Overtime Strategies for Extended Hatches

This is an article I wrote for the 2015 winter issue of Fly Fusion magazine. Fly Fusion is probably the most informative and artistic fly fishing magazine in print. I have not written for them lately but I do still have a paid subscription - something I would suggest to anyone with an interest in fly fishing.

   There are defining moments in every aquatic insect’s life-cycle when they are at their most vulnerable; nymphs caught in the drift and tumbling with the current, pupa rising to the surface to emerge, emergers in the surface film struggling to get free of their shuck, crippled adults that have failed to get free of their shuck, and fully emerged adults floating helplessly as they wait for their wings to dry so they can take flight. Both trout and fly angler alike are well aware of these key moments and are quick to capitalize on them. But there are other vulnerable stages in some aquatic insect life-cycles that many anglers are unaware of. If you know what to look for, these stages can provide additional opportunities for some exciting action by extending the hatch into extra innings. 

   One such stage for some aquatic insects is the act of oviposition (the process of laying eggs), or more specifically their behavior surrounding oviposition. Many adult aquatic insects lay their eggs using a rather haphazard approach, fluttering on or over the water and randomly dropping eggs individually or in clusters. The eggs settle to the bottom, or get trapped among aquatic vegetation, where they may reside for a few hours to a few weeks before hatching. Other aquatic insects have evolved egg laying strategies that take a more controlled approach, where eggs are carefully placed in locations that are relatively safe from hungry scavengers and where environmental conditions are most likely to result in successful development and hatching. Some of these strategies can involve laying eggs on overhanging vegetation – as the eggs hatch the juveniles simply drop into the water (a few species of caddisflies are known to do this), while others have adults hiding eggs in rotten logs using a modified ovipositor designed for drilling (this one is common in dragonflies). But many of these behaviors do not result in significant exposure to feeding trout. The egg laying behaviors that are of greatest interest to trout are those found in insect species where the adults physically crawl under water to deposit their eggs. 

Blobs of caddisfly eggs (Nemotaulius sp.) laid on a willow branch overhanging the water along a wetland.

   Underwater oviposition is known to occur in at least 50 species of Baetis mayflies (blue winged olives), along with over 100 species of caddisflies in the groups Brachycentridae (Grannoms), Hydropsychidae (Spotted Sedges), Rhyacophila (Green Sedges), and Glossosoma (Turtle-case makers). This behavior can also be seen in a few other groups including some species of damselflies, but the best fly fishing opportunities are generated by the afore mentioned groups found in rivers and streams. 

An adult Caddisfly (Brachycentrus sp.) making its way under water to lay eggs.

   This unique egg laying behavior presents the adults to feeding trout in ways not often considered by most fly anglers. Shortly after mating, adult females return to the stream and seek out riffles and runs with larger rocks or logs that are partly protruding from the water. As they climb down the side of the object and enter the water air is trapped on or under their wings. This air bubble will sustain them while they crawl to the underside of the object to deposit their eggs. Once a suitable location is found, eggs are laid in small clusters. Oviposition can take just a few minutes, or the adult may be submerged for an hour or more. When egg laying is complete the adults will either swim back to the surface and fly to nearby vegetation, or simply release their hold and drift with the current. It is these swimming or drifting adults that trout will often selectively feed on in the days following a hatch. 

An adult Baetis mayfly spinner penetrating the surface film as it makes its way under water to lay eggs. 

   Since much of the activity takes place below the surface this extension of the hatch often goes unnoticed. As usual, careful observation is required to determine when this opportunity presents itself. It helps to know what has been hatching (or was expected to hatch) in the day or two preceding your time on the water. If hatches included groups that are known for underwater oviposition, look on the undersides of rocks or logs to see if any egg masses are present (be sure to carefully replace objects exactly as you found them so the eggs can complete their development). Egg masses will be seen as either thin opaque sheets (usually Baetis), or small gelatinous masses (most common in caddisflies). Of course the presence of eggs alone will not be enough to tell you that egg laying is actively in progress – also look for adults entering the water, or clinging to the underside of those same objects. Most egg laying activity will take place in the morning hours although some species will lay eggs at any time throughout the day. Baetis will typically return to the water to lay eggs in the mid- to late afternoon on the day following the hatch. Active egg laying means that submerged adults will be present in the drift.

An adult Baetis mayfly spinner in the act of laying eggs on the underside of a log.

