Dealing with Daphnia
How do you take advantage of a major trout food that's too small to imitate?
know what you're thinking: "This guy is nuts. I'll put up with arcane talk of alderflies and inchworms, but this is ridiculous. Come back to the real world."
I sympathize; I didn't want to delve into another trout food either! If it weren't for dozens of British angling experts extolling the seasonal importance of Daphnia to average and larger trout, I never would have paid attention to this miniscule crustacean.
However, about three years ago I started looking for signs of them while fishing. I have since landed many trout in the 16-inch-plus range--a quality fish in my book--with Daphnia lining their mouth and esophagus. Obviously trout think Daphnia are important, even if anglers ignore them.
Tiny But Abundant
If you ever took a biology class, you probably remember looking through a microscope at tiny reddish critters kicking around in a drop of water. Those were Daphnia. Those critters probably ran .5-1.5mm in length, which is too small for heavy trout predation. But Daphnia can reach 4mm in length, more commonly 1-3mm (3mm is about 1/8 inch). It's these larger specimens that are important to trout.
When Daphnia undergo their seasonal population explosion, some waters can see densities approaching 500 Daphnia per liter. About 200 specimens per liter is more normal, however. Many lakes have populations pushing 14,000 per square meter of open water!
Trout and trout anglers take notice when insects occur in this kind of abundance, regardless of size. For example, we all recognize small Trico mayflies and midges as viable "hatches," yet those bugs are in the 2-4mm size range. Daphnia are certainly in that ballpark.
It's not enough, of course, for Daphnia to occur in abundance. To be important to trout anglers, trout must eat them. So do they devour food that is typically less than 3mm in size? In one study of a large Washington stillwater, Daphnia were the most important component of the trout diet until the trout reached 14 inches in length, whence they turned to baitfish as their primary food source. But even then, Daphnia were the second most important food source.
A study by Robert Behnke, one of the most distinguished fish ecologists around, showed that Daphnia made up a whopping 68% of the diet of Pike's Peak cutthroat trout, and "only" 32% and 28% of the rainbow and brook trout diets, respectively. The same study said that the Pike's Peak strain fed almost "exclusively" on Daphnia in August and September.
Lifestyles of the Small and Unnoticed
Daphnia maintain a fairly Spartan existence by slowly poking about eating algae. They tend to feed at the surface under low light and nighttime conditions, when trout (and other predators) are less likely to gobble them up. During the daylight hours, Daphnia usually move down to the 15-35 foot level where trout often reside during the warm summer months as they seek cool, well-oxygenated water. When trout are down deep in relatively open water, the only available food--save phantom midges--is Daphnia.
This light-induced migration is the critical factor that fly anglers must consider when imitating Daphnia. Color, size, locomotion, and reproduction all are relatively mundane. But understanding the migrations can put you where the trout are in open water/deep water situations. Even when trout aren't consistently found at the same depths as Daphnia, they may take "dives," plunging to the Daphnia-dense depths to graze for a few minutes before ascending to their normal cruising level.
The other important thing to think about is population fluctuations. Calm waters, low-flushing reservoirs, and cool water temperatures all benefit Daphnia populations. Although Daphnia are filter feeders, high levels of suspended sediments can be harmful to their well being. Reservoirs that see large influxes of silt-filled water (say below clear-cut forests) can see depressed levels of Daphnia and Daphnia-related movements on the part of the trout.
Daphnia are important to trout and fly anglers in the sense that trout can be located based on movement and densities of Daphnia populations. A wise stillwater angler will keep these diminutive creatures in mind when normal tactics prove futile, especially in the late summer months
How to Imitate?
Hold your thumb and forefinger about 1/8 inch apart. Imagine tying a fly that small. So how the heck do you imitate a single Daphina, one that might be half that size?
Simple: you don't.
Instead, use small- to medium-sized flies in red or orange. These flies are effective for Daphnia-feeding trout. I don't know exactly what they take it for, but it works. The color is rarely the deciding factor, though these colors are widely agreed upon for Daphnia. Other fish that are feeding on Daphnia are receptive to Chaoborus (phantom midge) imitations, as these insects follow the Daphnia around and prey on them throughout the day.
There are two styles of patterns that I prefer for Daphnia situations. The first is based on the popular British models that incorporate marabou and Krystalflash. These materials are excellent to construct flashy and active attractor-style patterns used for Daphnia situations. A small beadhead can be added for flash or depth, but with a type-V to type-VII line, a 1/164 oz. bead won't help all that much.
The other style that works well is also fun and easy to tie. It was designed by Westfly regular, Ronn Lucas Jr. This fly utilizes a sparse dubbing and, of all things, fingernail polish. This polish, as it dries, will clump up into small balls which look as much like little Daphnia as about anything could. I like to add a body of copper wire so that I can fish it near the surface in the evenings and early mornings on an intermediate line and still have it get down a bit.
However, what matters most is that you put your imitation where fish are suspended. The easy way to locate deep fish in open water is a depth finder/fish finder. The other way is to find a likely cove or point, and cast out allowing your fly/flies to sink to various depths before starting your retrieve. Having two or three other anglers with you can quickly cut down the prospecting time, as everyone can fish different depths at different speeds. Fish can be extremely deep (20-30 feet) when making daytime feeding runs on Daphnia, so don't just quit prospecting with Daphnia when 20 feet proves unproductive.
Once you locate the proper depth, check the water temperature and note that along with sunlight conditions in your angling log. Daphnia on certain bodies of water tend to move to consistent depths depending on the water temperature and amount of sunlight. Also, look in the gullet of the fish you catch at any depth for presence of Daphnia--they look like they have been eating Nalley's Chili. This will let you know whether you caught a random fish or what you were targeting. Good note-taking, or a good memory, will save prospecting time when targeting Daphnia-feeding trout on future trips.
Retrieve techniques when targeting Daphnia-feeding trout are varied, but I think a slow, hand-twist, retrieve is best. Summertime trout feeding on Daphnia are often lethargic (they are in the cool depths for the oxygen, not the food or scenery) so patient retrieves are recommended.
While Daphnia like moderately open water, trout don't. This leads me (and many others who follow Daphnia), to focus my efforts near points and small bays. Here trout can cruise at comfortably shallow depths, then dive to the Daphnia-rich deep water that is nearby. I have never done well on broad, straight, light-sloped banks, as trout likely have to wander far from prime feeding zones to seek out Daphnia populations.
Common sense may lead one to think that wind drives Daphnia to the downwind side of lakes, but this isn't as common as you would expect. Daphnia spend much of their daytimes at depths of 15-30 feet, where wind action has little effect. Even if they do get pushed to the windward shore, the heavy wave action makes them tough for trout to see, plus trout get a mouth full of silt if they try to feed in these conditions. I strongly recommend concentrating your angling effort along downwind banks, but think leeches, snails, and baitfish rather than Daphnia.
A depth finder can be a useful, nay, irreplaceable, tool in locating Daphnia feeding trout. I like to scan the depths of open water 20-30 feet deep looking for "random" fish while I munch on my lunch. If you start to see a trend, odds are that those fish are feeding on suspended Daphnia. I try not to mount a depth finder on my pontoon boat or float tube, but on a larger boat (in a larger lake) the depth finder can save hours of random casting.
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