Department of Fish and Game

Lake Davis Fisheries Management

Return to Lake Davis Fish Sampling INDEX

Preliminary Summary of 2000 Field Season Work

The Managing Northern Pike at Lake Davis: A Plan for Y2000 outlined two areas of work for the 2000 field sampling season; 1) Monitoring of the fish populations, and 2) Control and Containment of northern pike in Lake Davis. This is a brief preliminary summary of our work during the 2000 season. Specific results for each activity, assessment of effectiveness, and recommendation for modifying activities in future field seasons will be addressed this winter in a more thorough report.

Monitoring

The Y2000 plan recommends several approaches to monitoring the northern pike population at Lake Davis:

  • electrofishing
  • trap nets
  • gill nets
  • "pike eggs and larvae" sampling (using a variety of methods)
  • backpack electrofish monitoring of tributaries
  • angler surveys

By definition, true "monitoring" would entail regular sampling of a random or representative selection of stations throughout the lake to generate an accurate picture of the distribution of the population throughout the lake. Because we wanted to emphasize control and containment during the field season, we did not want to expend too many resources sampling areas where pike are not yet found. However, a sampling design was developed for electrofish monitoring of the entire lake.

Boat electrofishing

We were able to conduct monitoring of the entire shoreline of Lake Davis on a bimonthly schedule, beginning in April 2000 and concluding in November of 2000. Generally, two electrofishing boats were used to sample all shorelines of the lake. The lake was divided into six sampling segments (Figure 1).

All fish were collected during the electrofishing boat monitoring and were identified to species and counted, with sub-samples taken for total length. The location of each northern pike capture was recorded either on a map or by use of a GPS unit. All fish were released back into the lake, with the exception of northern pike which were retained for additional data collection, i.e. sex, weight, stomach content, and otolith collection for aging. The pike were then frozen and sent to the lab in Rancho Cordova for DNA analysis. Additionally, pulsator output for each boat was recorded so that the catch per unit of effort (CPUE) for each species could be determined. The CPUE will be used to determine trends in numbers for the various fish populations.

Trap nets

As described above, a lake wide sampling design for trap net monitoring was not developed. However, locations for all trap nets used in control and containment activities (see below) were recorded, and data on all northern pike and other fish were collected as described for the control and containment activities.

Gill nets

As described above, a lake wide sampling design for gill net monitoring was not developed. However, locations for all gill nets used in control and containment activities (see below) were recorded, and data on all northern pike and other fish were collected as described for the control and containment activities.

Pike egg and larvae sampling

Manual beach seining was occasionally conducted in shallow areas along Mosquito Slough, and Cow and Freeman Creeks.

Backpack electrofish monitoring of tributaries

Cow, Freeman, and Big Grizzly Creeks were monitored approximately each month, between the lake and the Forest Service Road 24N10. During most of the field season, the entire length of the creeks were electrofished, with records kept of length and species of each fish caught. Location of any northern pike (only found in the lower reaches of Big Grizzly Creek) was noted. During the latter portion of the field season, the monitoring design was altered to one of specific monitoring stations. This alternation will be easier to implement and make the data more easily comparable with that collected by the Department of Water Resources (all portions of the creeks, however, were still electrofished for control and containment purposes).

Angler Surveys

Regular angler surveys were conducted on both week and weekend days.

Telemetry

A radio telemetry study was incorporated as part of the monitoring program for northern pike. In order to monitor daily and seasonal activities (including migration and spawning) several northern pike have been implanted with radio transmitters.

Control and Containment

The Y2000 plan outlined 12 management activities for the control and containment of pike in Lake Davis. The following is a brief summary of the various activities and the accomplishments for the year 2000. Figure 2 depicts number of northern pike caught using sampling methods.

