A Strange Fish Story

More than sixty species of fish make their home in the Credit Valley watershed, but many of them are strangers to all but the most avid observers. Let us introduce you.

March 22, 2007 | | Environment

You might mistake it for a scene from a sci-fi movie: Invasion of the Fish Snatchers. Otherworldly waders, arms clad in rubber, features obscured by dark glasses, advance slowly through the turbulent waters of the Credit River near Belfountain. The leaders, strapped with metal backpacks, wave wands through the water and, ominously, scores of fish float to the surface, belly up. The waders erupt in a flurry of shouting and jostling. They jab nets into the frothy river, scooping up a harvest of stunned fish – specimens to take back to the Mother Ship?

Well, not exactly. What’s going on here is a little more down to Earth, though it does have a futuristic-sounding name. It’s called “electrofishing.” It involves no malevolent aliens, just earnest biologists and energetic volunteers determined to further our knowledge of fish and water quality in the Credit River watershed.

Electrofishing is carried out by passing electricity through the water to stun the fish, which are then transferred in buckets to the stream bank for identification and measuring. After these indignities the fish are released back into the stream to resume their aquatic existence.

Credit Valley Conservation conducts electrofishing at about fifty sites in the Credit River watershed each year. And you can be part of the action. Several of the sessions are open to the public during the summer.

According to Jon Clayton, an aquatic biologist with CVC, the volunteers are a diverse group. “They range in age from high school students working towards their forty hours of community service to retirees. Naturalists, members of fishing clubs and biologists from other countries looking for Canadian experience come out.”

I’ve participated in electrofishing several times and can vouch for its appeal. It’s an exciting way to learn about a largely unknown part of our fauna.

CVC supplies the essential elements of the electrofishing wardrobe: chest waders, rubber gloves and polarizing sunglasses. The gloves and chest waders keep you dry, but rather more crucially, they keep you alive, by insulating you from electric shock. Part of the rigorous safety protocol at each electrofishing session involves testing the waders for leaks before electricity is activated. The sunglasses have a more prosaic purpose; they cut the glare on the water surface so you can see the fish beneath.

Electrofishing might seem like another example of humans meddling needlessly with the lives of wild creatures. Why not leave them alone to conduct their fishy affairs unmolested? Good question – to which Jon Clayton responds: “Electrofishing is one of the most effective means of sampling fish. Like all sampling techniques there is a degree of intrusiveness and even some mortality, but used properly, these negative effects are minimized. As for the need to monitor fish, in this time of rapid environmental change, fish sampling gives us an indication of not only the health of fish communities but also the health of the entire watershed.”

Monitoring fish life can provide clues to the magnitude of the effects of such environmental assaults as urban growth, deforestation, pollution and global warming. Changes in the varieties of fish or in the population of a particular species can tell us that something is amiss. Conversely, if actions have been taken to improve an environmental problem, changes in fish life may indicate that they are on the right track.

After seven years of data collection, says senior biologist Bob Morris, “some statistically significant trends are already emerging and these trends will gain greater validity with every passing year.”

In general the upper Credit is in “pretty good shape,” says Jon Clayton. Some particularly healthy stretches include the West Credit upstream and downstream from Belfountain, the main river through Forks of the Credit and upstream of Cataract.

There are over 140 species of fish in Ontario and over 60 species in the Credit Valley Watershed. Aside from brook trout, these are fish that most people know little about and include darters, sculpin and many minnows – the largest family of fish in North America. (There are hundreds of minnow species, including various types of dace, shiners and chub.) It is important to note that not all small fish are minnows and, contrary to popular belief, not all minnows are small. Far from it: the introduced carp is a minnow and it can tip the scales at over fifteen kilograms.

Many of the fish in the Credit are too small to be caught with a hook and line, but they are still of tremendous value. Trout, salmon and bass grow fat on them. The iconic great blue herons and the raucous kingfishers depend on them. Mink, sleek predators of river and stream, would suffer in their absence.

Following are some of the fish I’ve had the pleasure to encounter in the Credit River over the past several years.

Electrofishing record: Credit River at Highway 24,
August 13, 2001. Air temperature 24°C, water temperature 19°C.

285 blacknose dace


If you have peered into the waters of the Credit River or just about any other southern Ontario stream, you have likely seen schools of a minnows called blacknose dace. These very common fish turn up more frequently than any other in electrofishing operations. A reason for their abundance, according to Erling Holm of the Royal Ontario Museum and co-author of an upcoming field guide to Ontario fish, is that “this is a species that can survive practically anything. They thrive in clean, cool headwater streams but also seem able to flourish in warm, polluted urban waters as long as their requirement for moderately flowing water is met.”

