Cultus Lake Pikeminnow and Milfoil Update
Posted on May 10th, 2006 by David Barnes, Fisheries and Oceans Canada

Fisheries and Oceans Canada has secured funding to support ongoing Northern Pikeminnow and Eurasian Water Milfoil removal programs for 2006. A summary of what we propose is below.

Northern pikeminnow

Removal - Pikeminnow prey on juvenile Cultus sockeye, contributing to a "predator pit" that may prevent their recovery from current low abundances. We estimate that there are approximately 60,000 to 70,000 adult pikeminnow (>20 cm) in Cultus Lake. We have developed fishing techniques that resulted in the removal of 4,000 to 5,000 pikeminnow in the late spring and early summer of 2005 when the fish were onshore. Due to the presence of non-target species (gamefish such as trout and char and other native species such as suckers and shiners) we are limited to techniques that will have a low bycatch and allow the live release of non-target fish. We intend to increase our catch of northern pikeminnow by expanding current techniques and investigating different approaches. These include:

1. Stationary Traps - Four trapnets (1 more than last year) will be fished from early May until mid-July. These were one of the most effective methods used in previous years. As well hoop-nets, modified black cod traps, and modified prawn traps will be fished in the spring and summer and at other times of the year on an exploratory basis.
2. Angling - Angling has been a very successful technique for capturing northern pikeminnow- 40% of the fish caught last year were by rod and reel. As well as the annual pikeminnow derby on the lake, we will conduct one or possibly two additional pikeminnow outings. These are sponsored by the Fraser Valley Salmon Society and supported by the local guides as well as DFO. We may also cover expenses (boat gas and tackle) for up to five FVSS experienced volunteer anglers. The DFO netting crew will also angle for northern pikeminnow as part of their duties.
3. Purse seining - Purse seining was used in the 1990's with reasonable success. This method appears most successful when spawning and feeding aggregations located in shallower waters are targeted, generally at night. Commercial fishers will be engaged in June and July to purse seine in the lake. The presence of our regular trapnet and angling crew will be able to inform the seine crew on aggregations as they appear in the lake to target fishing effort. An observer will be placed on the seiner to collect biological information and tags from the catch.
4. Other methods - We are investigating other techniques that have proven effective elsewhere. One method, long-lining, has proven effective on pikeminnow in some areas on the Columbia River, but has never been used on Cultus Lake pikeminnow. We will investigate its utility for our circumstances and, if warranted, we will conduct pilot programs with the aid of commercial fisherman and others. Past reports and data have a wealth of distribution and behavioural data that will be examined for potential harvest opportunities that are not currently utilized.

Assessment - All fish collected by the removal programs will be subject to biological sampling. Continued recovery of tags that were applied to adult pikeminnow in 2004 will allow the refinement of our estimates of abundance, survival, movement, and gear selectivity.

Last year's work showed that northern pikeminnow show exceptional site fidelity and were found at the same feeding and spawning sites from one summer to the next, but because of the limited spatial and seasonal extent of our sampling we know little about abundance in other locations in the lake, their winter habitats and daily diel movements. Early work by Ricker suggests that pikeminnow predation on juvenile salmon is likely greatest in the fall and winter months, and our fry surveys indicate that juvenile mortality is higher in the winter than in the summer. We intend to use acoustic tags and an underwater receiver array to track the seasonal and daily movement of northern pikeminnow. Knowledge of their location and movements at all times of the year will assist us in determining their impact on sockeye and will allow us to more effectively target removal efforts.

Locating the pikeminnow during the fall and winter months with the acoustic tagging program will enable us to assess whether this a time of significant pikeminnow predation on juvenile sockeye. This will be done by sampling for diet, the use of stable isotopes and by comparing the distribution of pikeminnow with the distribution of sockeye as determined by seasonal acoustic and midwater trawl surveys. The tagging work will also enable us to determine if there are concentrations of pikeminnow in overwintering areas that will provide opportunities for targeted removals in the winter.

Along with the seasonal acoustic/trawl estimates of sockeye fry abundance this portion of the project will increase our knowledge of pikeminnow/sockeye interactions and allow us to better design future control efforts: by refining our pikeminnow population estimate; by increasing our knowledge of pikeminnow life history and by refining our pikeminnow-sockeye interaction model.

