Discovering Daphnia

It was a nasty, windy, rainy day but that is just ambiance to the dedicated stillwater angler. My back was into the wind and a steady cadence of the fins kept my kickboat in place, despite the 20-plus-miles-per-hour breeze. My slow-sinking fly line cut into the lake’s clear waters, and I began to slowly retrieve my fly.

The line pulsed with life, came tight, and I set the hook. A girthy rainbow shot from the depths and vaulted up through the choppy surface, making several screaming runs before it slowly came in—a beautiful 8-plus pounder. The Daphnia Cluster Fly strikes again!

In the ’80s I stumbled onto a situation that illuminated a new aspect of fishing for me. The trout I was catching were so full that their gullets were gushing little critters that looked like green and reddish-orange mush. From my copy of McCafferty’s Aquatic Entomology I learned the “mush” was actually hundreds of small crustaceans called daphnia or water fleas. I was aware of plankton as part of the food chain but unaware that trout use this food resource so much.

I had often seen large, fat, and hard-to-catch trout in high mountain lakes and always wondered how they could be so healthy despite an eight-month-long winter under the ice. I finally realized that many of them were fat from eating daphnia.

Daphnia As Trout Food

Daphnia are part of the zooplankton in many lakes, ponds, and streams. Just think of daphnia as #30-#64 scuds. While you can’t possibly imitate daphnia individually, you can imitate a group of them. Trout gulp clusters of daphnia or they swim around with their mouths open, much like a whale or manta ray might do to filter plankton.

Daphnia are a fairly universal food that stillwater anglers should be able to imitate. Much of my early research took place at Strawberry Reservoir in Utah and in some high mountain lakes in Wyoming’s Wind River Range. Since then I’ve done well with the daphnia patterns and techniques I developed from the Arctic to Patagonia; in the high mountain lakes of North America and Bolivia; and lowland lagoons in Chile. Daphnia also inhabit many European lakes.

Few anglers seem aware of daphnia and their importance to stillwater trout. Many anglers have caught trout that are feeding on daphnia that hit their leech, streamer, nymph, or scud incidentally while feeding on daphnia.

I first wrote about daphnia fishing techniques in my book Nymphing Strategies, published in 1995 as part of Lefty’s Little Library of Fly Fishing, and it got lots of interest from curious lake fly fishers. Through guiding and slide shows, I’ve had the opportunity over the years to introduce many anglers to daphnia techniques and patterns.

European anglers are well-known for their lake-fishing skills and knowledge. Recent articles and a section in the recently published book Trout Fly Fishing An Expert Approach by Cairncross and Dawson show that they too are currently developing techniques and patterns for daphnia feeders. Some ice anglers also know trout rely on daphnia through the winter and will try to catch them with small, brightly colored jigs.

Daphnia are a favorite trout food when other food sources such as insects or leeches are scarce. Many minnows and insects survive on daphnia, so there is a secondary benefit to trout. Some trout and Kokanee salmon seem to feed on nothing but daphnia year-round. My Daphnia Cluster Flies have caught numerous trout up to 14 pounds, confirming that even very large fish can feed on them. That should not seem incredible when you consider the biggest creatures in the sea also feed on micro-organisms. Stomach pump samples show that some daphnia feeders feed only on these miniscule organisms, while others prey opportunistically on scuds, midges, damselfly and dragonfly nymphs, minnows, and other foods while feeding in a daphnia bloom.

Daphnia Biology

There are at least 135 species of daphnia in North America and possibly over 800 worldwide. Luckily, because of their small size and abundance, imitating individual daphnia species is not necessary, although color can be critical. Daphnia do not have distinct segmentations like scuds, sowbugs, or crayfish, although they are all crustaceans. Daphnia are enclosed in a folded shell called a carapace and have a helmetlike head with a distinct, dark eye spot. They have antennae that are also used for swimming and often have spikelike tails. They are generally 0.2 to 3 mm long.

Daphnia are often small and translucent enough to be nearly invisible in lake water. They appear to be about the size of the head of a pin and are often almost clear to gray, green, brown, red, or orange. In most areas, they turn light green in summer and turn red or orange in cooler temperatures. They are often mistaken for an algae bloom. You have to look closely to see them as individuals swimming slowly in vast clouds or plumes on the downwind shallows of lakes or where upwellings (places on the lake surface that look oily or slick in an otherwise choppy surface) bring them to the surface. Once you learn to identify daphnia close up, you can see clouds of them at some distance with polarized glasses.

Despite their ability to swim in a bobbing or jerky fashion, they are largely at the mercy of water currents and wind. Their limited swimming ability allows them only to rise or descend in the water column. They are photosensitive and will descend during the day and rise at night or during cloudy periods.

Daphnia feed primarily on algae, microscopic animals, and organic debris; populations increase proportionate to their sources of food. They require vitamins and oxygen, but too much oxygen will kill them. Numbers of larger daphnia decrease as trout feed on them heavily.

Daphnia are not always visible from the surface and are often down by the thermocline (the layer of water between the warmer surface and the colder depths where the water temperature changes rapidly), about 25 to 40 feet deep or more in summer. Spring and fall, daphnia are often found at or near the surface. You may have to fish different depths with sinking lines to locate daphnia-feeding trout when they aren’t near the surface. If you wonder why trout are feeding over the profundal zone (deep-water areas) where little insect life is, they are most likely feeding on daphnia.