Vancouver lamprey (Lampetra macrostoma) recovery strategy: chapter 3

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3. Threats

Given their extremely restricted distribution, Vancouver lamprey can be considered especially susceptible to a variety of local threats.  Much information is lacking on the general biology and habitat use of the species, which makes a thorough threats assessment difficult.  Nevertheless, it is possible to identify general threats, and these are discussed below.

Water Use.-- There are a number of active water licences on Cowichan Lake and its tributaries, and many more downstream on the Cowichan River and its tributaries.  Water licences on the lake and its tributaries may affect Vancouver lamprey.  Active water licences are summarized in Table 1. 

There are two storage licences and 60 diversion licences on Cowichan Lake, plus two storage licences and 173 diversion licences on tributaries.  The largest licences by far are the two storage licences owned by Norske Canada (now Catalyst Paper), which together sum to 61,304,950 m³ · year^-1, and are used to provide flow into Cowichan River for diversion to the pulp mill in Crofton and a minimum flow to the river for ecological purposes.  Storage is provided by a low weir (0.96 m of storage) at the outlet of Cowichan Lake.  In the winter wet period lake level is typically higher than the top of the weir, so maximum lake elevation has not been significantly altered from the pre-weir state.  The weir has a series of four gates that control flow in the river from April 1 to September 30.  During this period, lake level and river discharge are controlled according to the MoE storage rule curve and the release schedule, which ration water stored in the spring season for release into the river during the summer (M. Vessey, Catalyst Paper, personal communication).  In essence, the weir and its operation simply release Cowichan Lake water more uniformly through the dry season than would occur naturally.  Before construction of the weir, river flows would often fall to 1 m³ · sec^-1 or less, but can now be maintained at 7 m³ · sec^-1 during a normal summer (M. Vessey, Catalyst Paper, personal communication). 

Dam construction and water storage has the capacity to alter lake levels and thereby affect aquatic habitat.  Impacts may occur if lake levels are raised or lowered, or if fluctuations differ from normal.  To help assess whether lake levels have been markedly altered we examined lake level data since records began being collected in 1913.  We compared lake level data from before and after the weir was constructed in 1957.  Results are summarized in Table 2.

Annual fluctuations are approximately the same in both periods, and have been somewhat more stable during the lamprey spawning period after the weir construction.  Diversion amounts are only approximately 6% of the storage amounts.  Both storage and diversion appear to pose at most a minor threat to Vancouver lamprey.  It should be noted, however, that this assessment is subjective, and has not been ground-truthed.

There are two water licences on Mesachie Lake, one a consumptive license for 8,018 m³ year^-1, the other for the purpose of constructing and operating a fish counting fence.  The consumptive licence equates to approximately 1.4 cm of lake depth, an amount that seems unlikely to cause substantial impacts to Vancouver lamprey.

In addition to licensed users there are likely unlicensed water users in the Cowichan and Mesachie watersheds.  The threats to Vancouver lamprey posed by unlicensed water users is not known, but seem unlikely to exceed the threats posed by licensed users.  Taken together, licensed and unlicensed water use does not seem to be a substantial threat at this time.

Increased development in the Cowichan Valley has and will continue to put pressure on the ecosystem.  The Cowichan Valley Regional District is currently leading the development of a water management plan that attempts to balance the requirements of fish and wildlife with human needs.  Success of the water management plan in protecting the conservation requirements of Vancouver lamprey will need to be assessed in the future, perhaps through monitoring lamprey populations and habitats.  The needs of Vancouver lamprey should be considered in water use decisions, at present and in the future.  To ensure that long-term changes in water use do not adversely affect lamprey recruitment, it would be particularly useful to assess potential impacts of drawdowns, and set bounds for water level fluctuations that would protect eggs and ammocoetes.

Land Use.-- Some land-based activities have the capacity to alter aquatic habitat directly (e.g., impacts to riparian habitat, alteration of run-off rates) or indirectly (e.g., changes to water quality through introduction of pollutants).  These activities include forestry, mining, and land development for residential or industrial uses.

