Given how long the natural world has been evolving, it is not surprising that different species of fish have found uses for just about every kind of aquatic ecosystem on Earth. Fish live in beaver ponds, marshes, lakes, agricultural drains, shaded woodland streams, roadside ditches, rivers, and just about every tributary that fills them. Each type of water body differs from the others in what it has to offer fish, from its variety of food and cover to its temperature range, water clarity, and the amount of dissolved oxygen available to pass over their gills. A natural mix-and-match combination of characteristics determines which species live where. Just as we look for different qualities in our homes, have our own opinions about the settings on the thermostat and prefer some foods over others, so too do fish.
Why stay put in the same familiar habitat when you can explore and reap the benefit of others? Waterways are all connected, and many fish use this to their full advantage by moving from one freshwater neighbourhood to another as they pass through different life stages. By understanding the wide-ranging needs of fish that are served by creeks, streams, rivers, ponds and lakes, we can better appreciate what fish are up against when we interfere with their environment.
Much of the water that makes up fish habitat starts out as raindrops striking the earth and flowing across the land into small streams, rivers and lakes. From marsh to creek and river to lake, the character of the habitat changes along with its role in a fish’s life cycle. Even intermittent creeks found in the uppermost reaches of a watershed can be habitat for part of the year. A roadside ditch or the low area in a farmer’s field or a forested area might dry up in the hot months of summer, but for short periods each year, water flows over the land and may play an important role in habitat. Northern pike will migrate upstream in the spring and spawn in flooded areas which are dry for much of the year. When the youngsters hatch, they quickly move with the receding water to a more permanent watercourse to avoid being left high and dry. Even the tiniest stream may offer a refuge or a spawning area to smaller fish during the wet season. But even if a stream is not accessible to fish, it may still provide both water and food to downstream fish populations.
As the smallest threads of water merge into permanently flowing streams, more habitats unfold. At its headwaters, a higher elevation stream will be cold with high levels of oxygen – perfect for trout to live and breed. Further downstream, as the landscape opens onto the prairies, the channel widens and begins to meander, slowing the current and allowing sediment to settle to the bottom. No longer good for trout, these warmer waters now appeal to white suckers and northern pike.
Continuing on, the small streams join together to form rivers. With their larger size, rivers provide habitat for many unusual and interesting fish like lake sturgeon, goldeye, mooneye, sauger, several species of suckers and numerous species of minnows and other smaller fish.
Creeks, streams and rivers generally provide a variety of habitat types, including the more commonly known pool and riffle formations. A riffle zone is often characterized by shallow rapid areas of rivers that tumble and bubble over rocks and boulders to trap oxygen in the water. Trout, walleye and white suckers prefer to lay their eggs in this oxygen-rich water, which helps the eggs breathe. Riffles also play a key role in food production as these areas are abundant in insects and other invertebrates. Downstream of the riffle you can generally find a pool. Pools are deeper, slower areas that often act as feeding areas for larger fish.
When we alter the land bordering creeks, streams and rivers, we may inadvertently alter fish habitat. Shoreline vegetation acts as a natural filter, removing contaminants such as fertilizers and pesticides. Excessive fertilizers entering the watercourse fuel the growth of algae, which in turn uses up the precious oxygen in the water needed by fish to breathe. When shoreline (or riparian) vegetation is removed, the banks become unstable and can easily erode. The result is higher levels of silt being added to the watercourse, which can damage fish gills and lead to suffocation of the fish. Also, as water temperatures increase, the oxygen levels decrease and the stream may become unsuitable for species that prefer cooler conditions.
From a fish’s point of view, lakes and reservoirs open up all sorts of possibilities for fish use, from deep-water feeding grounds to shallow shoreline nurseries. Lakes and reservoirs certainly fit this description, although some are more conducive to fish than others.
Most people can easily identify a lake by its surface features - the islands and bays, the pine tree on a point and the beach where we suntan. But not surprisingly, the features nearer and dearer to the hearts of fish are invisible to our eyes. The diversity of fish found in a lake is dependent on many things, including the lake depth, temperature, abundance and type of plants, and the mud, sand and rocks that make up the lake bed.
Deep lakes are generally cold lakes as the sun is unable to warm the waters all the way to the bottom. As sunlight penetration through the water is limited, plant growth along the lake bottom is also limited. These deep, cold lakes often provide ideal habitat for lake trout and lake whitefish. Both species hang out in the deeper cooler waters during the summer, and then move to more shallow waters in the fall to spawn. Lake trout need the rocky shoals for their spawning, while lake whitefish make use of the hard or stony bottoms. When these habitats are in short supply, removing or harmfully altering them will have devastating impacts on the survival of the population.
The opposite lake type is shallow, warm and rich in nutrients. The sun warms up the water quickly, and since the sunlight easily penetrates to the bottom, there is an abundance of plant life growing in the water. Many fish use this plant abundance to their advantage. For example, yellow perch are often associated with extensive aquatic vegetation, which perch use for both feeding and spawning. A unique characteristic of perch spawning is that it attaches its eggs directly to the vegetation, making this habitat feature an absolute necessity for perch survival.