Along with other highly successful minnows, like creek chub and shiners, blacknose dace are low on the food chain and are food for a multitude of larger creatures. However, they get their payback by avidly devouring the eggs of other fish. Unfortunately, they’ll also eat eggs of their own species. But to get to the eggs they have to get past the males who fertilized them. The males defend the eggs vigorously, ensuring the species’ survival.

September 1980, Glen Williams. On the bank of a small tributary of the Credit River
I watch a school of redside dace swim in a limpid pool. As the males twist and turn the angled rays of the afternoon sun glint off of the brilliant red stripes highlighting their sides. There seems to be a school of at least twenty of these lovely minnows.


With regret I admit to pickling two of these fish on that long-ago afternoon in order to include them in a fish collection I was assembling for a university course in field biology. Though I didn’t know it then, I do know now that redside dace are in trouble in this province. Nationally they were designated as a species of “Special Concern” by COSEWIC (Committee on the Status of Endangered Wildlife in Canada) in 1987. Provincially they are designated as “Threatened,” denoting even greater concern for their future in Ontario.

“Redside dace have experienced dramatic declines in certain river systems,” Erling Holm says. “They used to be found throughout the Don River, for example, but are now restricted to its headwaters. They have also declined or been eliminated entirely from several other streams in the GTA.” While the redside dace has also suffered some setbacks in rural areas, it seems clear that urbanization is not friendly to these fish.

Redside dace still inhabit parts of the Credit River watershed although they seem to have disappeared from the Glen Williams site I found twenty-seven years ago. A population has recently been found along a tributary of the Credit River in Caledon raising hopes that other locations for these fish may yet be identified in the less urbanized upper watershed. These rare fish also inhabit parts of the Humber River watershed.

These dace are really quite unlike any other minnow we have. They feed by leaping out of the water to capture low-flying insects with their large upturned mouths. Bob Morris has been monitoring a population on a stream that runs through a cattle pasture. One afternoon he watched as a breeze swept flies from a cow patty out over the stream. The water erupted with the leaping bodies of the dace.

Excellent water clarity is critical for these fish. They need to be able to peer upwards out of their watery domain to see minute insects and properly gauge their jumps to catch them. Take a look at the lower Credit after a rainstorm, or any of a hundred other GTA streams for that matter. Mud eroded from tree-impoverished landscapes and flushed into the water off asphalt and concrete surfaces give streams the appearance of chocolate milk. Imagine being a fish reliant on eyesight to find dinner in such waters. You would starve. So it’s not surprising that redside dace are disappearing from sections of urban streams.

A Redside Dace Recovery Team has been set up to work toward improving the chances of this fish in Ontario. The Toronto Zoo is involved in this effort, offering public education to promote stewardship. Controlling road runoff, using environmentally friendly household products, and maintaining vegetation buffers along the edges of watercourses will help redside dace as well as other fish species. The Zoo also offers lesson plans according to the Life Systems strand of the Ontario Curriculum for grades one and seven. They can be found at www.torontozoo.com/conservation/fish.


Electrofishing record: Credit River at Terra Cotta,
August 23, 2001. Air temperature 27°C, water temperature 21°C.

131 rainbow darters


Years ago while seeking fish on the Credit River for my university project, I swung my net idly through the fastest rapids in the centre of the stream. I didn’t really expect to catch anything. After all, I thought, what fish would be able to exist in such turmoil? When I withdrew the net from the water however, a small fish squirmed in the mesh. It was a lovely creature, its fins and body tinted with blues and oranges. I had captured a rainbow darter.

Later I discovered that during their breeding time in the spring the males become especially vivid, rivalling the beauty of tropical aquarium fish. Like males from most species, including our own, rainbow darter males expend great energy to impress females. “The colour is a way of advertising good health,” suggests Erling Holm. “I would imagine that the more colourful a male is the more attractive he is to a female.”

Rainbow darters do not have a broad range in Ontario, but healthy populations exist in many of the streams flowing into western Lake Ontario and Georgian Bay. “Life in the rapids gives them the ample oxygen they need to survive, and also assists their feeding,” says Jon Clayton. “Rather than swimming to find food, rainbow darters can wait for it to be delivered by the current.”

