Abstract
There are many reasons why resource managers move plants and animals from one location to another. Understanding how the organisms disperse after relocation is an important aspect to any relocation action. This study was done to evaluate the post-stocking movement of Eagle Lake rainbow trout (Oncorhynchus mykiss aquilarum). This trout is endemic to Eagle Lake, California and has many characteristics that no other rainbow trout have. The unique traits of these fish are thought to be a result of the complete isolation that these fish historically enjoyed in the terminal Eagle Lake watershed. To study the post-stocking behavior of this strain of trout, eggs and sperm were collected from wild fish spawning at Eagle Lake, California. The offspring were raised in a hatchery for about one year and then transported to the nearby Lake Davis watershed, which had recently been treated with a piscicide by the California Department of Fish and Game in an attempt to rid the lake of northern pike (Esox lucius). Eight hundred of these wild strain Eagle Lake rainbow trout were tagged and stocked into Cow, Big Grizzly, Freeman, and Old House creeks, which are the four main tributaries of Lake Davis. The tags used were Floy T-bar anchor tags that each had a unique number used to identify individual fish. The location and size of each stocked fish were recorded at the time of stocking. After the fish were stocked in May of 2008, the tributaries were sampled for fish approximately once per month, ending in September of 2008. Each time a tagged fish was captured, the GPS location and size of the fish were recorded. These data were used to evaluate post-stocking growth and movement patterns of Eagle Lake rainbow trout in the Lake Davis Watershed. Additionally, the effect that the tags and electrofishing had on the condition of fish was evaluated. Rainbow trout are often observed moving downstream after being stocked into a stream or river, but other types of trout have been observed to move upstream following stocking. This study evaluated whether the unique Eagle Lake strain of rainbow trout moved downstream like many other strains of rainbow trout after stocking. The average post-stocking movement of fish in three of the creeks was 365 meters in the downstream direction. The fish that were stocked in the fourth creek moved an average of 178 meters upstream, but it is thought that this was caused by a barrier to downstream movement. On average, the Eagle Lake rainbow trout in this study moved in a downstream direction shortly after being stocked. A few fish moved long distances, up to 3000 meters, but the majority moved slightly downstream. The downstream movement appeared to be short-lived because the fish remained relatively stationary following the first sampling event, which was 27 day after stocking. It is often postulated that trout populations include static individuals and mobile individuals, and that mobile trout are presumably those that are unable to establish territories, and static trout are those that can establish territories. Additionally, after trout are stocked it is often noted that some individuals stay near the stocking location, and others move great distances away from the stocking location. Because stream-dwelling salmonids compete for space in streams, some of the variation in movements may be attributed to competitive ability. To evaluate the influence that competitive interactions have on post-stocking movement, size at stocking was compared to dispersal distance. The results of this study did not indicate that relative size at stocking was related to post-stocking movement; however, design limitations may have led to the uncertainty. The creeks in this study were electrofished once a month for five straight months. The main driving force for this electrofishing effort was to monitor for pike. While conducting these pike surveys, the recently stocked trout were captured and data was collected from them. The effects of electrofishing and handling on the trout were evaluated by comparing the condition of fish that had previously been captured to fish that had been captured for the first time during each sampling event. The results of this study indicate that repeated electrofishing and handling significantly reduced the condition of the trout. This research also evaluated the effect that the Floy T-bar anchor tags had on the condition of the Eagle Lake rainbow trout. During each of the sampling periods, the condition of tagged fish was compared to the condition of non-tagged fish from the same size range. There was no significant difference in the condition of tagged versus untagged.