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.
