Cultus Lake sockeye recovery program

Photo: Trapnet in Cultus Lake

We intend to aid the recovery of the Cultus Lake sockeye population by modifying factors affecting their survival in freshwater. The objective is to improve both freshwater survival of sockeye salmon, monitor and assess the improvement, and increase our understanding of what the factors affecting survival.

Objective

We intend to aid the recovery of the Cultus Lake sockeye population by modifying factors that affect their survival in freshwater. The objective is to improve both freshwater survival of sockeye salmon, monitor and assess the improvement, and increase our understanding of the factors affecting survival.

Context

All of these proposed projects are aimed at meeting "Recovery Objectives 1 - 3" in the "National Recovery Strategy for the sockeye salmon (Cultus population)". In particular, this project will further address components of Objective 3:

Is freshwater productivity adequate to support recovery? Analysis of historical data provides some evidence that, when spawner abundance is less than about 7,000 fish, freshwater productivity is lower (20-30 smolts/spawner) than in years of higher abundances (>60 smolts/spawner; see Biological Limiting Factors). The low productivity at current abundances will limit the population's potential for recovery or sustainable use. An increase in productivity as the population recovers is, therefore, an important indicator of recovery.

The project will also implement (at least partially) the approaches outlined in Table 2 of the plan:

Project components

The project has four components:

  1. Northern pikeminnow
  2. Eurasian water milfoil
  3. Sockeye residence freshwater period
  4. Project assessment and documentation

1. 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 (game fish such as trout and char and other native species such as suckers and shiners) we will only use 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:

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 Dr. 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 areas of pikeminnow overwintering concentrations 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.

2. 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 rototiller 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 labor intensive and slow (4 divers removed about 0.1 to 0.3 ha/day depending on bottom characteristics). Hand pulling assisted with Geoduck harvesting stingers may be a more efficient technique and will be investigated. The time required for milfoil to recolonize from this form of removal is unknown but recolonization is expected after one or two years.

We will continue with some form of milfoil removal in the winter of 2006/2007 in partnership with the Cultus Lake Parks Board, Soowahlie First Nation, the Fraser Valley Regional District, and provincial Ministry of Lands and Parks, 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 analyzed. 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.

3. Sockeye freshwater residence period

Adult Pre-spawning sockeye 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 SFU that will allow us to track tagged fish, map the lake bottom using the Quester Tangent system, and recover spawned-out fish using their 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.

Juvenile sockeye assessment - Juvenile sockeye will be assessed periodically using standard hydroacoustic and midwater trawling surveys. These estimates will enable us to evaluate the abundance, survival, distribution and growth of both wild and supplemental sockeye and will assist in the determining the potential impact of northern pikeminnow on sockeye, by allowing us to determine density dependent seasonal mortality and the seasonal overlap in distribution between sockeye and northern pikeminnow. These surveys will also provide information on the SARA listed Cultus Lake pygmy sculpin, which is the subject of current research by a graduate student of Dr Rick Taylor of UBC.

4. Project assessment and documentation

The Cultus Lake recovery program involves a large number of components. It is essential for improving future efforts and that proper evaluation, analysis and documentation is conducted on an ongoing basis. Some of this will be done through the normal course of the project but here we identify funds needed for mapping of appropriate data and for the development of interpretative models necessary for placing the data in context.

Timeframe

Ongoing efforts are required in order to maximize gains and to ensure that recovery of the sockeye population is assisted by northern pikeminnow and Eurasian milfoil removal efforts. Northern pikeminnow are highly productive and a concerted and continuous effort will be required to significantly reduce their abundance. We anticipate that efforts will need to extend over approximately 8 years, 2 complete life history cycles (2005--2013). Effective methods of milfoil removal can be accomplished only in relatively small areas, and as complete eradication is not possible, cleared areas will need to be redone every 2-3 years. The hypothesized benefits of milfoil will not be evident until the cumulative impact of the removal affects the adult pikeminnow population: this could take upwards of 8-10 years.

Participants

Lead DFO Investigators:

Partners

Proposed deliverables and impact on recovery

  1. Aggressive removal of pikeminnow during the spring and summer of 2006 could contribute to improved juvenile survival. This will be measured by the 2006/07 and subsequent smolt/spawner ratios derived from assessment project results. However, the actual impact of pikeminnow on salmon survival remains unclear. Assessment activities that are focused on the sources of salmon mortality will assist in determining the role of pikeminnow relative to other predators.
  2. Tagging of pikeminnow will provide us with the knowledge of their location and movements at all times of the year. This will assist us in determining their impact on sockeye and will allow us to better target control efforts.
  3. Tagging of adult spawners will provide us with knowledge of their distribution and movement during the lake resident period. This is currently unknown and could provide information of habitats that need protection. Locating spawners and recovering them will assist in evaluating methods for estimating reproductive success.
  4. Removal of milfoil could lead to sockeye survival benefits if the removals are significant and sustained. However, the impact of macrophyte removal on pikeminnow juveniles will not assist sockeye recovery in the short term, as results will be lagged due to the approximate 6-8 years it will take pikeminnow to reach sufficient size to prey on juvenile salmon.
  5. Improved understanding of spawning/incubation habitat may allow us to direct our recovery activities for the greatest benefit. For example, the observed site fidelity for pikeminnow suggests that if spawning is concentrated into a few areas, pikeminnow removal should be focused there to locally deplete the abundance of predators.

Risks compromising success

We have experience in most aspects of this study resulting in a high probability of successfully executing the project. The ultimate measure of success - sufficient increase in understanding to direct recovery efforts, as well as an actual increase in survival will depend on the underlying assumption of this project: that alterations to the habitat and biota of the lake can increase salmon survival. There are many scientific uncertainties that could refute this assumption.