Sex of a Trout - How To Determine Males from Females | Fly Fishing Insider Podcast

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A particularly intriguing aspect of trout is the distinct changes that occur in trout during the spawning season, especially in terms of differentiating between males and females. This period, often in late fall or early winter, marks a significant transformation in these fish, particularly noticeable in species like the Rainbow and Brown trout. However, there are key differences in determining the sex of a trout beyond the spawn season to look for.  Let’s take a look at spawning trout as they are typically more easily identifiable then dive into observation beyond those times of year.

Female and male rainbow trout

Female rainbow trout on top and male on the bottom.  

For male trout, the most prominent change is the development of a pronounced, hooked jaw known as a "kype." This feature, which becomes more pronounced as the spawning season approaches, is not just a physical transformation but also a functional adaptation for the competitive environment of spawning. The kype aids males in jostling with rivals and asserting dominance, a crucial factor in the mating ritual. Alongside this jaw development, male trout often exhibit a striking shift in coloration. Their bodies become adorned with brighter, more vivid hues, a visual spectacle that serves to attract females and signal their readiness to spawn. When you hook up with one of those buttery browns in fall its all colored up to present itself to the ladies. 

Females, while less dramatic in their transformation, undergo changes too. As they prepare for egg laying, their bodies become rounder and fuller due to the development of roe (fish eggs). Unlike males, females do not typically show significant changes in coloration or jaw structure. However, their physical transformation is equally vital, as it indicates their readiness to reproduce and the health of their potential offspring. Often you’ll see and hear hens jumping and flopping onto their sides in longer runs. This is commonly a heard slap and is allowing them to loosen up their egg sacks and preparing them to be laid in prepared reds.

Beyond spawning season you can identify the sex of a trout being males or females through a multitude of physical traits. At first glance, male and female trout appear similar, sharing the same streamlined shape, speckled pattern, and general coloration. However, upon closer inspection, several distinct differences become apparent. When looking to understand the visual differences between males and female trout that are not spawning, here are a few tactics. 

Typically, male trout develop a more elongated and streamlined body shape. They often grow larger and have a more pronounced muscular appearance, especially around the head and jaws. Female trout generally have a rounder, fuller body shape. This is again, particularly evident during the spawning season when they carry eggs. Females are also often smaller than their male counterparts as well in length but can often be a bit thicker.

During the spawning season, male trout often exhibit brighter and more vivid coloration as described above. Their colors become more intense, with reds and pinks becoming more prominent, especially on the belly and gills. The spots and markings on their body may also become more distinct. Whereas female trout maintain a more subdued color palette. Their colors are usually more muted, leaning towards silvers and grays, which helps in camouflage, especially when guarding nests. 

Head and jaw features are often my most common way of distinguishing between the two sex of a trout. One of the most notable differences is seen in the head and jaw structure of male trout. During the spawning season and carrying over into the remainder of the year, males develop that pronounced hooked jaw, known as a "kype," which is used in displays of aggression and dominance. Female trout do not develop this hooked jaw so that is often a dead giveaway. Their heads and jaws remain relatively unchanged throughout the year. The maxillary or upper jaw bones in males are longer, wider and thicker than in females. Males tend to have a longer upper mandible jaw line where as females have a downward sloping jaw line in the upper mandible.  

Male brown trout post spawn.
Male brown trout post spawn.

Fin size and shape is another determining factor for you to observe and can be a tue breaker for you. Males may have larger and more angular dorsal and pelvic fins. Female fins tend to be smaller and rounder in comparison. The easiest way I have found to determine the difference is between the anal fins.  Male tend to have a concave fin, where the back side will have an inward curve and females have a convex fin with a slightly outward curve. This is a quick observation that can be relatively accurate in observation compared to some of the more subtle signs discussed prior. 

After a while you’ll start to see the traits for determining the sex of a trout far more easily. Recognizing the visual differences between male and female trout is always fun as an angler and people often want to know. Whether for fishing, conservation, or simple observation, understanding these differences is a step towards appreciating the complex life of trout in our freshwater ecosystems.

Christian Bacasa, Host of the Fly Fishing Insider Podcast

By Christian Bacasa
Host of the Fly Fishing Insider Podcast

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