This is another fish that needs clear water to survive. Their intense colours would not be of any value in murky water, nor could they see the tiny organisms that they feed on. Scientists have determined that rainbow darters are very sensitive to silting and chemical pollution, so this common fish can help us monitor changes in the health of our streams.

Electrofishing record: Credit River at Cheltenham,
August 11, 1999. Air temperature 18°C, water temperature 14°C.

17 stonecats

Sharing life in the fast lane with the rainbow darters are these smaller relatives of the bullhead catfishes commonly caught on childhood fishing expeditions. Whereas bullheads tolerate or even thrive in warm, stagnant, muddy waters, stonecats prefer the highly oxygenated waters of rapids. Though their habitats differ, stonecats and bullheads share similar feeding habits. According to Erling Holm, their signature whiskers are rich with taste buds. They use them to hunt in total darkness. When the whiskers brush a tasty morsel – a larval insect perhaps or a crayfish or minnow – stonecats and bullheads react by swiftly engulfing the prey in their cavernous mouths.

Stonecats along with two other small Ontario catfish species, evocatively named “madtoms,” are protected by sharp spines on their pectoral fins that can penetrate flesh and deliver a jolt of poison. (Pectoral fins are found low on the body, just behind the gills.) While Holm has never been “stung” by a stonecat, he has painful reminiscences of encounters with madtoms in Ontario streams. “The worst instance was one that got me under a fingernail,” he says. “It hurt like a bee sting for a couple of hours.”

Electrofishing record: Forks of the Credit Provincial Park,
July 29, 2005. Air and water temperature not indicated.

281 mottled sculpin

With wide mouths, bulging eyes, and heads that appear too large for their bodies, these fish are well advised to avoid piscatorial beauty contests. What they lack in looks, however, they more than make up for in character. Mottled sculpins and their almost identical cousins, the slimy sculpins, thrive in cold temperatures and are repelled by warmth. Slimy sculpins range north of the Arctic Circle and the mottleds thrive in Ungava, keeping company with Arctic char. But both sculpin species are also found in southern Ontario and mottled sculpins are fairly common inhabitants of the Credit watershed.

Sculpins and brook trout are neighbours in the cool, clear waters they both prefer, and being quite small, sculpins often end their days in the stomachs of large trout. Noting this, anglers offer trout a sculpin-like lure referred to as a “muddler minnow.”

Sculpins face the same water quality issues as other fish in our region. However, because of their need for cold water they – with brook trout – will likely suffer to a greater degree from climate warming.

Another threat to sculpins are round gobies, exotic fish that have arrived in ship ballast from the Black Sea. Like starlings of the aquatic realm, gobies are well pleased with the new world and are spreading at an alarming rate. This is bad news for the sculpins, according to Erling Holm, because “the gobies pretty much occupy the same ecological niche. Sculpins used to be common in the St. Clair River, but since the arrival of the gobies they seem to have disappeared.” Gobies have now arrived on our doorstep. Electrofishing has revealed their presence in the lower Credit, and they will probably move north.

Electrofishing record: Mill Creek Orangeville,
August 8, 2002. Air temperature 25.5°C, water temperature 17.4°C.

18 brook sticklebacks

At university I learned about the pioneering studies into the nature of aggression conducted by a famous Dutch ethologist (animal behaviour expert) named Nikolaas Tinbergen. One of his favourite subjects was a feisty little fish called the three-spined stickleback.

Tinbergen studied instinctual aggression in these sticklebacks and wrote extensively on how this could help us understand aggression in other animals. Since these early studies by Tinbergen, three-spined sticklebacks have become a laboratory mainstay, with scientists often studying not only aggression, but the elaborate breeding behaviour of these fish.

Three-spined sticklebacks are not found in this area, although some do inhabit the Great Lakes proper. However, their close relative, the brook stickleback, is a common resident of our neighbourhood waters.

Highly adaptable, these fish are found in urban streams, beaver ponds and spring-fed source waters. In larger watercourses such as the main Credit, sticklebacks find it prudent to keep to the margins, where they can hide among aquatic plants. That’s because their small size makes them bite-sized morsels for larger fish.

According to Jessica Ward, a PhD candidate in ecology and evolutionary biology at the University of Toronto, the behaviour of brook sticklebacks is every bit as interesting as that of the three-spined sticklebacks. “At times during their breeding cycle males respond aggressively to all territorial intruders. They will even try to bite me if I put my hand in the tank!” Ward says. This is rather audacious for such a small creature.

