Why Scud Color Matters, What Trout Remember and Other Weird Things to Mull Over

Why Scud Color Matters, What Trout Remember and Other Weird Things to Mull Over

February 12, 2012 8:38 pm Published by Leave your thoughts
Bighorn River Sowbug Photo SG

Big Horn River Sowbug

I was talking with Steve the other day about how many different colors of scud and sow bug patterns we use. My first thought was that if someone did know exactly why and which fly to use every time, for every fish they would make a bundle. My next thought was about how boring and automated it would make fly fishing. That said, experienced guides and anglers who are what we deem to be ’fishy’ seem to make correct fly color decisions all the time, often seemingly on a whim. In reality it is an ability developed over time by soaking in experiences to a point it becomes almost instinctual. If you ask these folks why they changed nymph color the most honest answer is often simply because the other one was not working. It was just time. Or they may offer a complicated logical reason for the new color selection, but it still can seem down right wizardly. The best retort I have ever had from a client after a long winded explanation was “well why weren’t we fishing the pink one to start with?” I suggest you use that one on guides to keep their heads from out-growing their hats.  I kind of like going with my gut on these matters. It‘s great when the first re-rigged cast produces, but it never compares to telling your guy to be ready to set the hook a split second before a fish takes even if you have no idea why. It hardly seems professional when asked how you knew to say “I don’t really know” , but that is normally my answer unless fishing an obvious feeding lie where we catch fish all the time. Yes I know, I should have had them cast there earlier…


Color often can make quite a dramatic difference when selecting sow bug and scud patterns on the Big Horn. Gray tan, pink, orange and many combinations and tones of colors all seem to have their time, while many days they all work fine. My rule of thumb is to start with a slightly larger tan or gray pattern followed by a smaller pink or orange bug.  I know there is a catching rate bias on the lower fly over the top fly, but one can get a feel for what is happening pretty quickly running two vastly different colors. When sight nymphing I like a brighter more visible fly on top to aid presentation.  Water clarity can also make a huge difference in what fish will take. In the fall when the lake turns over and the river visibility drops I also like brighter scuds for obvious reasons. Orange seems to be the best color for getting fish to move to the bug.
Bighorn River insects

An Opportunistic Feeder Photo DP


Logically a natural grayish colored scud should work best given the majority of gammarus scuds and sow bugs are grayish and trout are notoriously selective feeders. But if you kick up the bottom and net some scuds and sow bugs you will find a nice variance in colors. Along with the gray ones, some are translucent with pinkish hues, many are greenish. Quite a few are two toned. At Lees Ferry back in the day we would even find bright chartreuse individuals that we represented with a pattern called the ‘Radioactive Scud’.  This wide range of colors is due to what they are eating, what depth they are at, light levels, if they are carrying eggs or not, water temperatures, nutrient loads…all sorts of reasons.  Dead ones will turn orange or pink from carotenes in the shell just as other aquatic crustaceans such as shrimp and crayfish do when they die.


Recent scientific research has shown selective feeding behavior in fish to be a function of memory. Behavioral ichthyologists (I made that term up), tell us trout may take a week or longer to start zeroing in on new food sources. Fish also anticipate hatches by learning the time of day the food will become available to them. (I think most trout anglers could have saved the tax payer some bucks  by stating the obvious eh? Amazing a guy can get paid to write down common knowledge in unreadable scientific vernacular.)  In my experience it also seems to be about preference for a certain food type. For instance when we have a multiple species hatch often the trout will take one or the other. They might be happily gulping big PMDs only to switch to tiny baetis when they suddenly come off amongst the PMD’s . In the case of the fish rejecting PMD’s, perhaps they have seen baetis all spring and revert to what they know better.


The interesting bit to me is that fish have a well developed long term memory ability. One paper described a carp that refused any food with a hook attached no matter how well disguised for a full year after being caught. Another study on bass showed that they could be caught on the same lure a few times over a 14 day period but the frequency of being fooled quickly decreased over the next month and finally they learned to avoid the bait all together and did so for nearly a year. One big brown I caught at the Pipeline Hole had six fresh Gray Ray’s stuck in his jaws and nose. I can’t tell you what it means definitively other than he seemed to like gray sow bugs as I also caught it on a Gray Ray… and that perhaps quite few guys needed some knot help. Over the years I have had guys catch the same fish a few times in a day. My record being one twenty inch YNP cutthroat caught three times in  half an hour during a green drake hatch. I have a hard time accepting that Big Horn trout develop long term avoidance behaviors. We catch too many to make the math work. Perhaps they have learned that being caught and released is not so bad. (I should apply for a grant!) I have fished syndicate managed chalk streams in England that require you kill any fish caught based on the belief that catch and release makes them too hard for the average member to catch. This thinking is finally changing as wild fish are becoming appreciated in the UK as much as they are here.


Watching trout eat scuds and sow bugs can be an interesting activity in it’s self. When the little protein pills are moving around or drifting with the current the trout simply take them like any nymph, but often make fairly big lateral moves to intercept them. When scuds are not active and stay buried in the vegetation trout will often grab clumps of algae or weed and sort the meat from the salad, or try to. Often the Big Horn’s famous green moss will get stuck in the fish’s teeth and they will shake their heads violently to dislodge it. Much of the greenery is inadvertently swallowed along with the scuds and comes out the other end pretty much unchanged.


 I feel brightly colored scud flies are also taken as eggs by Big Horn trout. With active redds being found from October through June and often into July, stray eggs are readily available. Last December I was fishing a pink and orange scud through a run containing no redds that we could see and poked a few trout. Steve pumped one of the browns and found eggs as well as midge larva had been eaten, both of which also come in all sorts of colors. That’s a whole other article…


Another theory of mine is that trout sometimes just like to eat bright shiny stuff as well as dull wiggly stuff. I sort of like that one the best.

-Bighorn River Guide Bob Bergquist

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This post was written by Bighornangler

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