December 10, 2021
By Staton Klein
This article was originally titled "Cannibal Trout" in the April/May 2013 issue of Fly Fisherman magazine.
The fish we most love to catch eat MEAT. By meat, I mean other fish. Fish that feed on other fish are technically referred to as piscivores. If you’ve ever thrown a Rapala at it, or jerked a giant streamer past it, that fish is a piscivore. But that’s only part of the whole meat-eating story. In heavily populated trout streams, a great deal of the piscivorous feeding behavior is bass-on-bass, brook-on-brook, and brown-on-brown violence . . . in other words, it’s cannibalism.
Some people might view cannibalism as evolution’s little sadistic twist, but it’s not. Cannibalistic interactions have beneficial consequences at both the individual and population levels.
Before we can understand how cannibalism is important to individual fish and to the population as a whole, we should take into consideration a few things. Typically, piscivores undergo several niche shifts through their lives. After absorbing their nutrient-rich yolk sacks, many freshwater fish feed predominantly on zooplankton (microscopic aquatic animals) before shifting to larger prey such as aquatic insects, worms, and crustaceans. In the quest for additional calories, the largest surviving individuals become more piscivorous. And the larger they become, the more reliant they become on smaller fish as food sources.
So how does this relate to cannibalism? From a scientific standpoint, cannibalism can reduce the temporal variation in population dynamics for sized-structure populations. In plainer language, it decreases the likelihood of boom-bust population cycles, or of most of the trout in the river falling into one size class. Cannibalistic feeding—in which the smallest size classes (young of year) are the prey—reduces competition for zooplankton and age-specific habitat. The survival probability of the remaining individuals increases because there are more resources per individual, thereby ensuring annual recruitment of new individuals into the population. Cannibalism on intermediate size classes can increase the per capita availability of invertebrates, smaller prey fishes, and open previously occupied habitat. And all of this serves to support the largest adult spawning fish.
Some waterways are more fertile than others, but all of them have a finite amount of resources. From an evolutionary standpoint, the best survival strategy is to begin eating your brother, sister, cousin, and anything else you can wrap your lips around as soon as possible.
This adaptive behavior not only provides a protein- and fat-rich meal, it also reduces the likelihood of future competition over food, habitat, and mates.
The frequency at which a sculpin, largemouth bass, or an old hook-jawed brown will engage in piscivorous or cannibalistic behaviors depends on a number of complex ecological interactions. To gain a better understanding of cannibalism and piscivory, one might ask: What is the predator species? What is the predator’s size versus its prey size? How easy is the prey to capture? Does the predator need to manipulate its prey because of its large size, bony fins, or rough scales? How are the predators and their prey distributed? Do they occur uniformly such that every square meter of riffle, or every square yard of aquatic vegetation holds one individual? Are they randomly distributed throughout the waterway, or are they spatially clustered, with high densities of individuals occupying species-specific/age-specific habitat types?
You could answer all these questions and write a master’s thesis in the process, but based on the many years I’ve spent on and under the water, it seems there are two basic, overriding rules that are always in effect—and these are the important trends fly fishers should be aware of. First, high densities of prey attract predators. This may occur in the form of striped bass working a ball of menhaden, smallmouth rising to White Flies, or large trout making fish tacos out of juvenile trout. Second, prey that is easy to capture or reacts oddly to the presence of a predator will get attacked.
Trying to understand the ecological conditions that influence these trends can pay off. For example, last fall a tropical storm system dumped a lot of rain in western Maryland. This unusual blessing contributed to the Savage River Reservoir turning over early, decreased discharge temperatures from the dam, and increased discharge volume from 50 cfs to 200 cfs.
Unlike most years, the river ran around 200 cubic feet per second during the spawn. Because of the higher discharge, the sections of the river typically used by spawning fish changed. One area in particular was more highly used than in years past. If you were to look at it, you wouldn’t think anything big calls this home. Except for the one short run approximately 150 yards upstream and a long complex of skinny riffle water just as far downstream, there are no prime lies, obvious feeding lanes, logjams, honey holes, bat caves, or any other physical habitat we’ve all been told are centerpieces in pig-pen decorum.
I decided to fish through this section of river one day in late September, looking for prespawn trout. That day was the first time in months I saw “Jaws.” I saw him charge my double articulated 6-inch Homewrecker pattern with no other intent than to kill. Did I put him in the net? Hell no. I was too quick on the trigger.
That was the last time I harassed Jaws that season but I did come back on several occasions to watch him do his thing. He was like a sailor off a ship the way he swam back and forth between four different females. I’m sure he probably ate a few other trout during this time, but except for the one time I watched him T-bone the head of another adult male, practically tearing its face off, I never witnessed successful cannibalism. I guess they all knew he was the baddest trout on the block, and that was why there weren’t any other trout around.
As fall faded into winter and winter into spring, Mother Nature threw a few more surprises. The tailwater discharges actually decreased and remained stable throughout much of the winter and early spring. This contributed to hatches starting almost six weeks early, and also provided favorable conditions for incubating trout. Soon I began to notice something even more odd . . . juvenile trout. Due to warmer water, these little guys were emerging from the riverbed more than a month earlier than normal. Within a short period of time these little guys were about 1½ to 2 inches long.
If you’ve never seen juvenile trout this small there’s a simple fix: Go out at night when they emerge from the gravel. Not only is observation the first step of the scientific method, but, as John McClure, manager at Galloup’s Slide Inn told me, “It’s one of the most important, yet underutilized tools an angler has.”
I generally don’t start spending a lot of time out at night until early spring when catostomids such as northern hogsuckers and white suckers congregate in large schools to begin their annual spawning migration. But last spring I didn’t pay much attention to the suckers because of the hyperabundance of juvenile trout, which seemed to be attracting more of the big fish.
