Steelhead passage at a mid-sized dam in California assessed using PIT-tags

Ecological traps can be caused when partial restoration leads organisms to make maladaptive habitat choices. One example of this is fishways (e.g., fish ladders) that provide upstream passage at dams, but are not paired with adequate downstream passage. We tested the hypothesis that attracting anadromous fishes to spawn above a dam, but blocking downstream passage of their offspring leads to an ecological trap. Using passive integrated transponder (PIT) tags, we monitored the movements of steelhead (Oncorhynchus mykiss) at a dam and fishway on the central California coast. We found that downstream passage for juveniles and kelts was limited by four factors: migration delay, loss in the reservoir, avoidance of the downstream bypass, and water depths on the spillway. Based on the spillway-passage depth-thresholds, we estimated that the ability for fish to pass downstream was limited to only half of the migration season in 55% of the past 20 years (2002-2021). Our results support the ecological trap hypothesis, which may explain why restoration using fishways has failed to produce recovery gains in this population and elsewhere.当开展局部生态修复时，生物可能做出适应不良的栖息地选择，进而引发生态陷阱（ecological traps）。典型案例之一为：在大坝处设置为鱼类提供上行通道的过鱼设施（如鱼道，fish ladders），却未配套足够的下行通道。我们验证了如下假说：若将溯河洄游鱼类吸引至大坝上方产卵，却阻断其后代的下行通道，将形成生态陷阱。本研究借助被动集成应答器（passive integrated transponder, PIT）标签，对加州中部海岸某大坝及过鱼设施内的硬头鳟（Oncorhynchus mykiss）的移动行为开展了监测。研究发现，幼鱼与产后洄游成体的下行通行受到四大因素制约：洄游延迟、水库内个体遗失、对下游旁道的规避，以及溢洪道水深条件。基于溢洪道通行水深阈值，我们估算得出：在2002年至2021年的过去20年间，有55%的年份里，鱼类仅能在半个洄游季内完成下行通行。本研究结果支持生态陷阱假说，该假说或可解释为何通过过鱼设施实施的修复工作，未能在该种群及其他区域的同类种群中取得预期的恢复成效。