Eurasian water milfoil

Removal - This invasive plant covers approximately 30 ha, or most of the littoral area of Cultus Lake. It adversely affects sockeye spawning ground quality and quantity, decreasing the available spawning habitat. It also extends the rearing habitat for juvenile northern pikeminnow, perhaps increasing pikeminnow survival and consequently predation pressure on sockeye juveniles, although this is an untested hypothesis. We intend to conduct annual removal of the milfoil for two purposes: 1) improve spawning ground habitat; and 2) decrease juvenile pikeminnow rearing habitat.

Of the possible techniques available to remove Eurasian water milfoil, only a few have proven to be feasible in Cultus Lake, these include the use of a small "harvester", equivalent to scything the upper 5 ft of the plant, and hand pulling. Harvesting creates clear swimming areas but has little effect on sockeye survival and may cause an increase in milfoil coverage because the fragments regenerate quickly. Previously, a floating rotatiller was used on the lake, however it is only feasible to use in very limited areas, is not readily available, requires an experienced operator and would be excessively expensive to build.

Eurasian milfoil was removed in March of 2006 by hand pulling using SCUBA divers. It was effective but labour intensive and slow (4 divers removed about 0.1 to 0.3 ha/day depending on bottom characteristics). Hand pulling assisted with suction dredging was suggested as a more efficient technique but excessive sediment mobilization and increased turbidity are likely to make this method unacceptable. The time required for milfoil to recolonize from this form of removal is unknown but recolonization is expected after one or two years.

While also investigating the use of suction dredging, we propose to continue milfoil removal using hand pulling in the winter of 2006/2007. Milfoil removal will be done in partnership with the Cultus Lake Parks Board (confirmed), Soowahlie First Nation (confirmed)), the Fraser Valley Regional District (tentative) , and provincial Ministry of Lands and Parks (confirmed), who will contribute financial, administrative, and in-kind support. We propose to continue contributing to this removal and use the additional diver time to target affected spawning areas in Spring Bay and possibly Lindell beach. Areas of concentrated juvenile pikeminnow abundances will also be targeted.

Assessment - we will monitor both the distribution of Eurasian water milfoil and the effectiveness of control mechanisms through annual surveys of milfoil removal sites to quantify the effectiveness and duration of removal efforts. The use of milfoil by fishes was investigated by DFO in 1991 but the data have never been analysed. We have secured these data and will prepare a technical report on the results. We will also assess the impact milfoil removal has on the utilization of sockeye spawning grounds, and if necessary we will conduct field studies of the effect of milfoil removal on juvenile pikeminnow habitat and its possible effect on northern pikeminnow survival.

In addition to the programs above, we are seeking funding for research into adult pre-spawning use of Cultus Lake. Adult sockeye enter the lake from early August to mid-November but do not spawn until November and December. Their distribution in the lake during this four month period is unknown. Spawning can occur at considerable depth and the location of spawning is not known with certainty, nor is the fate of spawned-out fish. Consequently, estimating spawning success (pre-spawning mortality) is very difficult. Estimating the success of spawning is critical because it could be an important contributor to the relatively low rates of smolt production (as indexed as smolts/spawner) in recent years. We intend to use acoustic tags to determine the survival, movement, spawning location, and final resting place of fish arriving during various stages of the run. Physiological sampling will be conducted on fish at the time of tagging at the adult fence. We have provisionally secured the use of a multipurpose research boat from Simon Fraser University that will allow us to track tagged fish, map the lake bottom using the Quester Tangent system, and recover spawned-out fish using their Remote Operating Vehicle (ROV). The Quester tangent system will provide detailed mapping of potential spawning ground areas by using advanced hydroacoustic analytical methods to classify the bottom substrate composition. Currently very little is known about sockeye redds in Cultus Lake and possible problems with the survival due to gravel quality or predation. We will use the ROV and divers to find sockeye redds and conduct basic assessment work on the survival of eggs within these redds.

Partners

• Cooperative Resource Management Institute, Simon Fraser University
• Cultus Lake Parks Board
• Fraser Valley Salmon Society
• Fraser Valley Regional District
• Ministry Of Lands and Parks
• Soowahlie First Nation
• Area E Gillnetters Association
• Cultus Lake Community Association

I am the local contact for these programs so please call with any questions or concerns.

Dave Barnes
Research Technician, Cultus Lake Salmon Research Laboratory
Phone: 604.824.4710 | fax: 604.858.3757
Email: barnesd@pac.dfo-mpo-gc.ca

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