It is no secret that forage plays a critical role on the Missouri River reservoirs in South Dakota, but exactly how that forage interacts both with itself and the fish is a complex issue. Gizzard shad, rainbow smelt, and ciscoe/lake herring all provide a forage base in the Missouri River reservoirs, but their presence within each individual and role they play within the fishery can vary dramatically.
To begin, one first needs to understand the individual environments present within these reservoirs. Lake Sharpe, Lake Francis Case, and Lewis and Clark Lake all feature warm water elements that set them up to be gizzard shad driven. Conversely, Lake Oahe is larger, deeper, and colder, which makes it more of a cool to cold water environment throughout the bulk of the system. On a game fish scale, this is why it provides a salmon fishery. On a forage scale, this sets it up to be more of a smelt and ciscoe based fishery.
Let’s start by looking at Lake Oahe, as this is the most unique and complex forage base. Lake Oahe, historically, has been a smelt driven fishery. These smelt grow fish to quality sizes, and results in a strong abundance of 23-27 inch fish within the system, but doesn’t necessarily grow fish to the trophy caliber sizes we have seen out of Lake Oahe in recent years.
These gigantic walleyes, the true teeners, were a result of a significant increase in the ciscoe population. Ciscoe are very large bodied preyfish, which means they quickly reach sizes that they are unable to be consumed by smaller, more eater caliber walleye. In fact, these ciscoe are near the same size as 2 year old walleye. That said, when a walleye reaches those upper 20 inch sizes they are able to eat these ciscoe, and that’s what grows the true giants.
This recent greatly increased abundance of ciscoe is a result of the 2011 flood. This flood washed the smelt through the system, significantly depleting the overall forage base in the process. While the results to the walleye population was noticeable, a change was taking place within the forage base that many anglers overlooked.
Smelt will naturally keep ciscoe populations in check. When there is an abundance of smelt, there will be greatly reduced numbers of ciscoe. This is because of the timing of spawn within these two species. Ciscoe will spawn in the late fall, while smelt will spawn in the spring.
This sets up a situation that as the young ciscoe are emerging, the smelt are moving up into these areas and the young ciscoe provide an easy meal for these smelt. The more smelt, the more mouths attacking these ciscoe, and it creates a system that naturally results in reduced ciscoe numbers when smelt numbers are high.
The smelt crash caused by the 2011 flood, and thus ciscoe emergence, meant there was very little for small bodied forage species for young walleye to key on, and thus was impacting growth rates. This is why South Dakota Game, Fish and Parks (GFP) biologists began gizzard shad stockings, which provide an excellent forage base particularly for those walleyes less than 20 inches.
Gizzard shad grow walleyes quickly, as they are abundant and suitable for walleyes of all sizes. These gizzard shad play a critical role in the recruitment and growth of walleyes in Lake Sharpe, Lake Francis Case, and Lewis and Clark Lake. As we mentioned above, all of these fisheries above are gizzard shad driven. These fisheries are known for lots of walleye, particularly in the eater class sizes, but less of the trophy caliber fish.
South Dakota is on the northern end of the gizzard shad range, and gizzard shad are incredibly susceptible to cold temperatures. This means throughout average to cold winters, gizzard shad populations will experience significant mortality. Throughout milder winters, greater over winter survival of gizzard shad can be expected.
A good rule of thumb is if there is more than 100 days of ice coverage on Lake Francis Case, increased mortality within the gizzard shad population is likely. Fortunately, gizzard shad are prolific spawners and it only takes a relatively small number of adult gizzard shad to produce a strong crop of gizzard shad for a summer and fall of forage.
One unique element of gizzard shad is they can also spawn multiple times. This means, generally speaking, if the water is greater than 55 to 60 degrees they will continue to spawn throughout the summer and into the fall until those water temperatures begin to dip down into the mid to low 50s again.
This means that as the summer goes on, and into the fall, there are often multiple different sizes of gizzard shad present. There could be relatively large shad, which were from late spring or early summer spawns and also smaller shad from late season spawns. These shad of multiple sizes means that walleyes, regardless of size, have meals that they can key on.
The drawback of gizzard shad is that once walleye reach about 20 inches or greater, shad really struggle to grow them to larger sizes. This is when switching to a larger bodied prey fish can continue to grow fish into the mid to upper 20s, or even larger. Unfortunately, this large body prey fish isn’t necessarily present in Lake Francis Case, Lake Sharpe, and Lewis and Clark Lake. Now, these fisheries still have trophy caliber sized fish, they’re just much fewer and far between.
However, Lake Oahe does have that option, and this is what GFP biologists are managing for. If Lake Oahe walleye can grow to 20 inches on gizzard shad, transition to smelt as they continue to grow, then key in on ciscoe this is the recipe for true trophies, similar to the South Dakota state record walleyes we saw come out of Lake Oahe in the fall of 2023.
The challenge with maintaining a stable population of gizzard shad in Lake Oahe is they must be stocked, as the cold water and lack of warm water refuge areas makes over winter survival very challenging for gizzard shad. This means gizzard shad must be captured and transported from one of these warm water refuge areas, Hipple Lake on Lake Sharpe, up into Lake Oahe.
This warm water refuge area is what makes gizzard shad such a strong forage base for Lake Sharpe. As discussed above, it only takes a small population of adult gizzard shad to survive each winter and produce the next crop of forage for the reservoir. These gizzard shad survive by finding warm water refuge areas from uncapped wells and other warm water springs. Similarly, American Creek features a similar winter refuge for gizzard shad on Lake Francis Case.
The final element to consider is that shad survival is going to play a major role in the bite anglers experience the coming season. For example, in cold winters when there is significant mortality there is likely going to be very little forage present throughout the spring and heading into early summer, which will set up for a very good bite for anglers.
Conversely, when there is a mild winter and reduced mortality, there is going to be a great number of gizzard shad that survive heading into the spring and early summer, which will set up for a forage option for fish. This means walleye will not be searching for food as they may have been following a cold winter, and the bite may not be as strong. Fortunately, in these situations the overall condition of walleye is going to be much better and generally speaking will have greater relative weights.
Reservoir fisheries, particularly reservoirs as large as the ones on the Missouri River, are complex systems. Water levels, spawning conditions, forage type, forage abundance, and abiotic elements all play a major role in the ultimate recruitment, growth, and mortality of game fish such as walleyes. One of the most dynamic elements of this cycle is forage, and the individual characteristics of each reservoir impacts the type and abundance of each forage species.
Anglers who understand the type of forage present in the fishery will have a better understanding of everything from their location, presentation, and the size of the fish they are targeting. Whether it’s deploying a large minnow style crankbait for trophy Lake Oahe walleyes or a small shad profile in the early summer on Lake Sharpe understanding the forage dynamics on the Missouri River reservoirs will make you a better angler! Good fishing!
