Lake Oahe, one of the most prestigious walleye fisheries in the country, experienced an up and down year in 2022. Anglers were able to enjoy some of the best, if not the best, trophy walleye fishing in the country. However, anglers targeting “slot fish”, or fish between 15-20 inches, experienced difficulty locating these fish throughout the reservoir. Lake Oahe is commonly referred to as a “boom or bust” fishery, but the reservoir itself is more complicated than this.
To fully understand this special fishery, one needs to consider the impact of water levels, the forage base, predator and prey dynamics, and the engineering of the dam structure itself. Looking into the past allows us to look at what the future holds and truly understand the dynamics of every aspect of the reservoir.
A Tale of Two Fisheries
South Dakota Game, Fish and Parks (GFP) splits Lake Oahe into two management zones, Upper Lake Oahe and Lower Lake Oahe. Historically, Lake Oahe had been managed by three zones; an upper, middle, and lower portion. Now, the 212 Bridge serves as the divider between Upper Oahe and Lower Oahe.
These two zones are managed distinctly because they have different characteristics. Upper Oahe generally consists of a higher abundance of fish, but a smaller size structure compared to the lower zone. This means there are a lot of fish overall, but they are not as big as the fish in the lower zone. Overall, there is good recruitment of new fish coming into the system in Upper Oahe and the system itself is overall more productive than Lower Oahe.
The lower zone of Lake Oahe is the exact opposite. Lower Oahe’s numbers of fish are generally lower, but the size structure is overall larger. Essentially there are fewer, but bigger fish. There is also weak recruitment, which means very rarely are there good year-classes fish on Lower Oahe and the system is overall less productive than Upper Oahe.
Understanding the History of the Reservoir
Historically, one of the most important factors to look at when evaluating the fishery is fish abundance. This is done via sampling through gill nets, which catch fish and biologists then essentially count the fish in each net. The catch per unit effort, CPUE, is the number of fish per net. CPUE is then evaluated over historical trends and time.
Through time, Upper Oahe generally has more walleye than Lower Oahe. The early to mid 90s experienced some of the highest numbers of walleye ever in the system. The first prey fish collapse took place at the end of the 1990s, in 1997. The number of fish dropped in both Upper and Lower Oahe. Through the 2000s, the fish populations were building until the flood of 2011.
Following the flood of 2011, the rate that fish were declining was the fastest ever seen. This decline took place via entrainment (fish being flushed through the dam), harvest, and natural mortality. At this time, the fish looked very skinny because they were essentially starving to death. They simply didn’t have any food available for them. Right after the flood Lower Oahe saw a steep decline in abundance, significantly more so than Upper Oahe.
The next metric to look at when judging a walleye fishery is the condition, or relative plumpness, of the fish. Biologists use Wr to measure condition, 80-90 is a good Wr, while anything above is a really heavy fish and anything below could be characterized as in poor condition.
When following the trend through time for both Upper and Lower Oahe, the fish are generally between 80-90 for Wr, but this measurement follows the same story for abundance. Fish were very healthy going into the late 1990s until the first prey collapse in 1997. Fish condition starts to plummet very soon after this first preyfish collapse.
The same story is said for the 2000s, fish were in very good condition right up until there was not enough food present in the system to support the abundant number of walleye. The 2011 flood caused fish condition to drop drastically. The drop occurred across the entire lake, but Lower Oahe experienced the most staggering of the drops.
The third component to look at when assessing walleye populations is length at age-3. To standardize these measurements, otoliths (ear bones that are used to age fish) are collected and all age-3 fish lengths are averaged to determine growth. This varies year to year based on forage and other conditions, but overall is a good indicator in fishery health.
Following the first prey base collapse in 1997, length at age-3 was about 14 inches in Lake Oahe. After the prey came back, growth rates were closer to 17 inches by age-3 through the 2000s. The second preyfish collapse hit in 2011 and length at age-3 returned to 14 inches. Overall, these trends for Lake Oahe run the gamut from the best growth in the state to some of the worst, depending on forage conditions.
Similar to condition, growth following the 2011 flood started returning in 2013, 2014, and by 2015 the fish were closer to 15 inches. There were fewer fish in the system, they were in good condition, and starting to grow well. In the mid-2000s, smelt were coming back but gizzard shad were very important for the fishery during this time.
In 2004 and 2005 Lake Oahe was at a much lower elevation. There are warm water springs in Lake Oahe and when the lake gets lower, these springs have a more significant impact on the water around them. The last time there was low water, gizzard shad persisted through wintertime. When water rises, the springs have less influence on the cold water around them.
