Arkansas Fishing-Norfork-Bull Shoals Lake-White-Buffalo-North Fork River
Radio Tracking Program & Minimum Flow Study
Purpose: Study Movement of Resident Brown Trout in the Bull Shoals Tailwater in Response to Differing Levels of Dissolved Oxygen and Spawning.
By John Stark, State Trout Biologist, District 2 Arkansas Game & Fish Commission
Behavioral responses of holdover or wild brown (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) were monitored to evaluate the effects of low dissolved oxygen on them. During portions of 1992-93, 44 TL 400-500 mm (16-20 in.) trout (32 brown, 12 rainbow) were successfully implanted with radio transmitters and followed until transmitter failure or trout mortality occurred.
Brown trout exhibited differential responses to various levels of dissolved oxygen. At lower oxygen levels (3-4 mg/l) positive selection occurred for areas of highest available oxygen. At higher oxygen levels (6-8 mg/l) areas of highest available oxygen were often not utilized; which corroborates that the laboratory derived "acceptable" value of 6 mg/l is accurate for "acclimatized" fish in uncontrolled environments.
Many trout seeking higher dissolved oxygen levels were not found in areas that had adequate current shelters; indicating that trout seeking higher dissolved oxygen levels were unable to locate areas that had both sufficient oxygen and adequate current shelters. Trout holding under these conditions faced greater energy costs. Additionally feeding efficiency, angler catch rates and therefore trout growth were probably detrimentally impacted below 6 mg/1 because current shelters are often excellent feeding stations.
Important information on brown trout movement and spawning migration was also obtained. Large scale pre-spawning movements of nearly all the tagged trout occurred in the latter half of October during both study years. Although largely stationary for much of the year, during the spawning run brown trout traveled upstream as much as 13-16 km (810 ml.) per day. Post-spawn migration occurred in early January and the majority of individuals returned to within 55 m (50 yds.) of the area in which they were located prior to the spawning season; which strongly suggests that brown trout in this system have well established home ranges.
Management recommendations included an operational target of 6 mg/1 dissolved oxygen particularly during the brown trout spawning season and instream habitat improvement including current shelter placements.