Case Studies in Fisheries Conservation and Management: Applied Critical Thinking and Problem Solving
Case 17: Managing Lake Oahe Walleye in the Face of an Imbalanced Food Web
Fisheries biologists are often required to make rapid management decisions during a crisis. Because swift action is usually demanded by the public, biologists rarely have the luxury of taking a long-term, conservative approach to making decisions, but rather rely on prior knowledge to choose a management plan and hope that they predict correctly. This case study will provide you with a real example of fisheries biologists who were faced with choosing a course of action during a rapidly developing situation. Fisheries biologists in South Dakota who manage Lake Oahe (a large Missouri River reservoir) were challenged to save a very large and popular walleye fishery after the prey base crashed.
Lake Oahe is a large impoundment of the Missouri River. It has a surface area of approximately 371,000 ac, has 2,250 miles of shoreline, and is 231 miles long. The walleye fishery in Lake Oahe developed during the 1970s, and was rated among the top 10 walleye lakes in the country from the mid-1980s until 1998 (the year that the prey base crashed). Walleye are supported by a non-native pelagic prey source (rainbow smelt), which became established in the system following escapement from upstream systems (where they were initially brought in as a prey source). When rainbow smelt were abundant, walleye growth rates were very high, reaching 20 inches by age 5. Following a drought during the late 1980s (where water levels and volume declined substantially), the Missouri River drainage experienced a series of wet years during the early-mid 1990s. These conditions enabled Lake Oahe to quickly recharge, and productivity increased dramatically resulting in abundant populations (much higher than levels that had ever been observed in this system) of both walleye and rainbow smelt. So, the mid 1990s were the “boom” years for anglers and biologists on Lake Oahe—anglers were very happy with the fishery, the local economy benefited, and the biologists had achieved a rare benchmark in fisheries science—producing LOTS of BIG fish… However, a compounding series of conditions resulted in the crash of rainbow smelt, starting in 1997.