Secondary contact and pre-zygotic isolating mechanisms between anadromous and freshwater fish populations
Secondary contact between recently isolated lineages can result in complex interactions that alter the evolution and ecological impacts of a species. The study of secondary contact in its early stages will provide valuable insight into the processes that inhibit or promote speciation. In addition, the ecological and evolutionary outcomes of secondary contact will pose an interesting problem for populations of conservation concern, particularly when maintaining genetic structure is a priority. Identifying pre-zygotic isolating mechanisms is key in determining if and when populations will hybridize. Here, I examine pre-zygotic isolating mechanisms in anadromous fish with a focus on the alewife (Alosa pseudoharengus), a federally listed threatened species. Alewife consist of an ancestral anadromous population and multiple freshwater “landlocked” populations that have been in isolation for approximately 300 years. Recently installed fishways are bringing the two life-history forms into contact for the first time in centuries. To identify potential barriers to hybridization, I used spawning time estimates from otoliths and anadromous run data to construct spawning time distributions for both life-history forms. I collected similar data for steelhead trout (Oncorhynchus mykiss) and threespine stickleback (Gasterosteus aculeatus) to explore the generality of spawning time relationships between landlocked and anadromous population pairs.
Preliminary results indicated that spawning in anadromous alewife occurs earlier and has a shorter duration than landlocked alewife. Spawning time is also less variable between lakes in anadromous populations. Despite the differences in the timing of reproduction, there is some overlap in spawning time between anadromous and landlocked populations. If hybridization does occur, it could alter the evolutionary trajectory and ecology of the anadromous alewife. Within the context of conservation, identifying the potential outcomes of hybridization, such as introgression of maladaptive genes, is critical for the successful management of the species. The trends observed in the alewife, with higher variability in landlocked spawning time contributing to some overlap in the timing of reproduction, may apply to other species of conservation concern. This is especially important in light of the river restoration movement, with restored river connectivity increasing the likelihood of secondary contact in many isolated anadromous-landlocked species pairs.