Balancing Fisheries Management and Water Uses for Impounded River Systems

<em>Abstract</em>.—Following the collapse of the primary prey fish species in Lake Oahe (rainbow smelt <em>Osmerus mordax</em>), liberalized harvest regulations were imposed on walleyes <em>Sander vitreus</em>, the primary piscivore, to decrease predation on smelt and facilitate their recovery. To evaluate the success of this regulation, we conducted a 5-year tagging study to evaluate fishing and natural mortality rates before and after the regulation went into effect. Total annual mortality for both male and female walleyes increased substantially in the 3 years following liberalized regulations (53% preregulation to 75% postregulation). Mortality rates were similar across the size range of female walleyes during the 2 years prior to the liberalized regulation but increased with female size after regulations were enacted. Mortality rates for males generally increased with total length but in a similar fashion before and after the regulation. Exploitation rates increased slightly after the regulation (18% preregulation to 22% postregulation), primarily due to harvest of smaller than 381mm walleyes. Although fishing mortality increased after liberalization, the overall decline in walleye abundance was primarily due to natural mortality of larger (>381 mm) individuals. Bioenergetics modeling showed that smelt accounted for 86–99% of consumed energy by walleyes when smelt were abundant (1993–1994) but only 20–46% after the smelt population declined (2001–2002). Size-specific patterns in net energy gain showed distinct differences between the two periods. In the 1990s, consumption of rainbow smelt alone resulted in positive net energy gains ranging from 370 to 2,304 kcal/fish/year. However, in the 2000s, consumption of rainbow smelt was insufficient to meet daily energy requirements, resulting in negative net energy values (–1,015 to –133 kcal/fish/year). To grow under these conditions, walleyes larger than 350 mm had to consume alternative prey that included macroinvertebrates and other prey fish taxa. Because the predator–prey imbalance in our study was largely compensatory, regulation changes aimed at improving the prey base (i.e., increasing smelt abundance) should be carefully planned and implemented. Our analyses showed that rapid implementation of regulation changes (i.e., minimize lag time) combined with increased harvest of older, larger predators provided the most parsimonious response to this situation by allowing harvest of fish that were otherwise lost to natural mortality.