Putting a Name to a Face

On my last fishing trip to the North Ram River I collected two adult stoneflies (Plecoptera: Capniidae) from the snow along the river (see blog post from 25-Oct-2011). I finally had a chance to identify the specimens and although they are both females, their characteristics are consistent with Bolshecapnia gregsoni. While knowing the exact species name will not add anything new to how one would fish this minor hatch (most members of the family Capniidae are similar in appearance and behavior), it does potentially add another piece to the entomological puzzle.

Capniidae – tentatively identified as Bolshecapnia gregsoni

According to Stewart and Oswood (2006) this particular species is endemic to British Columbia, having only been collected from a small handful of locations across southern BC. The nymphs have not yet been described and virtually nothing is known of the biology of this species, although there are hints that this species may be one of the few stoneflies that inhabit lakes. This leaves me with a few questions to be worked out before I can definitively say that the know range of this species can be extended to Alberta: It is remotely possible that the few adults I saw could have been carried into the Ram River drainage by a freak weather event that carried them over the Rockies. There is also a chance that these specimens may not have hatched from the river but instead may have come from a still water habitat nearby. And finally there is the possibility that the specimens are not Bolshecapnia gregsoni – while the female characteristics can be used for species identification in some stoneflies (including Bolshecapnia) it is the male genitalia that usually provide definitive proof.

 All of this is part of why I find the study of aquatic entomology so interesting. Given all that we know, we are still only scratching the surface – there are many species yet to be discovered and of those that are known, their full range and biology are often shrouded in mystery. There is ample opportunity for professional and amateur entomologists alike to add small bits of information to our collective knowledge; another piece of the puzzle – a puzzle so immense and complex that it will never really be finished. It is this tantalizing hint of a mystery to be solved that excites me. I will be out on the North Ram next fall with the goal of collecting more Bolshecapnia adults (and hopefully some males); and with any luck, I will add another piece to the entomological puzzle.

Stewart, K. W. and M. S. Oswood. 2006. The Stoneflies (Plecoptera) of Alaska and Western Canada. The Caddis Press, Columbus, Ohio. 325pp.

Bug Bits

The essence of fly-fishing can be distilled down this: Enticing a fish into believing that what we are offering them is something that they would normally eat, or at least something worth a quick taste. Ours is a sport of imitation; of mimicking, as near as possible, the appearance and actions of the various organisms that swim, crawl and wiggle in the trout’s world. Fly fishing is also a sport of deception – bits of fur and feathers tied to a hook that upon close inspection may at best have a rough resemblance to the food item we are trying to duplicate. Add to this the often selective nature of trout and the seasonal availability of most invertebrates and it stands to reason that knowledge of the habits and biology of the various food items that trout feed on is an important component of the fly angler’s tool box.

Baetis sp. nymph - an example of a simple yet effective fly pattern

The bulk of a trout’s diet is made up of aquatic invertebrates. Trout also eat the occasional small fish, or even the odd mouse or frog – but this component represents less than 1% of the average trout’s food intake. Granted, larger fish are more inclined to target larger food items when available, but even they do not discount the nutritional value in eating sufficient numbers of small bugs (a 200 pound black bear taking the time to tear open a rotten log for a mouthful of insect larvae is another prime example of this).  Aquatic invertebrates are an important link in most aquatic ecosystems; converting energy from green plants, organic debris, and microorganisms into nutrient dense biomass that larger animals are able to capitalize on.  This link not only allows trout to thrive in lakes and streams, it forms the foundation of the sport of fly fishing.

Claassenia sabulosa nymph (Plecoptera) - a common stonefly on some trout streams

So it is with the subject of aquatic invertebrates that our journey into aquatic “entomology” begins. I have put entomology in quotes because in the strict sense, entomology is the study of insects. But in this section (Bug Bits) I will be covering more than just insects – I will also touch on some of the non-insect organisms that trout feed on. My goal is to provide background information over the coming months so that when we begin to discuss current hatches over the summer, we can cover the subject in more advanced terms.

