful information for scientists and managers.

河流破碎化是威胁洄游性鱼类的主要威胁之一。本研究旨在构建一种新颖且简便的连通性指数，用以计算栖息地可达性，同时纳入鱼类的栖息地适宜性因素，并以西班牙北部的比达索阿河流域及大西洋鲑（Salmo salar）为研究案例。本研究以上游通行能力与河段长度为变量，计算了该流域的栖息地连通性指数（Habitat Connectivity Index, HCI）；随后，我们为每个河段计算了全新的栖息地质量指数，并将其与河段长度相乘，得到繁殖栖息地连通性指数（Breeding Habitat Connectivity Index, HCIb）。上述两类指数首先仅以上游障碍物通行能力为参数进行计算，随后纳入下游通行能力重新计算。两类指数在两种计算场景下呈现出不同的具体数值，但分布模式一致：无论是否纳入栖息地质量因素，距河口最近的干流障碍物均对连通性指数造成最为显著的影响。尽管本研究未开展空间追踪工作，无法将计算得到的指数与鲑鱼实际利用的栖息地面积进行比对，但我们认为，在河流连通性指数中纳入栖息地质量因素，可为科研人员与管理者提供更具实用价值的参考信息。