River watch: Leaves play vital role in health of salmon streams

Riparian vegetation includes the trees, shrubs and herbaceous plants that surround any stream. This vegetation is critically important for the Kenai River, Anchor River, Quartz Creek and most streams on the Kenai Peninsula. These plants provide dropped leaves, bud scales, twigs and even pollen grains that end up in the streams. This vegetable matter provides the energy that supports a majority of the aquatic insects and invertebrates that in turn are food for the fish populations of the receiving streams.

Leaves descend in a short but major pulse in the fall. If an average tree has close to 200,000 leaves, leaf fall can mean a huge organic material influx for the stream. These leaves are at first unusable by the insects until they are attacked by aquatic bacteria and fungi. A few days after inoculation by these bacteria and fungi, the protein content of the leaves actually rises and they become a favorite for a special group of insects. The nymphs and larvae of a couple species of stoneflies and craneflies will skeletonize the leaves and create large amounts of very fine particles that drift downstream. (We’ve all seen the nonbiting cranefly adults when we see a critter buzzing around that looks like a mosquito on steroids.)

These finely chopped-up leaves then become the food of choice for a great many aquatic insects found in these streams. The most common caddisfly in the Kenai River, Brachycentrus, sits on rocks in the stream and catches these fine particles with leg hairs. Some other caddisflies, like Arctopsyche, use specially spun underwater nets to trap the fine particles for their dinner. Many of the mayfly larvae, as well as a great many of the midges, use these leaf fragments as a major food item, too. Black fly larvae — we call the pesky adults “whitesocks” — have specially evolved antennal fans that are used to trap these chopped-up leaves as a major portion of their diet while in the larval stage. Additionally, there is a whole host of nearly microscopic invertebrates (many are related to shrimp) that also use these particles in our streams and rivers.

As every fly fisherman knows, these insects and invertebrates are a major source of food for the resident fish. So, we tempt rainbows, grayling and even lake trout with our fur and feather mimics of these insect larvae and adults. Less widely known is the reliance of our young salmon fry on the smallest of the invertebrates and insects. The more insects our salmon fry can eat, the bigger they will be and the greater their survival will be when they head out to the ocean.

It has been shown that streams with hefty riparian input will support large insect populations and in turn a large population of salmon and trout. The Kuparuk River on the North Slope has almost no riparian input and has very few insects that support only a very small population of grayling. Riparian input each year plays a major role in the overall food chain for our resident fish and the temporary salmon found in our peninsula streams.

There are federal- and state-mandated building and logging setbacks from streams and rivers for very good reasons. Obviously, we don’t want our structures to suffer flood damage and we don’t want our close proximity to the stream to be a source of stream pollution, either. However, the stream’s need for the leaf input each fall is often overlooked, even though these leaves are an important energy engine that fuels the stream and the fish populations. The take-home message is to preserve the riparian vegetation along all of our streams.


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.