   Matching submerged adults drifting in the current can be a simple matter of selecting a wet fly pattern in the appropriate size and color. March Brown, Hare’s Ear, and Blue Quill wet flies in various sizes will cover just about any situation you may encounter. While these traditional wet fly patterns will certainly catch fish there are some modifications that will improve upon the fly’s ability to elicit takes. For larger caddisflies, wet fly patterns tied with more realistic mottled turkey feather wings tied tent style seem to get a little more attention (one could even trim most of the hackle off a Quill-wing Caddis dry fly and fish it wet). The addition of a few strands of silver Flashabou tied over the wings and a rib of fine silver wire will add that sparkle that the trapped air in the natural creates. For smaller caddisfly and Baetis patterns (#16-20) replace the wings entirely with two strands of fine silver tinsel. Because these flies are fished subsurface, matching the body color of the natural becomes more important than when fishing dry flies, where the fly is seen by trout as more of a silhouette. This is an especially important consideration in Baetis where the color of the spinner will be different than the blue winged olive color of the dun (Baetis spinner body colors are usually rusty-reddish-brown or bluish-gray). In all cases it is important to keep the size of the fly consistent with the size of the naturals – many anglers are reluctant to fish the smaller Baetis and caddis patterns, thinking them too small to be noticed by larger trout, but all trout have excellent vision and they will pick out tiny flies in the drift even in somewhat murky conditions. 

With egg laying complete, this Baetis mayfly spinner is destined to become a part of the drift.

   In fact many fly anglers are often reluctant to fish wet flies in general, thinking of them as old school, or no longer relevant in today’s world of modern fly gear and advanced fly tying techniques. We may have advanced in terms of high-tech gear and modern materials but fly anglers in the early half of the last century were well aware of underwater oviposition and did not hesitate to develop flies and tactics to imitate this behavior. These flies and techniques have for the most part fallen by the wayside in recent years, and for those who shun them, they are missing out on some spectacular fishing.

A selection of size 16 and 18 wet flies to imitate tiny Baetis spinners in the drift.

   Most of the time you will be fishing these patterns on a dead drift using standard nymphing techniques but there are a couple of special situations to watch for. Trout feeding on submerged adults will sometimes move into surprisingly shallow water; often lying in the skinny edge-water of a run, or nosing right up onto a riffle to feed. Here, shallow water nymphing with a tiny yarn indicator a few feet up the leader can be a very effective method. This situation is quite common on the Bow River and other tail water fisheries the day after a Baetis hatch. Also, if you notice that submerged caddisflies are not just drifting, but are actively swimming back to the surface, position your cast so that the tail end of you drift ends up in the seam or slightly deeper water where the fish are holding. Dead drift the fly off the riffle and then stop the drift and allow the fly to ride the current to the surface. Takes on the rising fly are often very aggressive so you want to avoid pointing your rod directly at the fly – keeping the rod at a slight angle will absorb the impact of the take. 

The day after a Baetis hatch - hundreds of egg patches on the underside of a small, single piece of in-stream wood, a testament to the numbers of Baetis spinners in the drift on the day following a hatch.

   Like a hatch, underwater egg laying events draw the attention of feeding trout by virtue of the number of bugs in the drift. The day or two following a hatch may see thousands of adults crawling underwater to deposit their eggs. The reliable Baetis hatches we have grown to anticipate, that never seem to result in fishable spinner falls, do produce spent spinners. It’s just that the spent spinners are drifting subsurface, wings swept back and looking more like wet flies. Caddisflies also return to the river en masse to complete their life cycle, quickly skittering underwater and laying eggs, before making the trip back to the terrestrial realm – a journey that is often interrupted by hungry trout. These feeding frenzies can produce some fantastic fishing at a time when hatches may be absent and most anglers are plying the water with searching patterns. For the observant fly angler this is all a virtually untapped resource. It’s almost like fishing a new and unknown hatch all-together. Consider it a hatch that has gone into overtime.

Caught in the Act

A scene that normally plays out underwater, I flipped this log to find an adult caddisfly laying its gelatinous mass of eggs on the underside.

