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FOR IMMEDIATE RELEASE: Monday, March 6, 2006
CONTACT: John Lott, 223-7700
PIERRE, S.D. – Gizzard shad were found throughout the Missouri River system before the dams were built, but were not present in Lake Oahe after it was created in the 1960s. Now, many anglers are wondering why gizzard shad have recently become abundant in Lake Oahe?
For many years, it was believed that Oahe Dam was the northern edge of the gizzard shad range, and Lake Oahe was too cold for them to survive.
Adult gizzard shad were stocked into western South Dakota reservoirs, like Angostura and Shadehill, in the 1990s to help increase the food supply for walleyes. During the high water years of the late 1990s, gizzard shad passed over the spillways of these reservoirs and entered the Cheyenne and Grand Rivers, eventually finding their way to Lake Oahe.
“This shows that there are very few absolutes in this world,” noted Game, Fish and Parks Fisheries Biologist John Lott of Pierre. “The first gizzard shad sampled by Game, Fish, and Parks fisheries crews were caught in nets in 1999 in the Cheyenne River arm of the reservoir. By 2001, young shad were being caught during the standard shoreline seining survey, and from 2003 through 2005, the majority of small fish caught in seines throughout Lake Oahe were gizzard shad.”
Are gizzard shad “established” in Lake Oahe? Lott says only time will tell.
“In lakes like Sharpe and Francis Case, there is a history of annual production of young shad that goes back to the creation of the reservoirs,” he said. “In Lake Oahe, some of the shad produced each year have been surviving cold winter temperatures and reproducing during the past five to six years. However, the fact that substantial shad production has occurred the last four years is encouraging.”
The big question now is how important will shad become to predators in Lake Oahe, and how will walleye distribution, growth, and the ability of anglers to catch them change if they are feeding on shad?
How will gizzard shad affect walleye fishing? In South Dakota, gizzard shad generally begin spawning in mid to late May, and young shad become large enough to attract the attention of predators, like walleye, by early August. It is possible that the addition of young shad to the Lake Oahe food base may mean walleyes will have plenty to eat in shallow water during August, making them harder for anglers to catch.
“A slow walleye bite has traditionally occurred during August in the upper portion of Oahe, where young white bass, perch and drum often fill walleye stomachs by late July,” Lott explained. “The abundance of young shad throughout the reservoir the last few years may mean walleye will be harder to catch in the middle and lower portions of Oahe in August, as well.”
Another important factor influencing how easy walleye are to catch is water temperature. High summer water temperatures are often associated with a slow walleye bite. Water temperatures are usually cooler in the middle and lower portions of Oahe, than in upper Oahe during August. Lott thinks cooler water temperatures might mean a better bite, even if young shad are abundant.
Gizzard shad may also be influencing the walleye bite during the winter. “The walleye bite in the Mobridge area of Oahe this winter has been slow, and anglers report seeing dead and dying shad on the lake bottom when using their underwater cameras,” Lott said. “Walleyes caught by anglers have also been full of shad. While it’s nice to have lots of food for walleyes, it may mean fewer walleyes in the bucket or livewell at the end of the day.”
Will gizzard shad help walleye grow faster?Fish grow the fastest when water temperatures are optimal for growth and food is abundant. For walleye in Lake Oahe, that means staying in water between 55 and 65 degrees in the summer and eating rainbow smelt or lake herring. In Missouri River reservoirs like Sharpe and Francis Case that have shad as the main food source and warm water temperatures during the summer, walleye growth is slower than for walleyes that stay in cooler water all the time and eat fish like smelt and herring, like is possible in Lake Oahe. According to Lott, the middle and lower portions of Lake Oahe may have walleye that move into warmer, shallow water or go above the thermocline to eat shad and then go back to deeper, colder water to rest and digest their food.
“During times when smelt numbers are low, shad will help Oahe walleyes stay healthy,” he said, “but Oahe walleyes would grow fastest on a diet of smelt and herring than a diet consisting mostly of shad.”
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