Waterboatmen: Bi-Atheletes of the Fall

By Jeff Morgan

Water Boatmen (the family Corixidae) and their close cousins the backswimmers (family Notonectids) are a common aquatic insect that even non-fishermen have no trouble recognizing. These insects frequent the shallow margins of most stillwaters, as well as the slow-moving parts of many streams.

You can even find waterboatmen in saltwater because they can be very tolerant of saline waters. Ocean currents can transport estuarine species to nearby islands, which the little critters then colonize.

Though they can be found in the highest Sierra-Nevada alpine tarn and the warmest desert lake, I have yet to find large numbers of these insects in what we would consider a "trout" stream. However, the cosmopolitan nature of this insect, combined with its preference for shallow water, makes it important not just to trout, but to bluegill, pumpkinseed, bass, and a wide assortment of other fish.

Natural Bi-atheletes that Shine in the Fall

Water Boatmen are one of nature's best bi-athletes: they are equally adept at flying through the air as well as kicking through the water. The ability to fly is a crucial adaptation, which allows them to migrate between waters, particularly in the fall when their native locale may be drying up. Flying also allows them to populate lakes that may not have harbored waterboatmen earlier in the season.

Waterboatmen take on increasing importance in the fall. Dragonflies , damselflies , caddis , and Callibaetis emerged throughout the summer, so as the trout season winds up, only small, immature aquatic stages of these insects remain. Waterboatmen are one of the largest aquatic insects still available to trout at that time of year. A trout's final few cruising loops of the year almost inevitably result in the deaths of dozens of waterboatmen.

Oars for Legs

These insects are about 6-12 millimeters in length, have dark olive or brown backs, and a lighter cream, tan, or very light-olive belly. They are easily distinguished by their two rear pairs of long, oar-like legs lined with brushy hairs, hence their name.

These powerful organs propel the waterboatman in short quick bursts. The easiest way to distinguish between corixids (water boatmen) and notonectids (backswimmers) is that backswimmers swim on their backs, with their abdomens facing the surface. The backswimmers breathe by exposing their abdomens to the air, thus breathing atmospheric oxygen.

The bodies of both insects are so buoyant that they must swim when submerged, otherwise they float to the surface. Many insects battle this buoyancy by grabbing hold of aquatic vegetation in order to rest.

Waterboatmen feed on plant material, while backswimmers prey upon midge and mosquito larvae, freshwater shrimp, and even small tadpoles and young fish. Watch out when handling backswimmers--they can really give you a bite!

Waterboatmen usually reside in the upper foot or two of the water column, for they lack gills and must penetrate the surface film to acquire atmospheric oxygen to breathe. They also can take a little bubble with them to suck on while they cruise along under the surface.

Backswimmers often penetrate the surface with their abdomens in order to breathe.

Both waterboatmen and backswimmers love the weedy shallows that line the margins of many lakes, for it provides food and shelter for them. The inside weed edge is the most important location in a lake to imitate water boatmen.

Important Western Species

Two important western species of corixids and notonectids are:

  1. Arctocorxia alternata: a common corixid throughout the west, about 3/16-1/4" in length and generally dark brown or black, though the belly is often lighter.
  2. Notonecta undulata perhaps the most common American backswimmer, is between 3/8 and 1/2 inch long, with a brownish olive body and a dark brown wingcase. Remember that since these insects ride upside down, we must imitate their "top color," the exact opposite of what we do with every other aquatic insect!

Fly Patterns and Tactics

I usually don't cast waterboatman or backswimmer imitations until mid-July, after damsel and Callibaetis emergences have peaked. Starting about August 1st, however, imitating these food sources becomes a very important part of the stillwater angler's repertoire.

When tying imitations of these insects, remember that they FLOAT when they stop swimming. Most commercial patterns ignore this important biological fact, and by heavily weighting their patterns, render them much less productive. I use a foam underbody on all of my Water Boatman and Backswimmers to ensure their buoyancy.

To help your patterns catch they eye of trout, use a strip of Flashabou or a pearlescent bead to imitate the air bubble trapped along the insect's body when they dive.

Finally, make sure your backswimmer patterns are tied upside down, so that the brown or black wingcase (not the light underbody) is facing the fish below. Making sure that your flies follow these three guidelines of natural approximation should really improve the success of your Corixid patterns.

Imitations of waterboatmen and backswimmers should be fished in short, 2-6 inch strips, with a brief pause between strips. Naturals move in a jerky motion, and this type of retrieve is a good way to replicate that. Use a slow sinking line when imitating these insects, for a floating line will not allow you to get these buoyant patterns under the surface, and a fast sinking line will get your flies too deep.

The prime place to fish imitations of these bugs is the inside edge of a weedbed. Because the inside weed line is often located in 2-4 feet of water, it is a good idea to focus on this area during the low light hours when fish are less likely to feel exposed and be spooky.

These insects don't like to venture far from weeds (since they offer food and a place to rest), and they are rarely found deeper than 2 feet below the surface. If you fish these flies in open or deep water, you might never want to fish them again. But if you fish them in the proper place and in the right way, you may spend all winter tying up more imitations of these "miracle bugs."

Jeff Morgan has written many articles for Westfly, mostly on entomology and fly tying. He is the author of An Angler's Guide to the Oregon Cascades and Small Stream Fly Fishing.