Historically, following the flood of 2011 walleye abundance dropped significantly. Fish were in poor condition and there was not enough forage in the system for them. By 2014 and 2015, fish condition was starting to rebound and growth was coming back up. There were still few walleyes in the system, but growth and condition was on par. This led to a change in management of the fishery as a whole.
Fish Stocking Begins
Previously, aside from experimental stockings in the late 80s that concluded in 1991, GFP did not actively stock Lake Oahe for warm water species. Cold water species, such as salmon, had been the only stocking efforts. The abundance rates that occurred through the 1990s and 2000s were entirely through natural recruitment.
This trend changed when GFP started stocking efforts in 2017. From 2017-2022, there were 5.7 million fry and 5 million small fingerlings, 1 ½-2 inch walleye, stocked into Lake Oahe. Most of these stockings were focused on Lower Oahe, because the abundance dropped so significantly following the flood of 2011. While abundance in Upper Oahe was dropping, Lower Oahe was dropping much faster. Additionally, the condition and growth of fish on Lower Oahe was better, which meant there were more food resources in this section of the lake and enough prey to support these big stocking events.
Overall, there were 17 stocking locations during this period. The farthest north was Swan Creek, with the farthest south being at the face of the Oahe Dam. The vast majority of these stockings occurred on the lower ⅓ of the lake.
To ensure there was enough food in the system to support these stockings, GFP also started stocking gizzard shad at this time. Gizzard shad were critical for the recovery of the 1997 flood and became an important component of the walleye fishery in Lake Oahe. Previously, there were not an abundant number of shad in the lake, but GFP wanted to get those numbers back.
To stock gizzard shad, GFP utilizes trap and transport methods from Hipple Lake in Lake Sharpe. Essentially, biologists go into Hipple Lake and collect adult gizzard shad to move up to Lake Oahe so they can spawn and reproduce.
Gizzard shad are extremely fecund, meaning large female shad can produce up to half a million eggs. Due to this fecundity and the nature of these fish, there does not have to be a significant number of adults in a fishery to produce a lot of offspring.
Gizzard shad stocking efforts were focused on both Lower and Middle Oahe Oahe, but this was also during a period of high water events, which allowed GFP to work with North Dakota to stock gizzard shad in the upper portion of Lake Oahe across the North Dakota border with the goal of allowing shad to spread out along Upper Oahe.
2017 to Present
Since 2017, the pendulum has started to swing and GFP is now seeing the number of fish in Lower Oahe start to increase. The abundance of fish on the lower end are increasing from these stockings. On the contrary, the abundance of fish on Upper Oahe is gradually declining year after year.
In regards to the prey base, the smelt population is recovering and shad stocking efforts have led to an acceptable condition of fish. Currently, Wr is between 80-90 over the last six years for both Upper Oahe and Lower Oahe. This is an indicator there is enough food in the system, which means the need is now more walleye in the system.
Following the bottoming out point from the 2011 flood, from 2016-2020, the growth of fish is increasing, the prey base is building, and fish are growing faster. Walleye abundance is improving in Lower Oahe, but slowly declining in Upper Oahe. This leads to the primary focus of getting more fish into all of Lake Oahe. GFP’s plan to do this moving forward is to stock more fish.
2023 Stocking Plans
The stocking plans for 2023 is to stock 2.8 million walleye fingerlings, with a focus on the middle and upper end of Lake Oahe. This plan includes stocking 400,000 fingerlings per location at 7 locations. These locations range from as far north as Pollock Bay to as far south as Sutton Bay. Other stocking locations include Indian Memorial, Indian Creek, Walth Bay, and Swan Creek.
While there are other spots that would be desirable to stock, access to the lake is critical. These stocking locations are based on where there are boat ramps so the stocking trucks can get the fish to the water.
These fingerlings are going to be stocked in the beginning of June. Most the stockings on the Missouri River will be fingerlings, but extra fry are accepted from the hatcheries when they are available. One of the biggest stocking years was 2.2 million, which will be surpassed by 2023 at a requested 2.8 million.
Through monitoring, stocking efforts on Lower Oahe have proved to be working. Evaluations were conducted on the stocking year-class of 2018, which was 2 million fish. Following sampling, it was estimated that 48% of the fish in areas on Lower Oahe were stocked fish. This means without these stockings, there would have been half as many fish present. The 2023 stocking plan will equate to some of the largest walleye recruitment efforts in the fishery.
The 2023 stocking plan is one of the biggest stockings GFP has ever done, this means there is also a goal to continue stocking gizzard shad, primarily in Upper Oahe. There needs to be enough food in the system for 2.8 million walleye so they can keep growing and stay in good condition. To maintain these rates, the plan is to ensure there are plenty of gizzard shad.
