300 years ago these fish were the same species.
These fish are alewife found in lakes and rivers of Connecticut. The larger of these fish comes from Bride Lake, the smaller pair was found in Rogers Lake. Pretty amazing difference right?
Prior to development, these fish were quite literally the same species. These alewife live their lives migrating between rivers, lakes, and the ocean in this area. Prior to the arrival of humans, these fish lived part of their year out in the open sea and part in lakes. They would migrate upstream into lakes and feed on blooms of phytoplankton during the months when that food supply was available, turn around and head out to sea, feeding on other food supplies in the open ocean when the food supplies in the lakes became limited.
But, 300 years ago, settlers dammed the river that leads from Rogers lake to the ocean. The fish could no longer migrate downstream and any fish stuck in that lake could not reach the ocean.
The fish consumed virtually all the plankton they could feed on but that wasn’t enough to last the full year. As a consequence, most of the larger alewife in that system died off, but smaller ones were better-equipped to survive. On top of that, the fish switched food sources; smaller gilled fish capable of eating smaller types of plankton were able to flourish.
In only 300 years, the building of these dams created what are effectively completely new species of fish by changing their food supplies. These fish aren’t the only ones undergoing these changes; bluegill in these lakes have been found to show the same types of variation as well.
Image credit: Yale/ Post Lab