Ward is also amazed by the complexity of the behaviour of sticklebacks on their breeding grounds. “For example, on an underwater plant a male will build a round, ball-like nest with a single entrance similar to a bird’s nest. To build the nest, males collect algae, moss, leaves and twigs. Then glue it all together with a sticky, thread-like substance secreted from its kidney.”

After the nests are constructed the males get down to the important business of wooing mates. According to Ward they enhance their appearance by turning a “deep, velvety, black” and then they “dance.” A male will vibrate his fins, quiver his body, and even prod the head and flanks of a female with well aimed “snout butts.” A female suitably impressed by this obvious display of affection will enter the nest and lay her eggs.

The fascinating behaviour of these fish also includes all-male custodianship of the young. “When the eggs hatch,” relates Ward, “a male will take apart the nest and rebuild a looser version to be used as a nursery. He transfers the babies to the nursery in his mouth and continues to guard them until they are ready to swim away on their own. If the babies happen to fall out of the nursery, the father retrieves them in his mouth and spits them back inside.”

This is a mere taste of the interesting behaviour of these charismatic fish. Extraordinary things occur sight unseen in our local streams and wetlands.

Electrofishing record: Somewhere upstream
of the Niagara Escarpment on the Credit River, July 4, 2003.
Air temperature 30°C, water temperature unrecorded.

144 brook trout

These are fish of mythic status. They are breathtakingly beautiful, and perhaps best exemplify the pristine essence of tumbling brooks and the purity of forested headwaters. Even their scientific name, Salvelinus fontinalis, rolls off the tongue in a pleasing way. Fontinalis, meaning “living in cold springs,” speaks to the preference these fish have for cool waters. In fact, brook trout eggs perish if exposed to temperatures greater than about 11ºC.

In Trout Reflections, David M. Carroll calls the brook trout “a glacier child” and writes that, “Over the course of their history, they have been driven southward by the advance of glaciers, and have pursued them back to the north in periods of global warming, following in the very meltwater steps of retreating mountains of ice.”

It is quite probable that the brook trout that live above the waterfalls at Cataract have been part of a continuous fish community in that part of the Credit River for over 10,000 years, isolated from downstream fish by the waterfall. In fact, brook trout may have arrived in this area even earlier at about 13,000 years ago. At that time glacial meltwater thundered through the Violet Hill Channel, a vast watercourse that originated at the edge of the retreating glacier south of Georgian Bay. The torrent of water passed present-day Orangeville and the village of Cataract and then emptied into a large glacial lake occupying what is now southwestern Ontario. Brook trout may have fought their way north through this tumultuous channel to reoccupy former territory in south-central Ontario.

Regardless of when and how they arrived in the upper Credit River above the falls at Cataract, the brook trout in that northern stretch of the river are singularly fortunate. “The Niagara Escarpment has been a saving grace for the upper Credit River, because we’ve been able to maintain the native fish community up there,” says Bob Morris. “Below the Escarpment brook trout are forced to compete with introduced brown trout, rainbow trout and Pacific salmon. The upper Credit is known for being the best fly-fishing area for brook trout in southern Ontario because it is large enough to fly fish and because other similar rivers do not have a feature like the Niagara Escarpment to keep introduced species at bay.”

This situation offers an exception to the rule that high biodiversity is always desirable. “Some of the best brook trout streams have only two or three species of fish,” says Jon Clayton. While there are certainly more than two or three species in the upper Credit, there is less diversity than that found below the Escarpment and that’s good for brook trout. Add more species and the balance that has favoured brook trout for so long could be upset.

Regrettably one unwelcome addition to the upper Credit fish fauna has already occurred. Northern pike have entered the river from Orangeville’s Island Lake Reservoir. These ravenous predators are now taking up ambush positions in some of the quieter sections of the upper river. Their effect, if any, on brook trout is another puzzle on which electrofishing may eventually be able to shed light.

The pike were illegally introduced to Island Lake probably sometime in the 1980s. Misguided anglers have also introduced large sunfish called black crappie into Island Lake and, alarmingly, European carp have recently been found there as well. Black crappie are not stream fish so they are not expected to pose a threat to the upper Credit, but carp certainly may.

Perhaps anglers would stop these selfish, unauthorized introductions if they knew that a unique ecosystem in existence since the end of the last ice age was in jeopardy.

About the Author More by Don Scallen

Don Scallen enjoys sharing his love of nature through his writing and presentations. Check out his blog "Notes from the Wild".

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