With trout season just around the corner, understanding basic trout foraging ecology can give anglers an edge. Regardless of size, a trout is limited in what it can eat by the size of its mouth. Tiny fry eat tiny zooplankton. Without these barely visible animals, aquatic food webs would break down.
Many of the tiny animals that young trout eat are crustaceans. Though most crustaceans live in marine ecosystems, many are extremely important in freshwater systems, as well. Lobsters and crabs are the best known mega-crustaceans, probably because we consider them gourmet food. But small freshwater crustaceans such as isopods, copepods, water fleas and fairy shrimp nourish tiny trout fry and enable them to grow rapidly.
As young trout grow and their mouths get bigger, they can eat bigger prey. The primary source of food for growing trout in moving water is invertebrate drift. These are adult and immature invertebrates that literally drift downstream in the current.
Drift seems to be an ideal dispersal strategy in an unpredictable environment. Spring floods can scour out large sections of streams. Drift permits invertebrates located in protected stream sites to repopulate scoured stream sections almost immediately after flooding.
Trout dependence on drifting prey helps to explain the popularity and success of fly fishing. Experienced anglers often drift flies on the surface, just below the surface of across the bottom to simulate natural invertebrate drift.
To catch bigger trout (with bigger mouths), crustaceans again become important. Crayfish are the most familiar and largest crustaceans in freshwater habitats, and every kid who has ever played in a stream knows how difficult they can be to catch. And like trout, crayfish are quite small when young and much larger as adults. Regardless of age, however, crayfish are nutritious prey.
An effective crayfish fly or lure need not look so much like a crayfish as behave like one. In the water, it's up to the angler to make the presentation act like a crayfish. At this point long ago memories of catching crayfish pay dividends.
Biologist, author, and broadcaster Scott Shalaway can be heard 8-10 a.m. Saturdays on 1370 WVLY-AM (Wheeling) and online at www.wvly.net. He can be reached at www.drshalaway.com, firstname.lastname@example.org and 2222 Fish Ridge Road, Cameron, WV 26033.