Like many other anglers, I am always researching ways to catch more largemouth bass. While watching a fishing show featuring Rick Clunn, a legendary tournament angler known for his intellectual out-of-the-box approach, a point he made really struck home with me. That is, far too many anglers spend all their time researching the habits and behavior of largemouth bass. Time may be better spent researching habits and life histories of their prey species. Why? Because with the exception of the largemouth bass spring spawning period, prey species habits and movement control a majority of largemouth bass location and behavior (i.e., find the bait and you find the bass). This article focuses on the life history of the primary prey species at Sam Rayburn and Toledo Bend reservoirs: gizzard and threadfin shad.
Gizzard and threadfin shad are closely related species. Both are planktivores (filter-feed microscopic, free-floating plants and animals with closely spaced gill rakers), grow rapidly, have short life spans, and associate in large schools. Unique characteristics include an elongated, back dorsal fin ray and keeled midline from the throat to the anal fin. Under 4 inches in length, both species are similar in appearance with blue or gray backs and silver sides, but are discernable by the shape of the snout and color of the tail. The gizzard shad has a rounded snout with a subterminal mouth, whereas threadfin shad have a pointed snout and terminal mouth (i.e., can open lower jaw by sliding finger off snout). Threadfin also have a yellow-colored tail. However, gizzard shad reach much larger sizes (> 10 inches) than threadfin shad (4 inches = average adult size).
Threadfin shad are the more abundant species in both Sam Rayburn and Toledo Bend reservoirs. Threadfin shad are very prolific. Spawning begins in the spring when water temperatures reach 70 degrees and continues through fall until temperatures drop below 70 degrees. Some fish that were spawned in the spring will reach sexual maturity and spawn in the fall. Spawning usually occurs in large groups along the shoreline or vegetation edges, as eggs are adhesive and sink. Spawning is typically restricted to the first several hours of daylight. Threadfin shad are not tolerant of cold water temperatures, as die-offs occur if water temperatures fall to below 45 - 50 degrees. Young and adult threadfin eat phytoplankton and zooplankton, and primarily feed near the water surface. Average lifespan is 2 - 4 years. Average adults are 3 - 5 inches long and maximum length is 8 inches.
Gizzard shad spawning habits are similar to that of threadfin, but spawning is more likely to occur throughout the day. Although gizzard shad can tolerate water temperatures near freezing, die-offs can occur due to sudden temperature changes resulting from severe cold fronts. Gizzard shad seem to prefer plankton and typically grow very fast (4 - 7 inches within the first year). They also reach much larger sizes than threadfin shad, as adults exceed 10 inches and maximum size is 20 inches. This can be problematic in some reservoirs, as adults get too large to be eaten by bass and other predators and can comprise over half of the total fish biomass. In turbid reservoirs that have limited abundance of plankton, gizzard shad can adapt and feed on detritus (decaying organic matter typically found on the bottom).
Unfortunately, the scientific community knows little about habitat use or movement patterns of gizzard and threadfin shad. As you might expect, shad are highly mobile and tend to school in very large abundance during the day, likely for protection from predators. However, at night shad tend to break from tightly packed schools and scatter. Generally, smaller shad orient near the water surface, while adult shad tend to be evenly distributed from top to bottom.
However, I suggest that anglers can take advantage of three predictable patterns of shad movement. 1) As stated above, when water temperatures are above 70 degrees, threadfin shad spawn in the early morning around cover, especially on vegetation edges. It is common for largemouth bass to pattern feeding around these morning spawns, which can pay big dividends to anglers. 2) It is common sense to assume that shad distribution, like bass, is primarily controlled by their prey (plankton). Because of this, wind blown points and banks can concentrate free-floating plankton. Plankton attracts shad, which draws feeding bass. Many anglers get discouraged with high winds, but controlled drifts with the waves on wind-blown areas can put fish in the boat on an otherwise slow day. 3) I could find only one scientific study that examined seasonal movements of shad. During the summer, shad were primarily off-shore in open water. However, in the fall the majority of shad left the open water and hit the banks, both during the day and night. Shad tended to be uniformly scattered in shallow water, both in main lake areas and in coves. Therefore, beating the banks and covering a lot of water may be the most productive fall bass fishing pattern.
Todd Driscoll is a district fisheries management biologist for Texas Parks and Wildlife Department where he has worked for 10 years. He received a B.S. in Fisheries Biology from Kansas State University and a M.S. in Fisheries Management from Mississippi State University. His primary responsibilities include fisheries management of Sam Rayburn and Toledo Bend reservoirs in Southeast Texas. Todd is an avid bass angler and participates in approximately 25-30 local and regional bass tournaments per year. He also represents Lowrance Electronics as a Technical Pro Staffer, working tournament support and service at BASS, FLW, and owner’s tournaments across the country.
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