Most fly fishers have likely seen adult caddiflies skittering on the surface as they drop their eggs and allow them to sink to the bottom. Some caddisflies, however, take a more active role in placing their eggs where they are more likely to be safe from scavengers and are more likely to hatch. This video shows adult caddisflies crawling underwater to lay their eggs on the underside of a rock. As they penetrate the surface, they trap air under their wings. This air bubble will sustain them for the 5-15 minutes needed to lay their eggs. Once they have laid their eggs they will sometimes crawl back to the surface and fly away, but many will simply release their hold and drift away with the current. At times, there will be thousands of adult caddisflies in the drift. This is why wet flies can be so effective in the days following a caddisfly hatch.

Still-water Caddisfly Life Cycle

If you are not familiar with the life-cycle of the Caddisfly, this photo series should give you a few fly tying ideas to go on. 

A typical caddisfly life-cycle.

Anabolia sp. (Limnephilidae) larva - one of the larger caddisfly larvae found in Alberta lakes.

A close-up of Anabolia with its cool color pattern. As the larva grows it simply
adds on to the front end of the case to make it bigger.

When ready to pupate, the larva seals off both ends of the case with a sieve-like plug made from silk extruded from glands near their mouth. This allow oxygenated water to circulate through the case as the transformation takes place.

A cut-away showing the pupa developing within the case. If oxygen levels drop too low, the pupa will undulate
(similar to cased chironomid larvae) to help circulate water through the case.

This pupa has almost completed its development. Note the well-developed legs, antennae, and wing-case all bunched together on the front half of the pupa.

When ready to emerge, the pupa cuts its way out of the case and makes its way to the surface to emerge. The trip to the surface can be a quick one - with the pupa buoyed by air trapped under the cuticle, undulations of the body, and sometimes a sculling swimming behavior using their hind legs.

Once at the surface, the cuticle of the pupa splits along the back and the adult caddisfly crawls out onto the surface. 

If you see lots of these on the surface you know that caddisflies have been or are emerging.

A typical adult caddisfly with its characteristic drab color and long antennae.

Caddisfly hatches are more common late in the evening but you will
sometimes see an emergence in the middle of the afternoon.

Mating takes place on shore-line vegetation with
egg laying taking place a short time later.

Cased Caddisflies in Alberta Lakes

Most of the larger species of caddisflies found in Alberta Lakes are in the families Limnephlidae and Phryganeidae. Both groups have some interesting case building behaviors, with each case designed to help the larva blend into its environment and avoid predators. Natural materials are scavenged and arranged in ways the enhance the camouflage provided by the case. Note that these photos were purposely taken with each caddisfly larva in an unnatural setting to make them more visible in the photos.

Anabolia bimaculata (Limnephilidae) with a case of longitudinally arranged pieces of dead grass. This species tends to prowl along the edges of lakes and ponds where emergent sedges and grasses predominate.

Sometimes case construction is specific to a species and sometimes it will vary. This Anabolia bimaculata larva was collected from a pond lined with spruce trees - case construction using dead spruce needles made more sense than lengths of dead grass.

Members of the family Phryganeidae tend to arrange the case material in a spiral pattern. Fabria inornata uses short lengths of plant stems arranged in a lose spiral (better seen in the next photo).

Philarctus quaeris (Limnephilidae) larva have a habit of picking up seeds or snail shells to assemble their case. Some cases may be built entirely of small snail shells. This larva has decide to use Daphnia egg cases (ephippia) that have settled to the bottom.

Caddisflies are closely related to the Lepidoptera (moths and butterflies). And like caterpillars, caddisfly larvae can produce silk from glands near their mouth, which they use to line their case and hold it all together.

If you have the patience to fish a cased caddisfly pattern in lakes, it can produce some nice fish. The trick is to keep your fly on or near the bottom, and move it ever so slowly. Fly patterns are usually quite simple - I have seen woolly worms with the palmered hackle trimmed short work well. One of my favorite patterns imitates the larva of Phryganea cinerea (Phryganeidae) with its case of spirally arranged short pieces of grass.

A Bug in the Hand Beats Two in the Bush

When it comes to fly design it helps to have a good idea of what the actual bugs looks like. Being able to take your time to really assess the shape and color of an aquatic insect in its natural environment gives the fly tier a big advantage. But most bugs rarely sit still for very long, and aquatic larvae removed from the water tend to look darker and more clumpy than they actually are. A bug in the hand may beat two in the bush, but a good photo trumps them both.

A Dragonfly darner nymph (Aeshna sp.). You can tell that this one has
more growing to do by the small wing-pads.

A Dragonfly sprawler nymph (Cordulia shurtleffii)
This one has fully developed wing-pads and
is ready to emerge.

