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Biology

Migration: The Path Of Least Resistance

by Matt Straw   |  October 6th, 2012 0

We see a lot of writers today using old phrases and terms first coined by the In-Fisherman staff to describe fish behavior and movements. Imitation is the sincerest form of flattery.

A favorite phrase I’ve used for decades in seminars and print to describe the migratory behavior of river fish is the “path of least resistance.” Nothing new about the term. It’s used a lot by electricians, plumbers, roofers, and lazy people.

But, when I first used it to describe how steelhead and salmon  up a river during migration, it described some intricacies of behavior  nobody else had really taken the time to analyze. At least, not in print.

By fishing the same rivers day after day, in every imaginable water level, I was in a unique position to observe how big salmonids used the shallowest water with the least current possible when migrating upstream. That need—to follow the path of least resistance—is modified by other conditions. The first priority is to not be seen, so they move as shallow as confidence allows. As the water gets cloudier from rain, they begin feeling secure in ever shallower water. As the water rises, they do the same, avoiding the strongest flows on the outside of the bend and moving up river on the inside of the bend. Somehow or another, they know what you can or can’t see and wherever you can’t see bottom, a steelhead could be holding in that spot.

If you’re lucky enough to be there as waters gradually rise over a matter of days or weeks from very low and clear to flood stage and beyond, you can track holding fish right across the river. In low, clear water they’re in the deepest part of the pool. As the water rises and loses clarity, they move toward the middle of the river, closer to the inside of the bend. If the water keeps rising, pretty soon they’re holding in the spots where you stood to fish the deepest part of the pool. At flood stage, they hold against what used to be the bank, right where you generally step into the river. When the water begins flowing through the woods, steelhead are up over the old bank and holding on the foot path that winds through the woods on the inside bend.

River smallmouths migrating to wintering habitat around a plunge pool in fall follow the same rules. They duck behind lay downs and bridge abutments on their way upriver. Unless the water is low, they follow the inside of a bend, where the current velocity is about 25 percent (sometimes more, sometimes less, depending on gradient) of what it is on the outside of the bend. They tuck in behind ridges, wing dams, gravel bars, and whatever current breaks they can find to aid them in their progress.

Most (if not all) species of fish that migrate against current  in rivers seem to behave in a similar manner.

Smallmouths migrating downstream to winter in reservoirs follow entirely different rules. They won’t need a path of least resistance until next spring, when the begin heading back upriver to summer spots. During fall, they have the current to aid them in their endeavor.

Greater detail, in an upcoming post, on those downstream movements of smallmouths.

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