  1. Use of experimental control measures -
    1. Barrier Net - A barrier net was installed across Mosquito Slough in November of 1999. The net remained in position until April 2000, at which time it was removed due to the fact that the lake level had risen to the point where it had completely inundated the net, making it ineffective in containing pike within Mosquito Slough.
      In November of 2000, a newly designed block/trap net was installed across Mosquito Slough. It is hoped that this net will contain any pike within Mosquito Slough. In addition, it will allow for the capture of any pike which are present on the lake side when they move into the Mosquito Slough area, particularly adult pike moving into the slough in the early spring for spawning. The net is 20 feet deep and is designed to float as the water levels rise. We plan to leave the net in place to facilitate removal of pike from the slough area using electrofishing boats.
    2. Detonation Cord - The Department is investigating the use of detonation cord to remove pike on the upstream side of the net. It is not anticipated that the use of detonation cord would occur until the spring of 2002, pending the outcome of the CEQA review process and approval by the Steering Committee and local community.
    3. Encircling Nets - The use of encircling nets has not been used at this point. We have not found any concentrations of pike in any other areas of the lake and therefore it has not been deemed necessary to employ this method at this point in time.
  2. Install tributary barriers
    1. A rack-type barrier system was designed and installed in Big Grizzly Creek, Freeman Creek and Cow Creek in the spring of 2000. These barriers were placed in the limited number of areas where there was a hydrological drop available. Water passes through the racks and prevents fish from being able to jump over the water drop and gain access to the stream above the area. One rack was placed in Big Grizzly Creek at a gabion structure, approximately 3/4 miles upstream from the lake. Two racks were placed in Freeman Creek and Cow Creek, approximately ½ mile and 1 mile upstream from the lake.
  3. Block spawning areas.
    1. The barrier/trap net, as described in section 1, was placed in Mosquito Slough to block and capture adult pike moving into the Mosquito Slough area during their spawning run.
  4. Reduce pike food supply/stock predator fish.
    1. Pike prefer the fusiform shaped rainbow trout as a forage species. Historically the Department has stocked fingerling trout as a "put-and-grow" fishery. This program was terminated in 2000, as it was felt the smaller trout provided ideal forage for the smaller northern pike. To replace the fingerling trout plants, the Department has started a "catchable" trout stocking program to maintain the trout fishery in Lake Davis. The catchable trout continue to exhibit good growth and provide an excellent recreational trout fishery.
    2. The Department stocked approximately 800 "brood" stock sized brown trout during the summer of 2000 as a possible predator fish on small northern pike. It is not know whether this has had any effect on the pike population, but the brown trout have provided a trophy sized fishery for the anglers.
  5. Encourage pike fishing.
    1. Presently, the pike population is still small and would not provide a reasonable fishery for an angler. A few pike were caught during 2000 by trout anglers.
  6. Drag nets and purse seine (Encircling nets).
    1. Small mesh, ½ and 3/4 inch, gill nets were fished throughout the summer and fall. Most nets were fished in the Mosquito Slough area, with a lesser number of gill nets fished in the Freeman Creek and Cow Creek cove areas. These nets proved to be efficient in capturing young-of-the-year (YOY), 2000 brood year, northern pike. They also captured year old, 1999 brood year, pike. Use of the smaller mesh sizes minimized the number of rainbow trout captured in the nets.
    2. A commercial purse seine operator, contracted by the Department, conducted 10 days of purse seining in August 2000. A 1,100 foot long, 40 foot deep power purse seine was used to conduct the operation. A total of 5 pike were captured during the process. Purse seining was conducted at various locations and water depths at the lake. All the captured pike were found in the Mosquito Slough area. Dense aquatic vegetative growth decreased the efficiency of the seining operation. The Department is extending the contract for the spring 2001 period. It is believed that netting efficiency will increase due to the lack of dense aquatic vegetation at this time.
    3. Department personnel also conducted beach seining activities in the summer months to capture juvenile northern pike. We were successful in capturing a small number of YOY pike. We also capture a small number of one year old pike, 1999 brood year.
  7. Increase use of electrofishing.
    1. The Department used one to two electrofishing boats on the lake 3 to 5 days per week throughout the entire sampling season, April through November 2000. In addition, we performed stream back pack electrofishing in Big Grizzly, Freeman and Cow Creeks throughout the summer and fall months.
  8. Electrofishing to herd pike.
    1. This method was not utilized during the sampling season for 2000.
  9. Actions related to Grizzly Dam.
    1. The Department of Water Resources (DWR) continues to work with the DFG to prevent the release of northern pike over or through the outlet facilities of the dam. Water levels have been kept low to prevent spill over the dam's spillway. Additionally, the DWR releases water through the lower outlets to try and minimize the chance for pike, located most likely in upper levels of the water column, from being released. In addition, the "grater" on the outlet is checked to be sure that it is functioning properly. A summer study by DWR found that any fish passing through the grater were killed. Further sampling in December indicated that small brown bullheads were observed downstream, indicating that smaller fish, less that 2 or 3 inches in length, could pass through the grater and survive. This will require additional investigation into methods to preclude the passing of fish into the lower stretch of Big Grizzly Creek.
  10. Use of fyke/trap nets.
    1. Trap nets were placed in the Mosquito Slough, Freeman and Cow Creek cove areas, and coves north of Lightning Tree boat ramp, throughout the spring of 2000. No adult northern pike were captured using this method. Several one year old pike were captured. Trap net use was suspended during the summer months after pike catch rates dropped to zero. They were again deployed in the fall with only one pike caught. These nets will again be deployed in the spring of 2001 in an attempt to capture any adult spawning northern pike.
  11. Enforcement.
    1. The Department continues to patrol the lake checking anglers to be sure they are not moving pike from the lake to other waters, or putting more pike into the lake. A new warden has been assigned to the Portola area, commencing his patrol in December of 2000. DNA analysis of the pike from Lake Davis and other samples obtained from various states are being processed in an attempt to determine possible source of the pike in Lake Davis.
  12. Public education.
    1. A Town Hall meeting was conducted in February to present the proposed Y2000 plan. It was accepted by the public.
    2. Department personnel have met with several clubs and organizations in the area to discuss the ongoing project.
    3. News releases have been made by the Department. The local media has covered several of the sampling programs being conducted by the Department.
    4. The Department maintains a web site on its home page devoted to the pike issue. Steering Committee notes are posted on this web site, as well as a listing of number of pike captured during the field season.
    5. A newsletter was sent out in the fall of 2000 by the Steering Committee to the local area giving a brief description of the ongoing program at Lake Davis. In addition, the Department has proposed having a semi-annual newsletter published and sent out to the local area. This newsletter will be published in the spring and fall of each year giving an update of the project and results.
    6. Angler information signs were posted around Lake Davis and in local businesses which alerted anglers as to regulations concerning the capture of pike when fishing and the procedures to follow for turning the pike in to the Department. In addition, the Department obtained permission from USFS for installation of additional signs and bulletin boards around the lake.
    7. Public outreach was also accomplished during angler surveys and with the crews working at the lake. Personnel informed the general public of the project and the concern for pike. Also, the Department has staffed an office in Portola with full time employees who are available to discuss any concerns and answer questions regarding the work at the lake.
The Department is continuing its investigation into how Northern pike got into Lake Davis. If you have any information, we want to hear from you. You may be eligible for a reward and you can remain anonymous. You may call our 24 hour CalTIP line at 1-888-DFG-CALTIP.