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The Shad ZonePeter B. Mathiesen
Freshwater shad are arguably the most numerous of all baitfish in America. In many lakes the shad population accounts for more than 50 percent of the total fish biomass. There are two primary freshwater species, Threadfin shad, Dorosoma petenense, and Gizzard shad, Dorosoma cepedianum. Other common shad, American, Hickory and Alabama, are migratory species that commonly invade freshwater estuaries or rivers to spawn, but live most of their lives in salt water.
In the spring, shad school together to release massive amounts of eggs and milk. A single adult female releases more than 400,000 eggs during the spawn. The eggs are fertilized as they fall to the bottom of the lake and the tiny fry emerge in less than a week. Like most fish a shad’s first meal is microscopic zooplankton and protozoa. Later the primary food source becomes free-floating algae, (phytoplankton) and in many watersheds, insect larvae. In very clear northern lakes with sterile environments desperate adult gizzard shad have been known to actually suck algae and moss off the bottom.
Threadfin shad have little tolerance for low temperatures.
“Forty five degrees is about all they can take,” said Ken Weathers, Biologist for The Alabama Department of Conservation. “When the temperature falls dramatically, we have seen large shad kills in northern Alabama on the Tennessee River, although their numbers quickly recover.”
Despite their sensitivity to temperature, Threadfin shad flourish throughout warm water southern reservoirs and into some Midwestern lakes. As you move farther north and west threadfin become less numerous and are often stocked to maintain a viable food source for bass.
Growth rates for threadfin are surprisingly different than Gizzard shad. Most of the young of the year threadfin are two inches by late summer, the perfect bass meal. They will reach three to four inches by October in most of the near south and southern impoundments. Topping out at four to five inches, a threadfin will remain a potential bass food its entire life, particularly where fish in the 7-pound plus category cruise.
Shad are a clear barometer of water quality and bass populations. Parallels are just beginning to be understand in bass numbers and growth, relating to threadfin reproduction. When the shad numbers are strong, the bass are thriving. When the shad crash, the bass numbers fall.
Visiting the Shad Zone
To find shad much of the year you need to visit open water, where light penetrates above the thermocline. During the day shad will move deep enough to stay out of direct sun light, yet the water must have enough light to produce algae and plankton. These depths vary dramatically based on the clarity of a lake and the amount of algae bloom.
“The spot to fish in the summer on many lakes is where the thermocline in low light meets with structure like a drop-off or hump. Shad will stay there during a hot bright summer day and so will bass,” said Weathers.
It may not be popular to find bass in open water, but trolling can be efficient.
“Set a couple of rods with different depth crank baits, motor slow, and watch your depth finder for schooling bait fish. Once you establish a couple of hits at the same depth and see baitfish on your screen, mark the spot, note the depth and start casting. Key in on working birds and where shad are breaking water and then fish under the school. Most of the action on top is from smaller bass. The larger fish will usually hit the school from the bottom,” said Weathers.
You can find gizzard shad in most large impoundments in the U.S., but bass only chase them for a limited period of time.
“After the spawn the gizzard fry just explode,” said Elmer Heyob, Fisheries Biologist with the Ohio Department of Natural Resources. “They will be 1.5 to two inches in mid summer, but by late fall they’re often five inches long and a little too large for the majority of our Ohio largemouth bass. When conditions are perfect gizzard shad can actually spawn a second time.”
Gizzard shad, like threadfin, are working open water feeding on plankton and algae. Finding them requires a similar game plan. As you travel to more northern latitudes, due to temperature range and colder lake temperatures, gizzard shad will frequently occupy more shallow depths than in the south.
Throughout the summer months, depth finders can be the key to establish and zone in on schooling baitfish and to help narrow where you need to pitch your baits. By mid to late fall gizzard shad in most impoundments are just too large for all but the biggest bass in the lake to eat. To keep up the chase, change your pattern to a 2.5-inch bluegill, a readily available food source.
Roll it Over
Around Labor Day much of the Midwest will experience a big wind and a cool heavy rain will turn the lakes over creating dramatic changes in gizzard shad behavior. As the lake turns over you’ll find baitfish more evenly spread out on your depth finder. In lower-light conditions they will quickly congregate in shallow coves and protected water. Look for where a flush stream or creek empties. The turbid water and large amounts of silt and organic matter can transform into food and attract schooling baitfish.
“We’ve seen these coves black with gizzard shad. Largemouth will quickly maraud their way in and attack anywhere there is an edge or drop-off to deep water, it can be explosive,” said Heyob.
Keep in mind that if there is a low shad population, bluegill or perch (on far Northern Lakes) will play a much stronger role in summer and fall patterns and replace shad in the food chain.
The next time you’re reaching for a chrome Rat-L-Trap think about the lake you’re fishing and its population density of threadfin and gizzard shad. Locate where the thermocline meets the structure and spend some time idling watching your screen to establish a schooling pattern. If you’re in a primary gizzard shad lake, don’t wait for the fall temperature change, you’re already in the prime bite.