A River Never Sleeps

It’s not surprising that biologists experimenting with the fertilization of rivers and lakes on the West Coast of Canada have noted an explosion of insect life follows soon after watersheds are enriched with nutrients.

On the Keogh River on Vancouver Island, an explosion of caddisflies and mayflies has been noted in addition to the increased length and width of salmon fry.

A Keogh River Stonefly

The fertilizer used in the Keogh mimics the natural nutrients released by decomposing salmon carcases. At the same time, watershed specialists working in the Keogh rebuilt much of the streambank forest, as well as constructing backwaters where warmer waters served as rearing areas. With healthy forests, more vegetation and woody debris fell into the stream.

The fertilizer stimulated aquatic plant growth; the increased vegetation fall from the forest provided even more nutrients - triggering an increase in insect populations, which in turn helped grow bigger fish.

If the question is, which came first, the fat trout or the insect explosion, the answer is; neither. What came first was an increase in nutrients, which led to a better “plant crop” for the insects to feed on.

The dietary requirements of aquatic insects varies from species to species. Some have very strict dietary requirements, others are more wider ranging in terms of what they eat.

Aquatic insect larvae and nymphs which graze the stream bottom by scraping their mandibles over the stones and peeling away and eating minute plant organisms - diatoms and algae - are called grazers or herbivores. In general aquatic insect herbivores are found in faster flowing waters of a stream; in the rapids, riffles, and glides. Among the mayflies, the best known grazers are the Heptageniidae (ie Cinygma spp, Cinygmula spp, Epeorus spp, Rithrogena spp), the Baetidae (ie Acentrella spp, Baetis spp, Callibaetis spp) , and the Siphlonuridae (Ameletus validus). Among the stoneflies, the Giant Stonefly (Pteronarcys californica) and the Little Black Stoneflies (Capnia spp, Isocapnia spp) are among the most obvious grazers.

Grazing caddisflies include the Microcaddis (Agraylea saltesea) , the Grannon Sedge (Micrasema bactro), and the Little Plain Brown Sedge (Lepidostoma roafi). The Net-winged Midges (Blepharoceridae) and some Chironomid larvae also graze on underwater algal carpets.

Aquatic insect larvae and nymphs which feed on decomposing leaves, conifer needles, woody materials, even animal tissue are called shredders or detrivores. The detrivore eating mayflies include the Pale-morning Mayflies (Ephemerella spp), the Slate Winged Mahogany Mayflies (Paraleptophlebia spp), and some of the Epeorus species (eg E. nitidus).

An Alderfly under glass

Most of the stonefly species found in a coastal streams, and many of the caddisfly species inhabiting this system are shredders or detrivores. In fact studies show that terrestrial detritus, deciduous leaves, and conifer, needles account for up to 66% of the diet of the aquatic insects that are consumed by juvenile coho salmon.

In general, underwater shredders will be found in the slower flowing waters (eg pool and backwater habitat) where silt, leaves and decomposing debris has had time to settle to the stream bottom. It’s not surprising, then, that stream technicians on the Keogh and other rivers have built backwaters as rearing areas for salmon fry. The quiet waters are often rich in plant life - and in herbivorous insects.

Not all aquatic insects are plant eaters, of course. Nymphs and larvae which prowl stream bottoms in search of larger prey to kill and eat are called predators. There are predacious stonefly larvae (eg Claasenia sabulosa, Hesperoperla pacifica, Isoperla sp, Diura knowltoni), predacious caddisfly larvae (eg Arctopsyche grandis, Dolophilodes pallidipes, Cheumatopsyche oxa oecetis inconspicua, Polycentropus spp, Rhyacophila spp), predacious chironomid (Chironomidae spp) larvae species, predacious Cranefly larvae (Tipulidae spp) , and even a predacious mayfly (Drunella doddsi) .

Two of the most f earsome looking underwater predators are the megalopteran species - the Alderfly (Sialidae sp) , and the Fishfly (Corydalidae sp) . The Alderfly larvae grows to 18 mm, the Fishfly grows to 75 mm. They spend their time prowling though leaf packs, decaying vegetation, and spaces between rocks in search of any insect they can subdue with their large and powerful mandibles. The Alderfly and Fishfly are rarely seen, but I do look for the distinctively clumsy flight of the emergent adults along salmon streams in the months of June, July and August.

Aquatic insects which eat both living plant and living animal matter are called omnivores. Most aquatic insect larvae and nymphs actively search for their food, some wait in ambush for the food to come to them, while a few (eg predacious caddisflies like Arc opsyche grandis, Dolophilodes pallidipes, Cheumato syche oxa, Oecetis inconspicua, and Polycentropus spp) construct elaborate spider-like silk nets to trap insects floating in the stream currents.

Undoubtedly there is much we do not know, when it comes to the how, what, when and where the various aquatic insects species divvy up the available food supplies. But we do know that when the waters are enriched by decaying salmon carcasses - or by artificially introduced nutrients in streams where runs have been badly depleated - the plant life stimpulates insects, which are fed on by other insects and by salmon, trout and steelhead.

From the smallest things, the great fish grow.