In 2023, trap and transport efforts from Lake Sharpe will continue. The stocking plan includes 300 adult gizzard shad at each location at four different locations. Additionally, GFP is working with Kansas to stock an additional 4,500 total adults in Lake Oahe. Cumulatively, this is three times the number of gizzard shad stocked in the past. The gizzard shad from Kansas will be stocked at six different locations. These stockings are complementary to the gizzard shad stockings from internal trap and transport methods. Essentially, these are extra gizzard shad for the system.
Challenges of these gizzard shad stockings, particularly from Kansas, are focused on the ability to get the gizzard shad physically into the system. Gizzard shad notoriously do not haul well, they are a notoriously finicky fish and there will be lost shad. When shad are stocked at ramps, there will be dead fish present at stocking locations due to the difficulty of transporting these fish. The loss of gizzard shad is acceptable, because the fish that do survive do very well producing young fish.
The other greatest challenge to gizzard shad stockings is over winter survival. Significant numbers of gizzard shad die over the winter, which means these annual stocking efforts are critical until there is a mild enough winter for shad to survive and reproduce on their own.
Moving forward beyond 2023, the plan will be to flip flop Upper and Lower Oahe for a stocking focus. Lower Oahe was the part of the reservoir that was hit the hardest from the 2011 flood. Now, Upper Oahe needs the attention and in 2024 the stocking will be Lower Oahe. This will continue until the reservoir is back to where the population is desired. There will be no stocking unless there is prey, which is why gizzard shad are incorporated. The other reason for alternating Upper and Lower Oahe is stocking back to back years in one area is when you see a decline in growth and competition between year-classes.
Walleye Forage in Lake Oahe
A variety of species are present in Lake Oahe for a forage base for walleye. These include rainbow smelt, ciscoe (lake herring), gizzard shad, yellow perch, freshwater drum, crappie, and other warm water species.
In the perfect system, gizzard shad would be present to raise walleye to about 3-5 years old and then they would be able to switch over to the smelt and ciscoe to finish their growth. This would grow fish quickly to 15 inches, then they would be able to take advantage of the bigger prey resources as they reached more trophy caliber sizes.
Rainbow smelt estimates from 2021 are around 45-50 million fish in the system, of which half were adults and half were juvenile. While compared to the “hay days” of the rainbow smelt numbers, 45-50 million fish is still low, but the smelt populations are holding steady from previous years and slowly increasing.
Generally, these rainbow smelt populations have inhibited ciscoes. Ciscoe have always been in the system, in fact they are the dominant prey item in Fort Peck. It was in 2011 that ciscoes turned the corner after the smelt were flushed through the dam and were not around to eat the eggs and larval ciscoe. This has now led to an increased number of ciscoe in Lake Oahe.
Currently, the smelt are concentrated on the lower end of the lake because of the lack of cold water habitat above the 212 Bridge, where growth rates are looking positive. The greatest concern for growth rates is on the upper end of the reservoir, where growth rates are slower and smelt are lacking.
The recent low water levels are also creating poor spawning conditions for forage, such as smelt. In the upper portion of Lake Oahe, when there is flooded vegetation the spawns of freshwater drum, white bass, perch, and crappies all present a viable food source as age-0 fish for walleyes. The low water conditions are giving the opportunity to create vegetation along the shorelines and when water levels increase, there should be greater abundance of both food and walleye spawning.
Gizzard shad were selected as a forage stocking over other options for a variety of reasons. Gizzard shad grow very fast, in the first year by fall there will be gizzard shad that are as large as 6-7 inches in the system. This is because gizzard shad will spawn right away once they are put into the lake.
What makes gizzard shad unique is after the first initial spawn, which occurs at about 55-60 degree water temperatures, they will keep spawning throughout the summer, essentially as long as water temperatures are greater than 65 degrees. This means as summer progresses into fall, there will be gizzard shad of various sizes present in the system because the forage base keeps replenishing itself. The greatest challenge for gizzard shad, as stated above, is over winter survival. If none of the fish survive that winter, the next year stocking efforts will have to continue to maintain this trend.
As stated above, gizzard shad played a significant role in the return of the fishery following the preybase collapse of 1997. These fish were not stocked by GFP and the population took off because of a series of mild winters coupled with low water conditions, where warm water springs in the lake were able to produce warmer pockets of water. Gizzard shad are a boom or bust forage base, which is why biologists are constantly monitoring conditions and continuing stocking efforts as deemed necessary to continue the trend.
Contrary to gizzard shad, rainbow smelt would be difficult to stock for many reasons. The first is the need for a clean source of smelt, previously this had been the Great Lakes but diseases now prevent these fisheries from being an option. While local sources, such as North Dakota, may be an option, the only time biologists are able to stock smelt is right after they spawn or as they spawn to be successful. This means a significantly more number of smelt need to be stocked to get the return from gizzard shad.
