Belonging to the Corixidae family, the corixa, or lesser water boatman, are not dissimilar in appearance to beetles. Almost entirely aquatic in behaviour, a quick look at their anatomy reveals that they do, in fact, have wings, which are protected by an outer shell (wingcase).
Capable fliers, from time to time they migrate between waters to colonise and breed. However, one of their most striking features is a pair of paddlelike legs (hence the term boatman) that help them shift about at speed. Indeed, when wading the margins, many of these little bugs will be seen darting for cover.
Lacking in gills means they need to make repeated journeys to the water’s surface for oxygen. This oxygen capsule, held on the underside of their body, appears almost like a tiny air bubble. Diving back to the lakebed where they spend their lives, somewhat more buoyant now, they grapple with weed stems and any other vegetation to prevent themselves floating back to the surface. As their oxygen supply depletes, once more they have to undertake a perilous trip for a refill. With this in mind, it’s understandable why these little bugs seek out a living in relatively shallow water.
NATURAL SIZES AND COLOUR
Not to be confused with the water boatman proper that can be four times larger, corixa range from 6-12mm in length. Actually, the water boatman is quite different in its behaviour as it spends long periods of time attached to the underside of the surface film, where it hunts the margins. Examining samples in my area, corixa average about 8mm, which equates to a size 14-16 hook depending on model. Although geographic location always has a bearing on coloration, corixa generally have an olive-brown back with black barring and a pale yellow underside. However, once loaded with an air capsule, they often take on a brilliant silvery hue.
When it comes to fishing corixa patterns, I feel we fishermen often neglect them because, unlike many other aquatic insects, they don’t actually hatch at the surface as such. But let’s not forget that corixa are an available food to trout the year round at any time of day.
Corixa are not to be confused with the predatory and much larger backswimmers
HOTSPOTS AND TACTICS
Recalling that they prefer shallow water, fishing marginal regions should be our first port of call. Though do consider that if there’s been human/angler activity along a stretch of bank you intend fishing, the trout may have been pushed some way out. Given the luxury of an undisturbed bank, concentrate by casting almost parallel to the shore. Use a floating line and leader length that you’re comfortable with. Mine is usual around the 14ft mark.
My initial approach is a two-fly rig with a sacrificial pattern on a dropper. Tethered some 2-3ft behind this is a more buoyant fly. The idea is that the heavily-weighted fly has just enough ballast to momentarily sink the ethafoam fly when retrieved. Cast along the margins and let the flies settle before imparting a jerky retrieve with plenty of pauses and short draws. Such is the ballast of the dropper fly that it physically submerges the buoyant fly, giving it a nice reverse sink-and-draw path.
The idea being to either suggest an up-and-down motion of corixa or that of a fleeing bug. Naturally, such a weighted fly is in danger of hooking snags, especially in rocky or weedy areas. If this is the case then look to attaching a spilt shot or two on the leader. Yes, it goes against convention, but it is a deadly method on its day.
IN THE WEED
As corixa frequent weedy areas it can often be worth pitching a fly in and around the leafy fronds. Most people shy away from weed, especially if the growth is heavy. Such places harbour plenty of invertebrates and provide cover for fish. With both food and shelter, trout feed more readily and often accept a fly with confidence. Though this isn’t the territory for a threefly cast, as the trailing fly/flies are certain to snag, resulting in lost fish. It’s worth revising the leader breaking strain too, with 7lb being more acceptable and this might be stepped up if larger trout are expected. In extreme cases of menacing weed growth it is more a case of literally pitching a fly into a likely gap, then gently drawing it out with a raised rod tip. Depending on the size of the opening, sometimes the fly will only move a foot or two before catching weed, though there will be occasions when it’s instantly snapped up, making all that weed removal from the hook point worthwhile.
CORIXA MIGRATION TIME
Although corixa can be expected on the wing throughout a season there’s an increase in activity around August and September. Moving from water to water I’ve seen them in huge numbers. There’ll be a little “plop” nearby as a bug touches down. In any appreciable wave, corixa can penetrate a weakened surface film and dive for cover. In calm conditions, I’ve seen them scuttling across the water in a bid to break through the greasy surface. Such a commotion is easily noticed and trout home in.
Some of the corixa don’t even make it through the surface film under these conditions. Tired and weak during such times there’s often a high percentage of casualties, as dead and dying insects litter the water. At the mercy of the wind now, trout find easy pickings with a more leisurely rising form.
This is when a buoyant dressing really comes into its own. Two of these cast out and left to drift can be devastating. If sport is slow, or there are too many naturals about that your offering becomes lost in the crowd, give the fly an occasional tweak. Imitating a kicking natural often attracts the trout’s attention, drawing them to your fly. Alternatively, I’ll hang a spider pattern off a dropper approx 4-5ft from the point fly (diagram below left), and even when trout can be seen taking off the top it’s amazing how many of them succumb to a fly presented maybe an inch or two below the surface.
As discussed earlier corixa are constantly available to trout and, ever the opportunist, I’m sure fish seize the darting bugs whenever they chance upon them. That said, trout will sometimes give corixa their full attention. This usually occurs early and late in the day when trout feel safer in low light levels. Undisturbed they will happily forage for corixa along the margins. I’ve spent many a happy hour creeping along the bankside at first light, as fish can be seen bowing in water no more than inches deep. Spooning some of these trout did reveal hordes of corixa along with stickleback and minnows, no doubt a welcome addition to the trout’s diet.