Through the Eyes of a Trout

Trout Vision Part 2

If you think back to high school biology class you can probably recall the basic structure and function of the human eye: Light passes through the clear front covering of the eye (the cornea), and enters the eye through the dark central portion (the pupil). The diameter of the pupil, and consequently the amount of light entering the eye, is controlled by expanding or contracting the colored part of the eye (the iris). Behind the pupil is a clear lens that serves to focus the light onto the retina at the back of the eyeball. The retina contains a network of sensory cells that can perceive either low light information in black & white (rods), or color and fine detail (cones). To focus the image on the retina, the lens shape can be changed by contracting small muscles attached to the lens.

Human Eye



A trout’s eye is similar in structure to ours, but given that fish have evolved over millions of years to survive in a medium vastly different than air, there are bound to be some differences in the way a trout’s eye is structured, and in the way it functions.

Unlike the human eye, a trout’s eye does not have an adjustable iris; the pupil is fixed in in the open position. Combine this with the fact that fish do not have eyelids, and it becomes apparent that trout must have other mechanisms to regulate the amount of light reaching the sensory cells of the retina. Of greatest concern are the rods which are very sensitive and easily damaged by bright light. The cones on the other hand require bright light in order to see color and fine detail. In response to this, two mechanisms have evolved that allow trout to see effectively under changing lighting conditions: In bright light there is an increase in melanin pigments in the upper layer of the retina that shield the rods from bright light during the day (much like wearing dark sunglasses). In addition, the rods physically migrate deeper into the retinal layers where they are exposed to less light. During periods of low light, and at night, the process is reversed; the amount of melanin pigment decreases allowing more light to reach the sensory cells, while the rods migrate closer to the surface of the retina.



These mechanisms are so effective at regulating light that it allows many fish (including trout) to have a higher concentration of rods in their retina – the result is that trout have superior low light and night vision compared to us. The down side is that these mechanisms are much slower than the rapid response we get from adjusting the diameter of our pupils (studies show that the response of melanin, and rod migration, takes from 10 to as much as 70 minutes to adjust to rapid shifts in light intensity). This is not usually a problem since changes in light intensity at dusk and dawn are gradual enough that trout do not experience any visual deficits. But on dull cloudy days when the sun suddenly breaks through the clouds, feeding trout will sometimes move deeper in the water column or seek out shady areas until their eyes adjust, or the sun is once again obscured. Bright sunlight also will also encourage fish to seek cover due to the increased risk of predation.

Trout spend a considerable amount of time watching for predators, but they also spend as much, if not more time looking for something to eat. Much of what a trout eats is on the small side and consequently they need to be able to see and focus on tiny food items at very close range. A trout’s lens and focusing system allows them to do just that. Unlike our relatively flat and flexible lens, a trout’s lens is almost spherical, and rigid. The spherical shape is made necessary by the medium in which trout live – as light passes from air into a more dense medium like the human eye, the light bends slightly (as a result of refraction) thus reducing the amount of bending the lens needs to do to focus the image. Since trout live in an already dense medium, there is virtually no refraction as light enters their eyeball; trout therefore need a more powerful (i.e. spherical) lens to focus the image. With the lens being spherical and not easily deformed, the muscle attached to the lens focuses the image by moving the lens back and forth rather than by deforming the lens. The end result of all of this is that trout are able to obtain precise focus on a bug a mere inch or two from their nose (to fully appreciate this, hold you finger one inch in front of your nose and try focusing on your fingertip).

Trout Eye

We have already touched on the two basic types of sensory cells in the trout’s retina. The rods are designed to see under poor lighting conditions but can only perceive in black and white (really shades of gray). We know that the rods are used primarily for vision at dusk, dawn and at night, and since the rods can only perceive in shades of gray, fly color should become slightly less important when fishing a hatch at these times. I say slightly less important because it is still necessary to match whatever shade of gray that the fish perceives; and the easiest way to do this is to match the color of the fly to the actual insect. Despite the lack of color vision in low light, trout have excellent visual acuity in low light and have no problems picking out hatching insects.

So how important is fly color during the day? Like us, trout have several types of color sensitive cones in their retina, each type responding to a different wavelength (i.e. color) of visible light. The colors perceived by the different cone types are combined to produce a full color image in the same way that red, green and blue combine on your LCD computer monitor. Numerous experiments have shown that trout can not only differentiate between different colors, they can distinguish between different shades of the same color. While there is no way of knowing exactly how colors appear to a trout, they do have excellent color vision.