This is what Cordulia shurtleffii looks like as an adult. Its metallic-green eyes earned this
species the common name of American Emerald.

An adult Variable Darner (Aeshna interupta) laying eggs in a rotten log.

Weaving a Deception

I have been working on some woven-body dragonfly nymphs this week. Reg Denny taught me how to do this many years ago. It makes for a very realistic looking abdomen on any dragonfly pattern - including the longer and darker darner type patterns. The first one imitates the stubby sprawler type dragonflies during the spring pre-hatch migration.

A short, stubby woven dragonfly sprawler imitation.

I'm one of those tiers who doesn't always follow a recipe - most of the time I just wing it by memory. This dragonfly patterns start with two lengths of lead free wire laid along either side of the hook shank - this gives the body a somewhat wide and flat appearance like the naturals. Leave enough head space to attach the eyes. The hook for sprawlers is usually a 2XL #8 to keep the fly short and stout. I do up a bunch of the hooks with wire ahead of time and make sure the wire is tied down well and add a little head cement to keep it all in place. 

The weave is an alternating over & under using two strands of yarn in slightly different colors - one color for the dorsal side and one for the ventral. If I haven't tied them for a while it seems I need to re-learn the technique and practice a bit before I start on the actual flies. There are a few YouTube videos out there that cover the woven body technique better than I can explain it. My preference is for soft legs using tufts of dyed mallard flank - this allows the legs to sweep back when using a darting retrieve. The thorax is not woven - just a simple wrap spaced to keep the legs in position. I used bead-chain for the eyes on these ones but I also use glass beads on heavy monofilament with the ends melted sometimes. The wing case is traditional turkey quill. Colors for the sprawls range from light olive to olive-brown.

A longer-bodied woven dragonfly darner imitation.

The darner pattern is tied on a 4XL #8 hook using two strands of brown yarn in slightly different shades. Fine "nymph" rubber legs have a good action on the larger fly. Make sure to make the abdomen longer relative to the thorax  for this one.

Emerging Dragonflies - a video & photo essay

Dragonfly hatches are a big draw for larger trout. But many fly anglers fail to notice when a hatch is in progress, or realize that the nymphs will start migrating towards shore in the days leading up to a hatch. If you don't know what to look for, or how to properly work your fly, you are missing out on one of the best hatches of the spring trout season. This short photo essay will bring you up to speed.

Knowing what happens under water during a pre-hatch migration gives the fly angler a big advantage when fishing a dragonfly hatch. This older, low resolution footage shows how the nymphs make their way to the shoreline, using a combination of crawling and swimming. 

A dragonfly nymph (Epitheca spinigera) fresh out of the water and
making its way up shoreline vegetation in search of an emergence site.

It is not easy wiggling out of the nymphal exoskeleton - gravity is used to aid in the process.

Once free, it can take up to an hour for the body and wings to fully extend.

Wings and body starting to take shape.

Almost ready to fly - the wings just need to harden a little more and extend out to the sides.

A close-up of the large compound eyes.

As soon as the adults can fly, they make their way to nearby trees or bushes to
rest and allow for any final hardening to take place.
(Spiny Baskettail - Epitheca spinigera)

Adult dragonflies are very territorial - but during a heavy hatch they will congregate on lake-side shrubbery for a bit as they soak up some sun and complete the hardening of their exoskeleton.
There are 18 adult dragonflies in this photo.

Cased Chironomid Larva Respiration and Movement

In my last post I talked about how chironomid larva that build tubes or cases use body undulations to maximize oxygen intake. This behavior is used by not just chironomid larva living in the hypoxic zone on the bottom, but also by chironomid and caddisfly larvae that build portable cases. They do this when  oxygen levels are low or they need to refresh the stagnant water in their case. 

This video shows how a chironomid (midge) larva undulates in its tube to circulate water to aid in respiration. This particular genus (Cricotopus) builds a portable tube, while other chironomid groups construct more extensive stationary tubes in the mud. The video also shows how chironomid larvae that build portable tubes move by reaching out and pulling themselves forward. In the final scene, the midge larva gets caught on a strand of vegetation and uses a small pebble to leverage itself up and over.

Midges, Hypoxia, and Hemoglobin

Note: This post may not make you a better fly angler, but it will make you a more knowledgeable one.

