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Q & A
Spawning Trout, Male vs. Female
Question
This past weekend I caught a nice rainbow trout. While cleaning it, I came upon what I assume was the egg sack. It was approximately 2 to 3 inches long filled with small, round, soft balls. I was not happy because if this was the egg sack, I figured that the fish may possibly lay the eggs. I was under the impression that trout had already spawned for the season. Is this correct and what is the normal spawning period? Is there a way to determine female versus male trout so I will not remove a fertile female in the future?
Answer
In female trout, eggs are held within a thin membrane in the body cavity. Trout eggs range from pale yellow to orange in coloration. The size of the eggs depends upon the stage of development and the size of the female (in general, larger trout produce larger eggs). Nonetheless, when a female trout is ready to spawn, the eggs will fill much of the free space in the body cavity.

Rainbow trout - click for more informationIn Pennsylvania, brook and brown trout are fall spawners. Brook trout generally spawn between September and November and brown trout spawn a little later, usually between October and December. By nature, rainbow trout are spring spawners. However, by process of selection in the hatchery system, this species has been selected to spawn during the late summer and early fall (August - September). Basically, from the time that the eggs hatch, it takes about 16-18 months to produce trout of catchable size (averaging 9-11 inches in length) for stocking. Therefore, this process has allowed us to have relatively uniform size catchable size hatchery trout of all three species during a time period when they are in the most demand, or during the spring.

You shouldn't be too upset about harvesting the rainbow trout. This fish was most likely an older hatchery trout that had begun to develop eggs for the upcoming season or possibly had some old eggs remaining from a previous spawn (eggs that are not expelled are absorbed internally by the fish). It is highly unlikely that this hatchery fish would have successfully spawned with another rainbow trout in the wild in Pennsylvania waters. Although we have a wealth of reproducing wild brook and brown trout populations in the Commonwealth, only about a dozen streams support reproducing wild rainbow trout fisheries.

The methods to distinguish between male and female trout are difficult to determine to the untrained eye. Again, the older the trout is and the closer to spawning season, the easier it is to determine between the sexes. In general, mature male trout will develop a kype (or a hooked snout on their lower jaw). Male trout also tend to be more laterally compressed than female trout and during the spawning period may be brighter in coloration. Conversely, female trout tend to have a more rounded snout and body conformation. Of course the positive way to determine this would be as you did, to sacrifice the fish and examine for the presence of eggs or sperm.

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