Freshwater Fish Habitat

Habitat is the natural home of an individual. Fish habitat within a river includes the physical structures within the channel such as substrate, channel shape (pools/riffles), woody debris and the banks below the water surface. It includes the water, which surrounds a fish and the oxygen, other gases and chemicals dissolved within the water column.

The effects of light on the river through solar radiation resulting in varying water temperatures and photosynthesis of flora are all part of the habitat. The movement of water or flow is an intrinsic part of the habitat and is possibly the feature that most determines habitat quality and quantity. Flow is the engine that drives most ecological processes and maintains the physical structure of the habitat.

Changes in flow have the most immediate impact on fish and other biota in the stream. Flow determines obvious physical features of the habitat particularly water depths and velocity which have major impacts on the ability of fish to move and live in the stream. However habitat includes more than these obvious physical features. The energy which enables life to occur in streams, originates mainly from the input and then decay of terrestrial organic material such as plant material, from the surrounding land.

The ecological process of breaking down plant material into food relies on a chain of organisms being present most of which are never seen by the casual observer. Fungus, bacteria, diatoms, macroinvertebrates (larvae and nymphs of a myriad of insects) all convert the raw plant material into fine particulate organic matter, then into dissolved organic matter and ultimately CO2. A by-product of this process is the rich food source that each group of organisms provides for some other group. At the top of the food chain are fish feeding of the larger macroinvertebrates. Photosynthesis within the stream, which itself is affected by how much light penetrates the riparian vegetation, is also an important autotrophic energy input into a stream. Habitat is more than just a collection of all these features or ecological processes. Rather it is the complex interaction between numerous features and processes that make up the habitat. Even the presence and behaviour of other biota including other fish, all impact on the quality of the habitat of every individual fish.

If this is all too confusing then compare it with the place where you live and the activities that you carry out and the complexity of the myriad of things which influence, affect and determine your daily life. The basic features which fish require in their habitat are remarkably similar to ours or any other animal. These include a food supply, a place to rest, ability to move around, security from danger and satisfactory levels of oxygen, temperatures and an absence of toxic chemicals.

How does it all work together? A natural pristine river, through the action of flowing water, has developed its own set structure of meanders, pools and riffles; which has been determined by the width of its channel. This means that any channel of flowing water (including waterways damaged by man-made activities) can eventually restore itself back into a natural stream, with pools, riffles and meanders. Whole or parts of riparian trees, which fall into the channel, are distributed along the channel by floods.

This debris finally come to rest on the substrate, usually against the banks, providing protection of the banks from erosion during future floods and places where fish can rest, protected from high water velocities and predators. This woody debris also provides a home or habitat for many small biota. The debris traps leaves and plant material which after being broken down into small particles as previously described, is moved downstream by the flow, for other organisms to further process. The deeper quiet water in the pools provides space for large fish and biota not suited to life in the fast-water riffles. Riffles with coarse substrate material and well-oxygenated fast-flowing water, provide an ideal home for many macroinvertebrates. Small fish also find shelter among the rubble, safe from larger predators and protected from high velocity water flow.

Macroinvertebrates dislodged from the riffles float downstream and are taken as food. Other fish move into the deeper areas of the riffle to feed on the food it produces. The riparian trees not only provide instream woody debris and an input of organic material but shade the stream, reducing water temperatures during summer and maintaining higher water temperatures during winter. Fish such as salmonids that prefer cooler water (less than 170C) can feed and grow for longer each year with reduced levels of stress.

The natural UN-regulated flow, which is continually being oxygenated as it passes through the riffles, dilutes any input of chemicals, minerals and animal wastes originating in the catchment, to levels that the biota can cope with. The natural flow regime not only has daily/weekly fluctuations but periods of significant lower flows during summer followed by high flows and floods during winter/spring. These flood flows move fine material (sand/silt) off the areas of gravel and rubble, where it has accumulated during the low flow period (see sedimentation Basin 38), redistribute woody debris along the river, and restore channel shape and form.

The higher flows and resulting deeper water enable larger fish to migrate along the stream and also assists the downstream movement of those species that need to reach the estuary or sea for spawning. As flow decreases in late spring/ early summer it leaves the river and its habitats in excellent condition which coincides with major spawning activity of most fish, an increase in macroinvertebrate activity and opportunity for biota to recover from any stressful conditions resulting perhaps from droughts in the previous year.