The location of gizzard shad in the waterbody also is a critical component. Dropping water levels and lake elevation is causing the loss of cold water habitat, which is critical for smelt, in Upper Oahe. These dropping water levels are concentrating the cold water habitat below the 212 Bridge, which forces smelt to move both further downstream and deeper in the water column in search of this cold water habitat. Even if smelt were to be stocked in Lake Oahe, they would not be a viable solution for the forage base in Upper Oahe at the current lake elevation.
These smelt also will dive deep in the summer. In the spring, smelt run throughout the lake and may be anywhere in the water column as cold water habitat exists across the lake. As the water heats up they follow that cold water and will tuck into the cold water as deep as they can go and will either get trapped in warm water and die or move down river to where there is cold water. This is why the rainbow smelt population is increasing on Lower Oahe.
Contrary to smelt, gizzard shad prefer warm water habitat. This means shad will stay shallower for a great amount of time in the warm water habitat and keep fish shallower and more attainable to anglers for a longer duration throughout the year.
An additional forage option, ciscoe or lake herring, are recovering as well. Ciscoe grow fast and are available to most walleye up until ciscoe reach age-1, but following their age-1 year because they grow so fast the only walleye that are able to take advantage of ciscoe is those really big walleye, which is why the Wr of large walleye is so high. The ciscoe are self-maintaining and have not been stocked in Lake Oahe since the 80s.
The impacts of stocking specific species also has an impact on the fishery, even if they are just prey species. Any stocking will need to include adults that produce millions of offspring. One option may be yellow perch, but perch have specific requirements to spawn successfully. Perch hang their stainings on submerged vegetation and currently there is no vegetation in the system for this to occur.
Influence of Water Levels
Water levels play a major role for both the walleye and forage populations in Lake Oahe for a variety of reasons. These include the loss of fish through the dam, but also the dynamics that take place along the shoreline with these changing conditions.
The Oahe Dam is unique in that it has a mid-column release. This means depending on where the water elevation is, water is pulled from 25-50 feet. This is critical for understanding entrainment, because fish would have to be suspended in 25-50 feet of water to be swept through Oahe Dam. While there are some fish lost, they are primarily not walleye.
This is what led to the issues from 2011. Rainbow smelt, which do suspend in the cold water habitat at these depths, were swept through the dam while the walleyes were not, with the walleyes staying in the lake.This led to these walleye being too abundant for the forage available to them.
By contrast, Garrison Dam does not lose smelt like Oahe Dam in a flood. This is because Garrison Dam has overflow gates. This means the majority of water lost is the warm water at the surface, not the cold water in the middle of the column. When Oahe floods, it flushes out of the bottom of the lake, which includes smelt. When Sakakawea floods, it loses the top few feet of water, which allows it to keep its smelt. Sakakawea was still in really good shape following the flood of 2011 because it let loose the top few feet of water and kept all the smelt.
In contrast to flooding, dropping lake elevations also present challenges. These include reduced productivity, which reduces food available for young fish, and negatively impacts recruitment overall.
However, low water elevation is a positive situation for the long term health of the fishery. When the water levels drop, vegetation grows along the shorelines. When the water then comes back up, this creates nursery habitat and greatly increases productivity in the system.
These conditions set up abundant year classes, such as what was experienced in 2009. This was the biggest year class on Lake Oahe because of the low waters, then rapidly flooding vegetation. In a very short time, fish were everywhere. Another positive to low water is that wave action cleans rocks as water drops, which is primarily the top three feet of the water where fish will be spawning.
Moving Forward
Moving forward into 2023, the future is looking very positive for Lake Oahe. The management efforts taken by GFP will ensure there is both abundant walleye and baitfish populations available in the lake. A favorable winter and spring for precipitation would have a positive impact on water levels, causing the flooding of vegetation and increasing productivity, recruitment, and overall fish abundance in the fishery as well.
Lake Oahe is a special fishery, but it is also a complex fishery. Understanding how these dynamics impact the fishery itself throughout the course of time is critical to understanding the future of the waterbody as well. When at it’s peak, Lake Oahe is a national destination for anglers and all signs indicate the fishery will continue to live up to its billing as one of the best fisheries in the country.
Listen to Lake Oahe Public Meetings
In October 2022, GFP held a public meeting to discuss the status of the Lake Oahe fishery in Mobridge, South Dakota. A full recording of this meeting is available on the GFP YouTube.
In January of 2023, GFP held public meetings in Pierre, Aberdeen, and Rapid City to discuss the status of the Lake Oahe fishery. A full recording of the Pierre meeting is available on the GFP YouTube.
