The Catskill Fly Fishing
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August 5, 2003 2:44 PM

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Energy Considerations: The Flow Of Life Through The River

Macro Invertebrates
Macro Invertebrates Gallery

Macro invertebrates, particularly insects, are vital to the production of trout, creating the opportunity for the flow of energy from plants to insects, and then to the fish. The mayflies and caddis flies are two of the numerous examples of macro invertebrates that carry out most of their lives under the rocks and sand of most freestone rivers, and in the vegetation within limestone streams. Together with the stoneflies, they constitute the majority of macro invertebrates common to most coldwater rivers throughout the world. Immature forms of other groups of invertebrates - various diptera (e.g., deer fly and black fly larvae), aquatic beetles, snails and crayfish - live there, too.

Many of these species function as shredders, reducing the size of plant material (e.g., leaves, grasses, branches, trees; referred to as crude particulate matter or CPOM) into pieces (fine particulate matter or FPOM) suitable for filter feeding communities to consume. While most macro invertebrates prefer detritus for their dinner, a few species of stoneflies and caddis flies eat the nymphal and larval forms of many species of aquatic insects, thus diverting the flow of energy away from the trout. Taken as a whole, these diverse invertebrate groups interact to form food webs in every way as complex and inter-dependent as those found in coral reefs or rain forests.

When the nymphal and larval forms mature and transform into winged adults, the energy they represent leaves the river and enters the adjacent stream-side ecotones, supplying food for frogs, birds, insects, spiders and other insectivorous predators. Adult female insects that survive the onslaught of that gantlet give back some of the lost nutrients to the river when they return to it to lay their eggs. The females of most species of mayflies, for example, float downstream after depositing their eggs on the water and become an easy meal for a fish. The eggs they have laid sink to the bottom, separate, and then stick to the submerged rocks, maturing through their nymphal stages over the following year, completing their life cycle. But in fact, even some egg masses are eaten by fish, as well.


A trout stream’s bio-productivity (usually expressed as an annual amount of kilograms of trout per kilometer of river) depends heavily on a robust production of in-stream macro invertebrates. As already emphasized, the amount of invertebrate life produced in rivers that run through hardwood forests depends diectly upon the abundance of leaves and related plant material that falls into the river throughout the year.

Many things detract from the production of macro invertebrates - deforestation, pesticides, heavy metals, siltation, stream channelization, and thermal pollution - disconnecting the underwater life forms from their food webs. Without the smooth flow of energy upward through the four trophic levels, the number and size of the trout that survive will be far less than before the disturbance. These insults can largely be reversed by initiation of good ecosystem management practices, or can be prevented altogether. Rivers and streams cleanse themselves if returned to a reasonable level of intactness. All that is needed is the desire, and sometimes the money, to do so.

Millions of people depend upon a constant source of clean water the origin of which, in many cases, is a dammed up portion of a trout stream. This underscores the importance for maintaining them as functional habitats. When aquatic ecosystems break down, it threatens all who depend upon it. If the macro invertebrates and trout disappear because of adverse changes in the environment, then we not only lose a natural resource, we eventually may lose ourselves in the bargain. We are all connected. That is why the expression “We all live downstream” rings so true.

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