In addition, juvenile trout have specialized cones that give them the ability to see ultraviolet light (UV), a frequency just beyond what we consider to be the visible spectrum. This ability to see UV light is thought to help juvenile trout see the often transparent zooplankton that forms the bulk of their diet. But studies have shown that as trout mature and their prey source changes, their ability to see UV light is greatly diminished. Despite this, mature trout have one more trick to up their visual acuity; they have the ability to perceive polarized light. Light becomes partially polarized as it reflects off a surface (this is why we wear polarized sunglasses to see through the glare off the water’s surface), or as light refracts as it passes into water. The ability to perceive polarized light is thought to increases the contrast between similar earth tone shades in the subdued lighting of the underwater world.

What is clear from all of the research into trout vision, and from the experiences of fly anglers worldwide is that trout have excellent vision. To be able to single out an insect riding the surface tension in fast moving water, and be able to zero in on it and delicately pluck it from the surface, is nothing short of amazing. In some ways it is a wonder we are able to catch any fish at all – but as we delve deeper into other aspects of trout vision you will see that other factors like the physics of light in water, and the way a trout’s brain processes visual information work in our favor.

How a Trout Sees

Part 1: Introduction

Part of being a successful fly angler lies in being observant; in being visually aware of our surroundings: Seeing swallows swooping over the water is a sure sign of a hatch in progress. Spotting insect activity on the stream bank, lake shore, or water’s surface can provide clues as to what the trout may be feeding on. Catching a glimpse of expanding ripples on the water’s surface is a telltale sign of where fish are feeding. We can even infer where fish are most likely to be located by taking note of things like topography, weed growth or current patterns. And even though most of what takes place below the water’s surface remains shrouded in mystery, distorted or hidden from view by the rippling air/water interface, fly fishing is a visual game.

Things are not much different for the trout. Being observant can mean the difference between differentiating between edible morsels in the drift, and the mix of debris and artificial flies that are a waste of time and energy; between successfully securing their next meal, or allowing valuable calories to pass on by; between maximizing growth, egg production, and brawn on the spawning beds, and failing to compete successfully in the world of trout courtship; and between surviving to pass on their genetic material to the next generation, or falling prey to some unseen predator. Vision also plays a key role in the underwater world of the trout.

For the casual angler, understanding how and what a trout sees is an often neglected piece of the fly fishing puzzle. But with so much ridding on the trout’s sense of vision, it should really be a consideration in everything we do; from fly and leader design, to how we approach and cast to feeding fish. In this series I will shed some light on trout vision, and how we can apply this knowledge on the water.

Sometimes it helps to imagine how things appear to the trout:
The view upstream in a slow run on Stauffer Creek.
Note the Corixid scooting to the surface to grab some air

One More Cast

I knew I wanted to make one more trip to the North Ram before the season closes next weekend; and with the forecast calling for reasonable daytime highs (and overnight lows) this weekend seemed to be the best opportunity.  I usually try to avoid trips where the time spent fishing is less than the time spent driving, but in late October, with most of the action taking place between noon and sunset, this is usually unavoidable. 

As I left Edmonton at 6:00 am, snow tires were the furthest thing from my mind. I knew it had snowed in the foothills the day before and I was expecting to see the odd skiff in the trees, or shaded areas. But by the time I turned onto the trunk road, snow tires seemed like a no brainer; slow and steady got me to the parking area without mishap, but mountain biking the trail into the lower reach was out of the question.

Mid-morning was a cool -2 Celsius. The mist and cloud clinging to the hill tops, along with the contrast between the dark water and the white blanket of snow covering the banks, made for a very surreal scene. The tranquility alone was worth the trip.

By noon, the sun was breaking through the clouds, my rod guides were no longer icing up, and a few Baetis duns could be seen fluttering off the glass clear water. I did see a few fish rise, but the hatch was too sporadic to consider fishing top water. Most fish were picked up by casting a Baetis nymph to visible fish in their winter holding lies.

There were a few other bugs active along the banks. Winter stoneflies (Capniidae) could be seen crawling on the snow, along with Ameletus duns clinging to the rocks on the bank. Although both of these bugs emerge by crawling out along the bank and therefore do not cause any surface feeding activity, the migrating nymphs will sometimes get caught in the drift.

The day turned out to be a perfect way to close out the season on the east slope streams.