Next on our list is our second most popular game fish: the Rainbow Trout. One remarkable thing about these fish is there are two notable forms that exist in the Middle Fork and around the world: an anadromous form called steelhead and a freshwater form called rainbow trout. They are genetically identical, and it is a question of much research and significant debate as to why some Rainbows migrate to the ocean and others do not. To confuse matters further, it’s been shown that steelhead can have offspring that don’t migrate to the ocean, and resident ‘bows can have offspring that go on to be anadromous. Idaho River Journeys nymphing guru Montana Dave has more to say on the subject, but we’ll leave it there for now. In this portion of the article, we’ll focus on Rainbow Trout that live on the Middle Fork year-round. A staple of western fly fishing, the feisty Rainbow has a permanent place in most fly fishermen’s hearts.
Origin: Are Rainbow Trout native to Idaho? Yes and no. There is one line of Rainbows, called a Redband Trout, that evidence indicates has lived in Idaho for thousands of years. Clearly, these fish and their descendants are native and predate the arrival of European settlers. However, since settlers first arrived in Idaho there have been thousands of non-native Rainbow trout introduced into our lakes and streams for fishing, and these newcomers have interbred with the Redbands. Because of this, the Rainbows we find on the Middle Fork probably have some ancestors that evolved in Idaho streams for thousands of years and others that arrived much more recently.
How to ID: The appearance of Rainbows varies greatly depending on habitat. They may be silvery in lakes and reservoirs and range from olive to greenish-blue in streams. Their sides may show a red or pink streak, and there’s often a white tip on the pelvic (belly) and anal fin. You’ll also often see irregular spots on their back, sides, head, dorsal fin, and tail. The spots are generally more evenly distributed from the head than cutthroat trout. Even on the Middle Fork, Rainbows often exhibit very different coloration depending on size, age, and where the fish spends its time, ranging from very silver or chrome-colored to very dark deep red. These color variations occur naturally from genetics, gender, time of year, and food source.
How to differentiate Rainbows and Cutthroats: Rainbows usually lack the distinctive orange slash on the lower jaw that you will find on a Cutthroat. Rainbows usually have a broad, reddish stripe that extends along their lateral line from their gills to their tail, hence the name for the local strain “Redbands.” Cutties generally lack this red stripe. Lastly, spots tend to be more evenly distributed throughout the body of a Rainbow. This pattern allows for differentiation from the Cutthroats, where spots tend to cluster more around the tail.
Feeding Habits: In streams, Rainbows primarily feed on drifting organisms. They may ingest aquatic vegetation but are probably targeting attached invertebrates. Their diet changes seasonally, but insects remain the dominant food source throughout life. Bigger rainbows also find calories eating smaller baitfish.
Fishing: Middle Fork Rainbows, while not quite as voracious surface feeders as our Cutthroats, are often caught on dry flies throughout the summer season. Many of the patterns and approaches that are effective for Cutthroat work well for Rainbows. Additionally, because bigger ‘bows often consume more of their diet under the surface, anglers can find greater success with nymphs, soft hackles, and streamers. The larger Rainbows lower down on the Middle Fork often have a beautiful dark red color and are a joy to catch.