The threespined stickleback (Gasterosteus aculeatus) derives its common name from the presence of three small but distinct spines on its back. However, perhaps the most distinguishing characteristic of sticklebacks is their ability to adapt to different environmental conditions, including the ability to reside year-round in either freshwater or saltwater, or live in saltwater but migrate into freshwater to reproduce, much like Chinook salmon and steelhead. They also are said to have a unique characteristic of developing varying amounts of armor based upon predation pressures.
This particular fish was a rare capture in a downstream migrant trap. They are typically found in shallow water with sufficient cover, are 1-2 inches in length, and have a short lifespan, living only one to three years.
Photo source: FISHBIO