Inside Angles: Hydrodynamic Vortices—Fish Footprints, Fish Fingerprints

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It was my search for an explanation for why so many fish—walleyes, pike, muskies, bass, stripers, hybrid stripers—so often take boottail swimbaits so deeply that lead me to fishery scientist Stephen Spotte and his book, Bluegills—Biology and Behavior.

In-depth and enlightened writing about the biology of the bluegill might seem an unlikely place to find answers about fishing with swimbaits, but in no other scientific discussion have I seen such detailed analysis of swimming movements and how such movements are sensed by fish with their lateral line.

Spotte talks about the hydrodynamic vortices generated by fish as they swim. He says: “The wake left by a swimming fish or other aquatic animal slowly attenuates (dies), but not without alerting intercepting predators or prey of its owner’s location and distance. These hydrodynamic ‘trails’ have been playfully called ‘fish footprints.’ The size of a wake depends on the size of the fish, but its morphology varies by species. If ‘fish footprints’ are indeed unique to species and if they can be recognized and used to advantage by conspecifics or predators, they might better be named ‘fish fingerprints.’”

He says that trails remain distinguishable from the background noise for several minutes and may, in still water, remain detectable after 5 minutes. He mentions one process by which a muskie goes about feeding on a prey species—after noting that fish, without any visual cues, can continue to feed effectively using only their lateral line, but they can’t continue to feed effectively when the lateral-line sense is blocked.

Spotte says that a muskie often feeds in two phases. “It sulks stealthfully toward its prey using the pectoral and caudal fins, then strikes quickly. The process involves vision for initial target detection, but the lateral line assumes dominance during the strike.”

So, predatory fish happening through the “trail” of another fish can tell what fish species it is (fingerprint it) and likely even tell if it’s swimming badly—wounded or injured. Likewise, predatory fish wouldn’t have trouble tracking a school of minnows or other small prey fish, sensing the school in larger context as it moves.

This is what’s at work when fish see and then track and strike a lure—and, obviously, not just a swimbait. We know the lateral-line sense is for close-quarters sensory perception having to do with low-frequency vibrations. Once a fish sees something interesting it swims steadily toward the offering, gets behind it, and tracks it.

Once the lure or baitfish is in a zone about 1 to 2 feet in front of the fish, vision isn’t so important anymore. At this point the lateral-line sense kicks in and the fish almost goes on autopilot, relying on that sense to make the final judgement about whether or not to eat the thing. Again, it’s lateral-line sense, not vision, that likely tells a fish to break off the chase and avoid the lure at the last second—or go ahead and eat it.

This understanding of how important the lateral line is in getting fish to bite is at the very heart of the presentation process—how we choose and manipulate lures. The reason tighter-running crankbaits often are more productive than wide wobblers during early season has as much (or more) to do with vibration as it does visual appearance. It pays to tinker with crankbaits (or spinnerbaits or blade jigs) with different vibration patterns in order to find the pattern they prefer at that moment.

Think about the vortices left by a muskie spinnerbait with #10 blades. In earlier times, #7 blades were considered big and it was a surprise to many anglers how productive the giant blades could be. Again, the strike process starts visually, but we know now that those big spinning blades produce distinctive and appealing low-frequency vibrations.

This, too, is the reason it pays to experiment with various trailers on bass jigs. I have long categorized my trailers by the vibrations they give off, as much as much as by their visual appeal. Especially during summer, I find that bass often prefer the lighter, faster, flickering vibrations given off by a trailer like the Berkley PowerBait Pit Boss. Big flappers like those on a Chigger Craw (one of the best soft trailers of all time) can be good all season but tend to shine in colder water. Meanwhile, trailers like the PowerBait MaxScent Creature Hawg (a hot one for me the past two seasons) “go with the flow” so to speak—that is, they have little inherent movement beyond what you give them with rod tip movements. This category also can be exceptional all season long.

So it is that we need to calculate how vibration plays a role in our lure presentation. The reason certain lures, including so often paddletail swimbaits, so completely fool fish is because the fish is getting feedback via the lateral line, telling it that this thing feels perfectly natural as the fish closes in, traveling in the vortices of the lure. It’s alive—or the closest thing to it.

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