Evaluating the potential for prezygotic isolation and hybridization between landlocked and anadromous alewife (Alosa pseudoharengus) following secondary contact

Abstract

The recent increase in 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 prezygotic 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%–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 were 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 prezygotic isolating mechanisms are minimal, leading to potentially rapid ecological and evolutionary changes in restored habitats.

Ecological and Evolutionary Consequences of Environmental Change and Management Actions for Migrating Fish

“…Evolutionary divergence following reproductive isolation can occur in just a few generations (Christie et al, 2012;Thompson et al, 2019), and is thus potentially relevant for recent population sub-divisions. Anadromous fish populations that have been split into reproductively isolated subpopulations due to migration barriers have resulted in the evolution of freshwater forms in landlocked subpopulations (e.g., McDowall, 1997;Littrell et al, 2018). Evolutionary divergence and population genetic structure may also be modified by admixture associated with captive breeding programs, stocking, and escapes of farmed individuals (Christie et al, 2014), and this can affect migratory behavior and evolution of populations that have not previously been much influenced by gene flow.…”

Restoration‐mediated secondary contact leads to introgression of alewife ecotypes separated by a colonial‐era dam

“…in the year and for a shorter duration than landlocked populations (Littrell et al, 2018). The frequency of hybrids we observed (4.6%)…”

Loss of Coilia nasus habitats in Chinese freshwater lakes: An otolith microchemistry assessment

“…The disappearance of anadromous C. nasus in the lakes connected to the Yangtze and Yellow Rivers highlights the impact of anthropogenic activities on the life histories of diadromous fish species, such as becoming land-locked in Gucheng, Taihu, Gaobao, and Dongping lakes, and the loss of spawning and nursery habitats in Dongting Lake. Similar to the cases of other fish species in Europe [ 69 ] and North America [ 66 ], the disruption of habitat connectivity poses considerable challenges with respect to C. nasus resource management. The challenges are especially caused by the destruction of local lake ecosystem integrity, the effects of the altered ecological roles of different ecotypes on resource structure and ecosystem function, and the non-sustainability of high-value anadromous C. nasus fisheries in these lakes.…”

“…Unfortunately, anadromous C. nasus individuals have not been found recently in the lakes of Dongting, Gucheng, Taihu, or Dongping. If C. nasus does indeed have natal homing behavior, the loss of spawning sites could lead to the extirpation of some of its populations, as exemplified by local extirpation of the alewife ( Alosa pseudoharengus ) along the Atlantic coast in North America, which was attributed primarily to dam construction [ 66 ]. A similar case is the extinction of Macrura reevesi , another valuable anadromous species of economic interest in the Yangtze River, due to overfishing and the loss of spawning sites as a consequence of dam construction and water pollution [ 67 ].…”