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3. Some animations on how fish react to prey

Where do you need to put the fly to catch a trout?

Countless books about fly fishing contain whole chapters on this subject, but a couple of the main lessons are (1) get the fly down to the fish's level, if you're fishing subsurface, and (2) cast upstream of the fish, not right over its head.

Thanks to the hard work of my labmates at the University of Georgia building detailed datasets for the Drift Model Project, I was able to construct some 3-D animation videos showing where drift-feeding fish, such as trout, detect their prey. We're using these data to test models that predict how much energy the fish are gaining and how that depends on aspects of their habitat such as water velocity and drifting prey. Those models will eventually help us better predict the effects of habitat modifications and changing streamflow patterns on trout and other drift-feeding fish. We're far from done crunching all the numbers from last summer's data collection, but some of our intermediate results may be interesting to fly anglers.

A few key videos are embedded below, but the full set is available as a playlist on the Drift Model Project's Youtube channel.

The videos rotate around an animated fish representing either an Arctic grayling (14-19 inches long), dolly varden (6-8 inches long), or juvenile Chinook salmon (about 3 inches long). The yellow dots show the estimated positions of potential prey items (some real prey, some inedible debris) at the moment when the fish reacted to them. All the positions are shown relative to the position of the fish, which is shown facing straight upstream.

Here's one from a smallish (~14-inch) Arctic grayling during a moderately thick Cinygmula hatch:



Note the separate layer of detection positions on the top, all of which were prey taken from the water's surface (not quite in an even plane because of small measurement errors). Some goes for the following 19-inch grayling feeding during a Drunella doddsii hatch:



And here's a 7-inch dolly varden feeding without any particular hatch:



And a juvenile Chinook salmon:



Obviously this is a limited sample. We're continually expanding it, but even then we're only studying three species at the moment. So don't be surprised if you've seen some things that don't fit these patterns. Nevertheless, they give me reason to speculate a few possible insights into fly fishing:

First, the fish react most often to subsurface prey that drift really close to them. So there's probably a huge payoff for very accurate fly placement, especially when you're sight fishing with a subsurface fly. Getting down to the fish's level is clearly critical, but so is placing the fly laterally so it passes right by the fish.

Second, even though the grayling were surface feeding on hatches, they continued to take a lot more subsurface prey throughout the time we observed them. These fish seemed to be picky about what they took from the surface, judging by what flies they would accept when we caught them later to (harmlessly) pump their stomachs. Yet despite that apparent selectivity on the surface, they were pursuing and capturing a variety of items below the surface. Nymph aficionados, some of whom prefer to fish a nymph straight through a hatch and a good rise of fish, may rejoice at this news. But not so fast, because...

Third, the fish react to prey on the surface from much farther away than they react to subsurface prey, perhaps because it better stands out against the background. This explains why dry flies work so well, and it offers the dry fly angler a defense against the nympher who insists that it's better to fish subsurface because that's where the fish get most of their food. It's true that there's rarely a better way to entice a fish than to present a nymph on a perfect, accurate drift. But it can take most of us several tries to get that perfect subsurface presentation, especially if we don't know exactly where the fish is. When trying to just read the water and guess where fish might be, it makes a lot of sense to use a dry, because every cast effectively covers a lot more ground. Even when we're working a particular rising fish that would be receptive to a nymph or dry, it's easier to consistently cast a dry into the strike zone than a nymph.

Finally, I want to emphasize again that these are just my preliminary thoughts as an angler after seeing the data we've analyzed so far on this project. They are not conclusive scientific findings. But they are good food for thought.