Data from: Evaluating the potential for pre-zygotic isolation and hybridization between landlocked and anadromous alewife (Alosa pseudoharengus) following secondary contact

The recent increase of river restoration projects is altering habitat connectivity for many aquatic species, increasing the chance that previously isolated populations will come into secondary contact. Anadromous and landlocked alewife (Alosa pseudoharengus) are currently undergoing secondary contact as a result of a fishway installation at Rogers Lake in Old Lyme, Connecticut. To determine the degree of pre-zygotic isolation and potential for hybridization between alewife life history forms, we constructed spawning time distributions for two anadromous and three landlocked alewife populations using otolith derived age estimates. In addition, we analyzed long-term data from anadromous alewife migratory spawning runs to look for trends in arrival date and spawning time. Our results indicated that anadromous alewife spawned earlier and over a shorter duration than landlocked alewife, but 3% to 13% of landlocked alewife spawning overlapped with the anadromous alewife spawning period. The degree of spawning time overlap was primarily driven by annual and population level variation in the timing of spawning by landlocked alewife, whereas the timing and duration of spawning for anadromous alewife was found to be relatively invariant among years in our study system. For alewife and many other anadromous fish species, the increase in fish passage river restoration projects in the coming decades will re-establish habitat connectivity and may bring isolated populations into contact. Hybridization between life history forms may occur when pre-zygotic isolating mechanisms are minimal, leading to potentially rapid ecological and evolutionary changes in restored habitats.近年来河流修复项目的持续增加，正改变着众多水生生物的栖息地连通性，提升了此前隔离种群发生二次接触的概率。眼下，在康涅狄格州老莱姆镇罗杰斯湖安装鱼道后，溯河洄游型（anadromous）与陆封型（landlocked）的阿氏西鲱(Alosa pseudoharengus)正发生二次接触。为明确阿氏西鲱不同生活史型间的合子前生殖隔离（pre-zygotic isolation）程度及杂交潜力，我们通过耳石（otolith）推导的年龄估算方法，构建了2个溯河洄游型种群与3个陆封型种群的产卵时间分布。此外，我们还分析了溯河洄游型阿氏西鲱的产卵洄游长期数据，以探究其抵达时间与产卵时间的变化趋势。本研究结果显示，溯河洄游型阿氏西鲱的产卵时间早于陆封型种群，且产卵持续时长更短；但仍有3%至13%的陆封型阿氏西鲱产卵时段与溯河洄游型种群的产卵期存在重叠。产卵时段重叠程度主要受陆封型阿氏西鲱产卵时间的年度与种群水平差异影响；而在本研究体系中，溯河洄游型阿氏西鲱的产卵时间与持续时长在各年度间相对稳定。对于阿氏西鲱及诸多其他溯河洄游鱼类而言，未来数十年间鱼类通行类河流修复项目的增加，将重建栖息地连通性，可能促使原本隔离的种群发生接触。当合子前生殖隔离机制作用微弱时，不同生活史型间可能发生杂交，进而在修复后的栖息地中引发潜在的快速生态与进化变化。