Another fish that prefers these shallow, warmwater lakes is the brook stickleback. Swimming and hiding amongst the underwater jungle of plants, the stickleback stalks its prey of aquatic insects, snails and worms. The stickleback spawn in these shallow areas, using bits of vegetation to construct tiny nests attached to the stems of submerged grasses and reeds.
The streams and rivers that enter and exit lakes are also a critical part of the picture for fish. Some species that use open lake water habitat to support their adult lives also depend on these adjoining watercourses to reach their spawning grounds. In the spring, the white sucker may migrate from the lake into gravelly streams to spawn. The lake sturgeon also migrates into rivers to spawn in areas with swift water or rapids, often at the foot of low falls that may prevent any further migration. As far as these and many other fish species are concerned, a lake’s health extends well beyond its perimeter.
When it comes to bigger bodies of water like lakes and rivers, our greatest potential impacts do not occur where we get in over our heads, but rather where we get our feet wet! This area is known as the nearshore habitat. Shallow and sheltered by aquatic vegetation, these areas are sought by many fish species for a number of reasons like laying eggs, lying low from predators, or even stocking up on treats such as crayfish, dragonflies and leeches. When we muck about in a nearshore area, building breakwalls or clearing a swimming area, we damage a very sensitive ecosystem and put fish at risk. Nearshore aquatic plants may look like weeds to us, but they are often key to this sensitive ecosystem, harbouring rearing areas for young northern pike, habitat for minnows traveling along the shoreline, reducing algae in the water, and helping to filter runoff and settle sediment. Any work we do at the water’s edge requires very careful planning “Working In and Around Water: How to Do It Right”).
Whether it is a creek or a lake, the strip of trees, shrubs, and grasses that naturally grows along a shoreline is important for fish habitat. This is the riparian zone and, if left alone to do its job, it acts as a buffer between land and water. The network of roots acts as both a shoreline stabilizer and a water filter to control erosion and remove impurities from surface water runoff (for example, phosphorus is a nutrient that occurs in nature, but it also occurs in human products and waste. Excessive phosphorus can throw off the nutrient balance of a waterway and cause algae and aquatic plant populations to explode). Leaves and branches break the force of falling rain, and runoff is slowed by the piles of leaf litter, pine needles and broken twigs. By slowing down the runoff, the riparian zone allows the water to be absorbed into the ground, resulting in less surface flooding and bank erosion. Without this green line of defence, the nearshore waters are vulnerable to both natural and unnatural forces.
Canada’s Federal Policy on Wetland Conservation (1991) defines a wetland as “land that is saturated with water long enough to promote wetland or aquatic processes as indicated by poorly drained soils, hydrophytic vegetation (i.e. plants that grow in water), and various kinds of biological activity that are adapted to a wet environment”. Simply put, wetlands are the mid-way environment between aquatic ecosystems and land-based ecosystems. Wetlands include bogs, fens, marshes, swamps, prairie potholes and shallow waters.
Sometimes dry and sometimes wet, wetlands share characteristics of both dry land and aquatic habitat. This zone harmonizes water, soil, nutrients and sunlight to form an extraordinarily fertile environment for innumerable species, including insects, fish, amphibians, birds and mammals. Wetlands are among the most biologically productive places on Earth!
Some fish, such as brook stickleback, may spend their entire lives in a wetland. There are occasional users who periodically swim into wetlands for cover or to feed on the forage fish that they harbour. For other fish, including northern pike, the use of a wetland is critical for the completion of their life cycle.
Wetlands are perhaps even more vulnerable to human activities than other types of fish habitats. Many of us have no idea that wetlands act like giant sponges for waterways, soaking up excess runoff and filtering sediment that would otherwise cover rock or gravel spawning beds. Yet there is a limit to what a sponge can absorb: our environmental abuses can overwhelm a wetland, sending pollutants downstream. Of course, some people simply want to “improve” their real estate by draining or dredging it. Unfortunately, this results in the complete destruction of a healthy and productive wetland.
While a marsh or floodplain may not seem like the most hospitable place to raise a youngster, the abundance of cover and food makes wetlands some of the busiest nurseries around. In their lake home, young suckers and walleye often migrate into these protected waters to feed. For other species, this is where life begins. In early spring, northern pike move into sedge (grasses that grow in wet areas) and grass-filled floodplains of rivers to spawn. In case you are not yet convinced of the fragility of wetland habitats, consider the delicate manner in which the northern pike gets its start. The northern pike leaves its eggs in the care of the wetland, attached to standing plants from the previous growing season. To us, the vegetation may look like an untidy mess, but it means everything to the eggs’ survival. The wetland vegetation keeps them from sinking into the bottom muck where they would either suffocate or be devoured by other aquatic organisms.