More Migratory Mayhem

Dragonfly and damselfly nymphs are two of the biggest draws (or at least the biggest bugs) as far as aquatic insect migrations go in lakes. But if your preference is to fish streams and rivers, it is the stoneflies that cause a stir at this time of year with their migratory antics.

Like the Odonates, stoneflies crawl out to emerge and so the adults are not available to feeding trout until they return to the water to lay eggs (or get blown onto the water).  Despite this, stonefly hatches can provide some fantastic fishing both during a hatch and in the days leading up to a hatch.

Things that lurk in the dark (Pteronarcys californica adult)

As the time for emergence approaches, many stonefly nymphs will start to migrate from their preferred feeding habitats to near-shore areas where conditions favor a smooth transition to the terrestrial world. As they migrate they become more available to feeding trout; both as they crawl along the bottom, or enter the drift to surf into back-eddies and slicks. Trout will station themselves along edges near the shoreline to intercept anything that wiggles or crawls within range.

When hatches are heavy, shed exuvia can be found on many surfaces along a stream
(Pteronarcys californica exuvia)

With more trout feeding on these often larger nymphs, search images are formed and retained as the migrations progress to the actual act of emergence (often taking place after dark). Knowing what species are present in your local waters can be a big help in fly selection. A quick search of shoreline rocks, bushes, or bridge abutments can provide clues as to which species are active. Golden stonefly nymph imitations are generally a good choice and can simply be drifted along any shoreline edge or seam, or slowly worked along the bottom in the slack edge-water below a riffle.  These are big bugs that bring larger trout to the feeding trough, so even if the water is a little high and off-color, working the edges with a stonefly nymph is always worthwhile.

Golden stones are common on many western streams and rivers (Hesperoperla pacifica nymph)

Migrating Damselfly Nymphs

The month of June is prime time for fishing damselfly hatches on lakes. Although I have witnessed trout specifically targeting the adults as they perch on overhanging bushes along the shoreline, it is a rare event. During a hatch, most of a trout’s attention is focused on the migrating nymphs as they make their way from submerged weedy shoals to emergence sites along the lake-shore.  

Depending on conditions, and the damselfly species involved, migrations can take place along the bottom or just under the surface. I have only encountered a handful of hatches where I observed nymphs migrating just sub-surface. Most of the time, the nymphs are swimming along the bottom or just above the weed-tops. If I see signs of recent emergence, or an active hatch in progress, I will work a Migrating Damselfly Nymph on a sink-tip line with a slow or intermediate sink rate. In either case, the mode of propulsion for the nymph is the typical and somewhat inefficient side-to-side undulation of the body.

A multi-exposure strobe photo of a swimming
damselfly nymph (Enallagma annexum)

Creating a fly that effectively mimics this undulating behavior has been the goal of many a fly tier over the years. Strategies have included everything from long tails of marabou to articulating bodies, and even the addition of a tiny crank-bait style lip to the fly – but all to no avail. While some of these fly patterns do provide a small measure of undulatory action (in the up and down dimension as opposed to the desired side-to-side direction), I feel the advantages are not sufficient to warrant the extra effort. Instead, I focus on the more important factors of retrieve rate and search image.

A newly emerged (teneral) damselfly with shed exuvia (Lestes sp.) - a sure sign
of a hatch in progress.

If threatened, damselfly nymphs can move at a pretty good clip – but only for short bursts. During migration, they typically plod along in a very slow but determined manner, often pausing for short periods to rest. Your retrieve should follow this pattern. A very, very slow hand-twist retrieve (¼” to ½” per second) with the occasional pause will bring more takes than a rushed retrieve. It takes a lot of patience to work a fly this slowly, and you will need to use a line with a slow enough sink rate to avoid constant hang-ups, but the higher percentage of takes makes it worthwhile.

Search image requirements are fairly simple. There is the usual size, shape, and color to consider. Beyond that, I like a fly with prominent eyes and just a short tuft of marabou to imitate the broad set of gills at the posterior end. A medium olive Migrating Damselfly Nymph in size 10 gets the job done under most circumstances.

Bait-fish Succession Planning

If you spend as much time prowling along the shallow margins of lakes as I do, you come across some very interesting scenes.

Early June is when most aquatic ecosystems really start to kick into high gear. Bugs are emerging and fish are spawning – including many of the bait-fish that larger trout like to feed on. If you have the patience, and move slowly enough, you can catch a glimpse of the spawning behavior one of the more interesting minnows in Alberta, the fathead minnow.

