Rainbow trout are spawning right now, cruising reservoir shorelines as they look for a place to lay their eggs.
The spawn peaks from mid- to late-April in rivers and streams with gravel bottoms. The flowing water in a river or stream provides oxygen, which keeps the eggs alive. Gravel helps protect eggs from predators. Also, a silty bottom could smother and kill the eggs.
When it’s time to spawn — based on daylight length and warming water temperature (at least in the mid-40s) — a female rainbow will clear a slight depression, called a redd, in the gravel. She does this by turning on her side and beating her tail up and down. Then she deposits 2,000 to 3,000 eggs in the redd. Next, she begins digging at the upstream edge of the nest, covering the eggs with gravel.
As she releases her eggs, a male rainbow will move alongside her and release his milt over the eggs, fertilizing them. In a month or two the eggs will hatch.
Rainbow trout in lakes must find a suitable creek or river with flowing water and a gravel bottom. Right now, they are slowly swimming along the shorelines, searching.
They often find the right type of gravel they need at a gradually sloping shoreline, like at a boat ramp. So if you hear somebody say they are near the boat ramp because they are returning to where the hatchery truck dumped them into the water, that’s not true.
There is also the theory that sunshine on the concrete of the boat ramp warms up the nearby shallow water just a few degrees, making it more attractive to fish.
In lakes where fish are stocked from the hatchery, it’s OK to keep the fish you catch because they usually won’t find the right place to spawn.