Late spring /early summer is also the time that juvenile fish such as common galaxias that have developed in the estuaries make their long swim upstream back into fresh water where as adults they will spend the rest of their lives. Riffles or other potential barriers will still have adequate water depths for the shoals of small fish to pass but the high water velocities previously present during the floods will have decreased. The changing water conditions throughout the year perfectly match the different habitat requirements of each species at every stage of their life cycle and their daily activities. Each individual species and age group has a specific range of conditions (temperature, water depth, velocity, amount of dissolved oxygen, etc) that it prefers in its habitat.

Each condition must also suit the particular activity that is going on. For example fish prefer low velocity water in locations where they rest during the day. However when feeding they are happy to move temporarily into fast water. They require adequate water depth and some overhead shelter when resting against the bank but will move into very shallow water with no shelter nearby, during darkness. The precise habitat requirements of many fish are well known and enable the biologist to assess whether or not various types of habitats are suitable for particular species (discussed further in brown trout habitat Basin 2).

A feature of any pristine stream is the huge variety of habitats present. This is the reason that so many different species and age groups of each species can live together because each individual is able to select the location in the river that suits them best. The species and individual fish present in any stream is therefore directly related to the type and quality of habitat present. One should not expect large-sized fish in a shallow creek or fish such as salmonids that prefer cold water, to be present or to reach their best size or condition in a warm-water stream.

It is now not difficult to appreciate that any changes to either the stream channel, river structure, flow regime, water quality, riparian vegetation, catchment or endemic community of biota, will have some affect on habitat and then on fish and other biota. The actual loss of some habitats or individuals in the food processing chain, can result in extinction of some species. However what often happens is that the changes are small and gradual. For example, a little more sediment comes into the stream from more bank erosion or water temperature becomes slightly higher each summer because of loss of riparian vegetation or flows are consistently less because of offstream diversions.

Water quality may slowly deteriorate because of more nutrients entering the stream from stock or farming activities, causing an increased growth of algae. The results of these changes on the biota are often not seen immediately but start a long-term process of deterioration in the fish populations. Possibly most serious is the excessive diversion of water from the stream causing extreme and rapid changes to the flow and therefore water depths.

Lower water levels result in large areas of habitat in the shallower areas of the stream such as the riffles, becoming dry and unavailable to biota for long periods each summer. Riffles support most of the macroinvertebrates used as food by fish. A reduction in food supply results in increased competition among fish and slower growth rates. Not only is less food available but the fish need to spend extra effort to feed and therefore derive less benefit from the food.

There may also be a change in the species composition of the macroinvertebrates and this always happens if sedimentation increases. Species such as mayfly and stonefly nymphs, which need clean substrate are replaced by others which can tolerate silt and mud. Excessively low flows and therefore a loss of deep water also results in areas that were used by fish as shelter or resting areas, (the banks or instream logs) becoming unsuitable and no longer able to be used by fish. Gravel-bottomed streams have numerous riffles where water is shallow and faster. If the water level falls too low, the riffles become barriers to fish movement, trapping the larger fish in the pools.

Fisheries scientists worldwide agree that habitat is the major determinant of the quality of fish populations. Good habitat results in a good population of fish. Successful fisheries can only be maintained if the habitat is present and not degraded by inappropriate actions. The same principle applies to any population of fauna or flora, whether natural or farmed.

Fisheries managers may have to control the numbers of fish taken by anglers in a particular stream, or protect some sizes of fish. They may even have to stock fish at times if natural spawning is inadequate but none of these measures will improve fishing or provide more and larger fish beyond what the habitat can naturally do.

Next time you go fishing have a look at the stream. If it is mostly shallow water that means there is insufficient deep-water habitat to allow many of the fish to grow to a larger size. Most fish will be of small size because that is all the habitat can provide. Stocking with more fish will not help. If you want to catch large fish then go to a river with extensive deep-water habitat. Have another look at the stream you are fishing, particularly at the condition of the banks and bed. If there are few places where fish can lie-up during the day in some deeper water sheltered by the bank or instream debris then don't expect a good catch.

Fortunately habitat loss and deterioration of our streams can be reversed. Exclusion of stock and replanting of native riparian vegetation to stabilise the banks will result in a reduction in bank erosion and instream sedimentation, and is the first step. Restoration of instream logs and debris is next followed by the more difficult task of increasing summer flows, if excessive water diversion is occurring. Such activities would significantly improve habitats in most degraded Victorian streams.