I use several approaches to observe fish at night. Without a doubt, night snorkeling is the most dangerous and spooky method. Believe me, a raccoon plunging into the river looks a lot like a bear leg. Snorkeling requires the most gear, most preparation, and the most common sense. However, it can be the most efficient way to really figure out what is happening in deep or broken water, and under grass beds or logjams.
The easiest method to observe trout at night is to wade and use an LED underwater flashlight attached to a hockey stick or a painter’s pole. With the light underwater, there is no reflection off the surface, and the pole allows you to stand upright. While they do make specialized diving lights, I just use an LED flashlight and wrap all of the threaded pieces with Teflon tape. This makes the light about 99 percent waterproof, and the whole rig cost me about $30.
I’ve tried using different lens colors such as red and yellow, but the fish don’t really seem to care. They swim off eventually when you shine the light on them, but if you kill the light for a bit, they’ll usually just return to their former activity.
By using these techniques and focusing my efforts on sections of the river with optimal nursery habitat, I counted almost 1,600 juvenile trout in only a few nights last spring.
Because we never had any high water to disperse these little guys, and the spawning areas were more aggregated then usual, the juveniles were almost always in large schools. Some of these schools contained hundreds of individuals.
Despite their small size, their abundance attracted many large trout into inches of water every night for weeks at a time. I can’t begin to describe the blatant savagery to which these juveniles where exposed. Not only did I observe trout as small as 7 or 8 inches chewing up freshly hatched juveniles, I also spotted some real whales.
I even observed my old friend Jaws from the previous fall—he was feeding on what were likely his own offspring.
At times there was only one trout working the school, but at other times there were two or three. In one location, I spotted a large brook trout patrolling the downstream edge of the shallows, while a large brown was just upstream doing the same thing.
The most ironic thing I observed was little juvenile trout with other smaller juveniles hanging out of their mouths. I’ve read about siblicide, and seen it in wildlife videos about lions and birds, but watching 2-inch brookies and browns chewing up their own brothers and sisters made me realize how important cannibalism is in the quest for food and survival.
Catching meat eaters requires a little different outfit than you might be used to. Rod brand is subjective, but a streamer-specific rod in the 6- to 8-weight range gets you into the game.
I throw a 9-foot 7-weight with a large-arbor reel, full-sinking line, and 4-foot leader tapered down to 10- to 15-pound-test. I often use tandem articulated streamers, and the smallest streamer I use is about 4 inches. When I really want that one “big boy” I may go as big as 8 inches on one of the flies.
To rig two articulated flies, I tie on the upper fly using a nonslip loop knot leaving a long tag end. Instead of clipping the tag end, I use it to tie the same knot to a second fly about 18 inches away. If you tie the second fly to the bend of the first fly, the rig might be easier to cast, but you’ll lose most of the articulated action of the top fly.
Keep in mind that in this setup, both flies are tied on the same line. If the bottom fly snags, you may lose both flies. You can fix that by placing a blood knot between the two flies, and reducing the terminal tippet strength. If blood knots aren’t your thing, or it’s too cold to tie one, tie a high-quality ball-bearing swivel between them, then reduce the breaking strength of the terminal tippet.
Bump & Grind
One of my favorite ways to fish this “cannibal’s delight” setup including an articulated juvenile trout imitation is a combination of drop-shot fishing, and bumping crankbaits, both of which put millions of fish in the net every year.
To pull this off with a fly rod, I use a 200-grain full-sinking line. The bottom fly should sink faster than the upper fly. To achieve this, I use slotted tungsten beads, lead/tungsten barbell eyes, or a 1⁄32- to 1⁄16-ounce poured-lead jig hooks for the bottom fly. It’s best if the flies are tied with the hook points up. For the upper fly, I use one of my Prostitot juvenile trout imitations— they almost never foul on the bottom because they are neutrally buoyant.
Once you get the flies down near the bottom, you can bump, jerk, strip, crawl, or jig them across the bottom without snagging. Depending on the substrate, this creates noise and kicks up mud, silt, sand, and algae. This substrate disturbance is a trigger. I don’t care if we’re talking about largemouth, smallmouth, trout, muskies, stripers, or snakeheads, they are going to kill it.
Quite often trout won’t strike the lower, bottom-bumping fly—it just gets their attention and they strike the upper fly, which looks a lot like a juvenile trout escaping from the disturbance. Later in the year, a crayfish pattern is a good choice for the bottom fly, but in the spring when they are emerging from the gravel, I use two baby trout imitations.
Just about all competitive anglers have a system they use to discover the feeding pattern of their quarry. It doesn’t matter whether they are a fly-fishing guide or competing in the Bassmaster Classic. This system may involve colors, styles, sizes, retrieves, but when you understand even just a little bit of the current ecological conditions, life history, or the millions of years of evolution that shaped that fish’s instincts, you’re going to be a better angler, and you’re likely to find a feeding pattern that helps you catch more fish.
I’ll admit that it doesn’t always work out the way you hoped, but sometimes it does pay to observe and experiment in order to find a feeding behavior, and cannibalism is one of those important behaviors. Remember Jaws? I targeted him for months, and on several occasions saw him making mincemeat out of juvenile trout that he probably fathered.
However, I realized what seemed like cruel and unusual punishment made perfect sense in an evolutionary context. So I took advantage of it, and on one late September night he latched onto one of my flies, I netted him, and then I released him.
Staton Klein is a fishing guide on the Savage River, Maryland. He has a degree in fisheries biology, and is working on a master’s degree in ecology and conservation biology with special emphasis on freshwater fishes.