We captured this “ripe” female rainbow trout (Oncorhynchus mykiss) while electrofishing in a California reservoir. You can tell if a female is ready to spawn by how easily eggs are extruded during handling. In this case, although we were gentle, the expulsion of eggs indicates that she was ripe for spawning.
Rainbow trout that inhabit lakes and reservoirs may either migrate into rivers and streams to spawn (adfluvial) or may spawn in the gravel at inlets and outlets (fluvial). They generally spawn in the winter/spring (January to June), depending on the climate. The timing is dictated by increasing temperatures and spawning usually occurs when daily water temperatures exceed 42-44°F (6-7°C). Trout, as with most salmonids, have relatively large eggs with more yolk material than most fish species and will lay about 1,000 eggs per 1.5 pounds (1 kg) of female body weight (Behnke 2002).
Photo source: FISHBIO