At this time of year, male fathead minnows start excavating nesting sites under objects resting on the bottom in shallow water. This usually includes rotten logs or pieces of wood, but they have also been known to nest (sadly) under bits of trash discarded by uncaring anglers. Once the nest is excavated, the male spends a lot of time defending the spot from other male interlopers intent on taking advantage of all his hard work.

Male Fathead Minnow (Pimephales promelas) guarding his nesting site.

Males are easy to identify at this time of year – they are thicker through the body and more colorful than the females, and they have dark heads with white nuptial tubercles. The tubercles are mostly used to tend and clean the eggs but may also play a role in defence as the male fights off other males or kicks spawned out females out of the nest to make room for the next suitor. Yes, the male allows multiple females to spawn in the nest and then guards all the eggs till they hatch – the better the nesting site, the more females the male gets.

Female Fathead Minnow (Pimephales promelas) in courtship.

Here is a short video that shows a male on his nesting site, followed by a male and female nesting together before the male evicts the female in anticipation of another hook-up.

A Day in the Life of a Damselfly Nymph

Recent posts have been getting a little heavy on the theory and math so I thought I would lighten things up a little. If you like to fish damselfly hatches, we are right in the thick of things now.

But have you ever wondered what damselfly nymphs are doing during their year of development before emerging? If things are going according to plan, they are spending most of their time lying in wait for unsuspecting critters to munch on. What’s really cool is how they capture their prey. Damselfly nymphs have a modified lower jaw (labium) that they keep folded up between their legs. When a small organism blunders within range, the labium shoots out with lightning speed and the snags the critter, pulling it in to be chopped into bite sized pieces as it is eaten alive. Yeah, kind of like something out of a horror movie (the movie Alien comes to mind). Here is a short video that shows how it all goes down (without sound):

The Fly Selection Probability Curve

In last week’s post we learned that although most trout will behave in a certain way under certain circumstances, there will always be some fish that tend to buck the trend. In a similar way, given a certain set of conditions, individual trout will have a high probability of taking a fly that matches the preferred search image, but there is always a possibility that the same fish may stray from the norm and take a fly that is far from a perfect match. The fly selection probability curve is a good way to visualize this:

In this version, the fish is feeding in a somewhat opportunistic mode. There is a reasonable chance it will take most flies if presented properly (zones B & C), but there is a somewhat higher probability that it will grab flies that match the search images of bugs it has been recently feeding on (zone A). Here, most fly anglers will do well regardless of what fly pattern they fish, but for anglers who take the time to select a fly based on recent bug activity, they will tend to catch a few more fish. Unfortunately, trout feeding in the near opportunistic mode is not very common. A more likely scenario is the non-hatch feeding mode:

In the non-hatch mode, the fish is unlikely to grab a fly that does not resemble natural food items that it has been exposed to at some point in its development (zone C). Flies that have a more natural and realistic appearance will get a little more attention (zone B) but certainly not as much attention as flies that match food items that the fish has been recently feeding on (zone A). In a scenario like this, the fly you choose can mean the difference between catching a few fish and catching a few dozen fish. Even in the non-hatch mode you can see that knowing a little about bug biology and what the trout have been recently feeding on can be a significant advantage. That advantage becomes magnified when trout move to the somewhat selective mode:

In the somewhat selective mode, the fly selection probability curve starts to tighten up. Most flies are ignored (zones B & C) and the fish focuses its attention on what is actively hatching or what is most active and available (zone A). It will still sample other food items, but the tendency is definitely more focused on specific food items. The somewhat selective mode explains why a fish may be feeding on a hatch but suddenly decide to grab a fly that is not even close to what is hatching, or even your strike indicator. When trout are feeding in this mode, the average fly angler who has selected his fly based on what worked well last week, or what his favorite “go-to” pattern is, will have a tough day – while the fly angler who selects his fly pattern based on observation and what is active will do well.

During selective feeding, the fish is focused on a single life stage of a specific bug. Other flies are generally ignored but there is always a very small chance that the fish may grab something out of character. This is where you need to know exactly what bug is hatching and what stage they are feeding on if you want to have any chance at hooking a fish. Not quite hyper-selective at this point but for some fly anglers, the fish might as well be.

The fly selection probability curve explains a lot when it comes to why one fly is working well while others get little attention. It also explains why selective fish may sometimes do the unexpected. But this is not the end of our journey into how mathematics and probabilities can make you a better fly angler. Other layers to add to all of this are how individual fly patterns are aligned on the probability curve, how the curve can shift slightly as feeding conditions change, and how each individual fish may not be feeding on the same probability curve as other fish are.