One of the pitfalls of living in the mud at the bottom of a wetland or lake is that there can be very little oxygen present. Oxygen diffuses much slower through water than it does in air, and even slower through the mud barrier where the decay of organic debris further depletes what little oxygen is present. Under these low oxygen (hypoxic) conditions, life is limited to organisms that are adapted to survive in these extreme environments.

Some chironomid (midge) larvae have adapted to exploit these hypoxic habitats through the use of hemoglobin. Hemoglobin is the protein component of red blood cells that circulates through our body and is responsible for delivering oxygen to tissues. It also gives blood cells their red color. Hemoglobin has a high affinity for binding to oxygen molecules making it the perfect oxygen delivery system where constant circulation is present.

There are several groups of chironomids that utilize hemoglobin to aid in survival under hypoxic conditions, but the two most common ones are in the genera Chironomus and Glyptotendipes (commonly known as blood-worms). The high concentration of hemoglobin in their body fluids (hemolymph) gives them their distinctive red color. While there can be some passive movement of the hemolymph through their body, it does not circulate like our blood does. Instead, it simply surrounds the organs and tissues.

Note that more evolved and larger insects may have an open circulatory system, but even here, the hemolymph does not circulate very fast and oxygen uptake and transport is primarily via the air-filled tracheal system.

A Chironomus larva with its typical bright red coloration.

So how exactly does the hemoglobin help blood-worms survive in hypoxic and sometimes anoxic (no oxygen) habitats? If there is little to no oxygen to bind to the hemoglobin to begin with, what advantage can there be?

Most blood-worms build extensive tube networks in and on the mud by tunneling and reinforcing the tube walls with mucus. These tubes provide protection from predators but they also allow the midge larva to circulate water through the tube by undulating its body within the tube (caddisflies also do this). This action pulls water with a slightly higher oxygen content through the tube. In some cases, the midge larva will also elevate the tube opening, even by just a few millimeters, to place the tube intake above the low oxygen boundary layer at the water-mud interface. Of course under low oxygen conditions they can't spend all of their time undulating to gain oxygen, they need to feed too. That is where the hemoglobin comes in - as the larva undulates and circulates water through the tube, the hemoglobin picks up oxygen (much like recharging a battery) - then when the larva stops undulating to feed; oxygen bound to the hemoglobin diffuses into the tissue to sustain metabolic activity. Once the hemolymph oxygen level falls to a critical point, the midge larva will once again take up its ventilation position in the tube to recharge its oxygen supply.

A Glyptotendipes larva out of its tube and foraging for food.

This species of Glyptotendipes builds its tube out of mucus and bits of organic debris. Here, it has chosen to establish its tube up in the vegetation rather than on the bottom. Note the red and bright green coloration – this is the impetus for the midge fly pattern known as a Christmas Chironomid.

Deep Water Chironomids Feeding and Swimming

The most productive part of a lake is the littoral zone where the penetration of sunlight allows for the rapid growth of aquatic plants and periphyton. Here is where you will find the highest level of bug biodiversity, including a hundred or so species of chironomid (midge) larvae. But this doesn’t mean that the deep water areas of a lake are devoid of life. In fact, there are some genera of chironomids (Chironomus and Glyptotendipes) that specialize in hiding in the low-oxygen layer of mud during the day, and coming out at night to forage for whatever tiny organic particles of food they can find.

While the diversity of midge larvae living in the mud 15 to over 60 feet down is low, the biomass (number of organisms) can be very high. This is because, with very little oxygen to support life in the hypoxic mud layer, there is little competition for food. The midge species that can live in this layer of mud are able to monopolize on the resources, much like the mono-culture of a wheat field.

One would think that there is not much to eat way down there in the depths, but as organisms living in the upper layers of the limnetic zone die (algae, zooplankton and cyanobacteria), or organic bits get blown onto the lake’s surface (pollen and organic dust), they settle to the bottom. This translates into a continuous shower of food for most of the year, even in the deeper parts of a lake.

This video shows midge larvae foraging for food on the lake bottom and their swimming behavior when they decide to move longer distances to find better foraging sites. Their swimming behavior is unique, and somewhat frantic, and not easily imitated by the fly angler.

The Curved vs. Straight Debate

 Every fly tier who frequently fishes scud patterns has an opinion on this; should scud patterns be tied on straight shank or curved shank hooks?

Scuds at rest tend to have a very pronounced curve. This observation has resulted in a large number of scud fly patterns being tied on radically curved hooks – what used to be called scud or shrimp hooks.

