METHODS

Radio transmitters were surgically implanted in 29 grass carp during the spring 2007 stocking at Lake Gaston. Fish were then tracked at various intervals from boats and airplanes.

DISPERSAL & MOVEMENTS OF GRASS CARP

We observed extensive movement immediately following release. Tagged carp moved only 28 yards on average during the first 2 hours, but 0.8 miles within 6 hours and over 2 miles within the first 24 hours. Overall dispersal was quite different between the two release points: 1.7 miles for the fish in the upper region of the lake and 5.1 miles for the fish in the lower region. These differences are most likely a result of the abundant shallow-habitat surrounding the upper release point, while the lower region of the lake consists of deeper water with limited shallow flats. Fish simply have to move farther in the lower lake to find their preferred habitat. Future stocking plans might consider planting fish at more stocking sites, in order to achieve more uniform dispersal of fish around the lake.

Our tracking efforts revealed limited movement following initial dispersal. Monthly movement averaged only 177 yards, and was slightly less during the summer months compared to the spring or fall. Daily movements averaged just over 55 yards, and the fish appeared to be slightly more active during the day than at night. The aerial survey in December 2008 showed that no fish have moved out of the Lake Gaston system.

ATTEMPTS TO TAG ADULT GRASS CARP

To allow us to compare movements between juvenile and adult grass carp, we attempted to capture and radiotag adult fish from previous stocked cohorts. We conducted electroshocking operations for 4 days on Lake Gaston. We successfully collected a number of adult grass carp (up to 80 pounds), but these large fish do not seem to tolerate the stress of handling and radio-implant surgery. We were unsuccessful in implanting and releasing any adult fish. The best way to characterize movements of adults may be to continue to track the presently tagged fish as they mature. Our radios should have 1-2 years of remaining battery life.

ESTIMATING MORTALITY OF GRASS CARP

Similar to other radiotracking studies that have been conducted with grass carp, we found that many fish went inactive or missing over the period of our study. Inactive fish (those that have not moved for 3 months) may be a result of mortality, or the radiotag may have been physically expelled by the fish; high rates of expulsion have been noted in some other studies. Missing fish may be the result of electronic failure of the tags. If missing fish had been removed from the system by predators (e.g., eagles or ospreys), we would have likely located these tags during the aerial survey; we found no evidence of such removal. Based on inactive and missing fish, we estimate mortality of radiotagged fish to be a maximum of 72% during the study period. Given that some tags may have failed or been expelled, true mortality is likely much less, perhaps in the range of 30-40%. Mortalities ranging from 25-90% have been reported in similar studies.

To confirm post-surgical mortality or tag expulsion, and to more accurately estimate natural mortality, it would be necessary to hold fish in a confined environment. To do such in a laboratory setting would not provide realistic results. Perhaps the only way to realistically estimate these factors would be to confine some tagged fish in a more natural setting, such as an enclosed cove of the lake.

Michael B. Duncan and Brian R. Murphy, Virginia Tech
February 12, 2008