Science of the seasons: Rocks star in creating good fish habitat

If you think about your favorite trout streams, or perhaps the secret fishing holes you only share with your very best and most trusted friends, there are probably some strong similarities between these streams.

These similarities are most likely going to be related to stream substrates. While stream substrates may seem to be an entirely esoteric subject, it is the characteristic and diversity of stream substrates that salmon check carefully when deciding where they will spawn. Not only do salmon use the gravels, cobbles and boulders as a determinant for spawning, but these same substrates are home to the community of aquatic insects that trout and salmon fry feed upon.

It is well-documented that aquatic insects prefer to be in streams with a mixture of various-sized stones. Insects are most abundant in streams with heterogenous substrates, but their numbers decline dramatically when the fine particles make up more than 20 to 30 percent of the substrate material. These fine particles are defined as being various-sized sands — 0.8 millimeters to 0.12 millimeters — or silt and clay particles — 0.006 millimeters or smaller.

As you can imagine, glacier outflow streams here in Alaska with high levels of fine sediment are poor habitats for most aquatic insects, and salmon don’t like to spawn there, either.

The amount of fine sediments in the substrate mixture is referred to as “embeddedness.” Spawning salmon look for stream areas with low embeddedness and prefer areas with large gravel, cobble and small boulder substrates. The most sought-after spawning areas are those just above a riffle section where water percolates down — downwelling — into the gravel substrates below. This water flowing through gravel brings oxygen to growing salmon eggs that settle into the lower, or deeper, substrates. The spaces between the larger gravel bits provide a home for aquatic insects that alevins and young salmon fry can feed upon. While seemingly unimportant, there is a lot of activity in the stream substrates that we don’t get to see.

The downwelling water flow can also carry fine sediments that fill in the spaces between gravel, especially if there were large amounts of fine sediments present. Too much fine sediment material would entomb and kill the young salmon residing deep in the gravel.

Over a period of years, fine sediment will clog the gravel below and make it less attractive to spawning salmon or trout. Fortunately, periodic high-water flows and stream flooding will wash out the fine sediments and re-sort the gravel. Then the area will become attractive as a spawning site once again.

Stream sediment composition can vary due to a large number of situations. Local geology and geomorphology plays a major role. Streams like the Kenai River that flow through old, well-washed glacier moraine gravel seem to have great substrate diversity. Next time you visit the Kenai River, notice the mixture of big cobbles, boulders, large gravel, smaller gravel and only a very small amount of fine substrates.

Unfortunately, there are a large number of ways that we as humans can change the mixture of substrate sizes in a stream. Some of the most problematic human activities are poor farming (tilling) techniques, road building, logging and large-scale mining. If poorly planned or improperly executed, all of these activities allow fine particles to wash into nearby streams.

In efforts to reduce the escape of fine particles from construction sites, low, black plastic fences and straw bales are placed in neat rows. These are arranged specifically to trap fine particles and prevent them from washing into a nearby stream.

While fine sediments in a stream can drastically reduce the aquatic insect population, at the other end of the spectrum, streams with only huge boulders also have reduced insect populations too. This interesting phenomenon usually occurs when there is a high gradient stream that has washed out most of the smaller gravel and cobbles. The major reason for this reduction in stream organisms is that the boulder substrates are fairly uniform and support only a small number of insects. The overall reduction in the insect population can then only support a small population of trout or salmon.

Diverse mixtures of stream substrates are critically important for the survival of a whole host of aquatic insects. These same rocky substrates create habitats below the surface of the stream that salmon search out for spawning. If these gravel, cobble and boulder substrates are changed, particularly with additions of fine sediments, salmon will avoid the area or have limited success in spawning.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the biology of the Kenai River watershed.