A Scud (Gammarus lacustris) in a typical resting position.

But scud patterns are generally fished with some sort of retrieve – imitating a scud in motion. Scuds actively swimming tend to straighten out their bodies to become more streamlined. In this situation, a scud pattern tied on a straight hook makes more sense. But do swimming scuds really hold their body as arrow-straight as a straight shanked hook? The answer is a definite no. While the degree to which a scud holds its body straight while swimming varies, the telson and last few body segments generally curve down a little, and there is usually a slight curve along the back (seen in the photos below of scuds actively swimming).

So what is the best hook for a scud pattern? You certainly want to avoid the radically curved shrimp hooks. But I am not sure it matters much if a scud pattern is tied on a straight or slightly curved hook. I have done both over the years and find them equally successful. When I do use a straight hook, I tie the body slightly onto the bend of the hook to simulate the turned down telson and last few segments. My preference now is to tie scud patterns on slightly curved pupa hooks. I consider it a compromise when fishing the fly with short slow strips followed by a pause - straight enough to fool the fish on the strip and also curved enough to look natural on the pause.

Zombies, Ghosts, and Multi-legged Hosts

Here is an article I wrote for the 2016 fall issue of Fly Fusion magazine. Fly Fusion is probably the most informative and artistic fly fishing magazine in print. I have not written for them lately but I do still have a subscription - something I would suggest to anyone with an interest in fly fishing. This article was written with Halloween in mind. I hope you enjoy it.

   It was clear from the get-go that something strange was afoot. Was this really happening? Could I be dreaming? This just didn’t make any sense...

  Strange indeed; knowing what was normal, what was expected, I was having a difficult time reconciling what I was bearing witness to. The funny thing was that I was not the least bit afraid. The fact that it was already mid-morning may have had something to do with my mindset leaning more to simple curiosity than to morbid misgivings, but with the seemingly sinister events unfolding before me, I simply had to know more.

   I cautiously waded a little closer to a small patch of weed growth in shallow water. Three brook trout circled above it like sharks around a wounded mackerel, their dark olive backs and white fin edges standing out in stark relief against the verdant background. Every so often a pale colored scud could be seen spiraling lazily towards the surface, only to be quickly picked off by the nearest fish. Suicide, I thought. What would possess these scuds to leave the safety of the weeds like that, in broad daylight no less? I knew they didn’t hatch into a winged terrestrial form like aquatic insects do. They really had no reason to head for the surface. It would be several more years before I would learn that those scuds were in fact possessed – infected by an insidious parasite and no longer in control of their own bodies. Yes my friend, zombies really do exist.

    Scuds normally lead peaceful and unassuming lives. Much of their time is spent simply scavenging among aquatic vegetation and along the bottom for bits of dead plant and animal material to eat. As scuds grow they periodically shed their plate-like exoskeleton through a process called molting. Once maturity is reached they will form mating pairs, swimming in tandem as they propagate the next generation. The eggs develop in a pouch on the female called a marsupium and the young are born looking just like their parents, only smaller. There is no larval stage or metamorphosis. The average lifespan for a scud is about one year and depending on conditions they may produce from one to several generations in a summer. That effectively sums up the serene cycle of life for a scud.

   Of course being a perfectly packaged piece of protein means that scuds are a sought after food source. Many birds and fish will go out of their way to chase down any scuds that have been careless enough to get caught out in the open. Unfortunately for the scuds they have little to offer in the way of escape strategies. Even with all of those legs scuds are relatively poor swimmers – often swimming in slow spirals, sometimes on their side, and sometimes even upside down. And with their Mr. Magoo-like eyesight they are constantly bumping into things. With antics like that it’s no wonder that scuds make up a significant part of a trout’s diet wherever the two coexist.

   For the slow moving scud staying hidden is really the only available defense. Cryptic coloration that matches their habitat is a good start. Being found in the weeds, or prowling in the muck, scuds tend to be shades of green, olive, or olive-brown. They can’t quite change their color with chameleon-like adeptness but scuds can adapt over time to changes in the background color of their environment using subtle shifts in shading. Staying out of sight is another strategy that keeps scuds safe. Scuds are normally photo-phobic – meaning they tend to avoid the light, preferring instead to frequent dark and shadowy places. During the day they can be found tucked deep in the vegetation or hiding under logs and debris. It is during periods of low light that scuds tend to become more active, sometimes venturing out into the open (even cloudy days can be enough to coax them out of hiding). Regardless of the lighting conditions, a scud’s instinct is to always dive for cover when threatened.

   Knowing all of this it’s easy to understand why I was so intrigued upon witnessing the exact opposite behavior on my earlier fishing expedition. And here is where our story takes an unsettling twist, one that fly anglers can use to their advantage.

   Parasitism is not uncommon in the invertebrate world. Small bugs often serve as intermediate hosts to even smaller bugs looking for a free lunch. One of the most common scud parasites is a tiny creature known as a spiny-headed worm (Acanthocephala). Scuds become infected with spiny-headed worms when the scud ingests eggs that are mixed in with the organic debris they are feeding on. The eggs hatch in the gut and the “larvae” (acanthella) burrow through the scud’s intestinal wall and into the body cavity where they continue their development. The acanthella develop into sub-adults known as cystacanths – at this stage they are fully formed worms but lack reproductive structures. These tiny worms migrate to a central region, usually along the back of the scud, where they encyst and wait for a definitive host to eat the scud. Depending on the parasite species the definitive host is usually a fish or bird. Final development and reproduction takes place in the gut of the definitive host and the parasite eggs are released when the host animal defecates. Scuds feed on the feces and the cycle continues.

   Once the first phase of development has been completed within the intermediate host, the life of the spiny-headed worm (or continuation thereof) hangs in the balance. If the scud is not eaten by a definitive host the scud will eventually die of old age, and all will be lost for the progeny of the spiny-headed worm. Not leaving things up to chance, the ancient acanthocephalan has evolved a few clever ways to beat the odds.  By manipulating the hormonal system of the scud, acanthocephalans are able to control the scud’s behavior. Instead of avoiding the light, infected scuds in the cystacanth stage become attracted to light and are more active during the day. Infected scuds will also develop a tendency to swim towards any disturbance in the water (such as feeding fish or birds) rather than head for cover. Acanthocephalans also make infected scuds stand out more by altering their coloration. Cystacanths are typically bright orange or yellow – they can easily be seen in infected scuds as a small spot along the back. But more importantly, the cryptic coloration of a normal scud often becomes muted and takes on a pale, bluish-grey, ghost-like hue as the infection progresses (freshly molted scuds will also exhibit the same bluish-grey color but they will lack the orange spot and will have an even greater tendency to stay hidden). These two factors combine to make an infected scud stand out like a neon sign. No guarantee that the scud will be consumed by the correct definitive host, but quite likely that some sort of fish or fowl will gobble it up.

A scud (Gammarus lacustris) in the early stages of infection, before the natural color has faded.

   In fact, it has been shown experimentally that fish will preferentially feed on infected scuds even when both infected and uninfected scuds are readily available in equal numbers. This should come as no surprise. Most fish rely heavily on search images to direct their feeding effort. If they are seeing and successfully capturing more pale scuds than normal ones, the search image for the pale scuds becomes a stronger feeding trigger.

A scud (Gammarus lacustris) with its natural color faded to a bluish-grey due to the infection.

   If infected scuds stand out more, and are eaten more frequently than their uninfected counterparts, doesn’t it make sense to use a fly pattern that specifically mimics an infected scud? Once I had all of the facts, it didn’t take me long to answer that question for myself. Several years ago I tied a bunch of simple baggie shrimp on #10-12 hooks, some using pale blue dun rabbit dubbing and some with the traditional olive dubbing. I added a small orange dot to the back of some using a marker pen but left a few of each color unmarked. After testing these patterns on several local trout lakes the olive scud with the orange spot turned out to be slightly more effective than the plain olive pattern, while the blue dun scud out fished both olive patterns by a wide margin. The presence or absence of the orange spot on the blue dun scuds didn’t seem to matter. This experiment was all very unscientific of course, but the results were enough to convince me to permanently add the Blue Dun Scud to my repertoire.

   In the intervening years I have come across fly tying recipes for other scud patterns in blue, light grey, and pearl – all good candidates for an infected scud imitation. I prefer the simplicity of the old-school blue dun baggie shrimp but some of the newer synthetic fly tying materials are certainly worth experimenting with. The goal is to get that translucent bluish-grey look of a zombie scud. Of course I still carry olive scuds for those times when olive is what the fish seem to prefer, but more often than not it is the blue dun scud that is in tatters on the end of my line.