Forest harvesting is a prominent industry in the Cowichan watershed.  The primary potential threats to aquatic habitat from forestry practises include deposition of sediments and fine woody debris, riparian habitat destruction, and changes in hydrology and water quality.  There is substantial literature demonstrating the negative effects of suspended sediments on fish and egg survival.  Moderate increases in fine sediment inputs associated with logging may be a benefit for lamprey ammocoetes in sediment-poor drainages, because ammocoetes rear in depositional habitats with fine sediments, conditions that are often lacking in high gradient coastal watersheds (Beamish 1998).  Excessive sediment inputs, however, may negatively impact spawning areas and perhaps other lamprey habitats. Given the long history of forest harvesting in the Cowichan watershed, and the apparently stable status of Vancouver lamprey, the risk of extinction from this potential threat is not believed to be substantial.

A review of information on mineral extraction indicated no mining activity at present or in the recent past in the Cowichan Lake watershed (Ministry of Energy, Mines and Petroleum Resources 2005).

Development pressures exist in the Cowichan Valley and specifically around Cowichan Lake.  This pressure comes in the form of increased demand for water, alteration of nearshore and surrounding lands through construction of buildings, roads, and docks, as well as recreation in and around Cowichan Lake.  Of particular concern is impact to littoral spawning areas.  Assessing this threat will require additional information on the distribution and utilization of spawning habitat for Vancouver lamprey.

Water Quality.-- There are no known water quality issues within Cowichan or Mesachie lakes.  Chemical spills associated with industrial or residential development have the capacity to negatively impact Vancouver lamprey, however this risk does not seem extreme.  The recent closing and dismantling of the Youbou Mill has likely lessened the inputs of industrial pollutants.  There has been a steady increase in recreation and residential property development in the Cowichan Valley, but the density of this development is still fairly low.  High winter rainfall in the area likely leads to a reasonably high flushing rate of both Cowichan and Mesachie lakes.  This threat may require additional assessment in the future as related information becomes available.

Recreation.-- Cowichan Lake is a popular recreation site for tourists and local residents. Fishing, boating, swimming and hiking all take place frequently in the watershed.  The primary potential threats from recreation activities are competition for prey species through fishing, negative effects on water quality via pollution from power boats, and disturbance or destruction of shallow littoral areas.  Given the large perimeter of Cowichan Lake and the fairly localized areas of recreation use, these threats do not appear to be substantial.  However, a proper assessment of these threats will require a greater understanding of lamprey habitat use and distribution of suitable habitats within the lake.  The extent of these threats on Mesachie Lake is not known.

A direct source of mortality for Vancouver lamprey is their destruction when incidentally caught by recreational anglers targeting salmonids.  Anglers are known to destroy lamprey that are parasitizing their catch.  The threat from this mortality source is unquantified, but thought to have an impact on the adult population.

Prey Base.-- Adult Vancouver lamprey are obligate external parasites of resident fish of Cowichan and Mesachie lakes.  It appears that salmonids, in particular young coho, are preferred over other species (Beamish 1982).  Human impacts on salmonid species (e.g., recreational and commercial fishing, habitat destruction) are therefore expected to affect abundance of Vancouver lamprey.  Ammocoetes are filter feeders, feeding on detritus and suspended organic matter while living in burrows in soft sediments.  Activities that alter the productivity of this food base are expected to also affect abundance of Vancouver lamprey.  There have been no formal assessments of the threats to the prey base of Vancouver lamprey.

Climate Change.-- Scientific evidence clearly indicates that climate is changing and animal and plant distributions are responding to these changes (Parmesan and Yohe 2003).  Since climate affects precipitation, water flow and temperature in many ways, it may also affect Vancouver lamprey abundance and distribution.  This threat is of concern; however, it presents a less immediate risk to the lamprey population than other threats, and at present the topic is considered beyond the scope of this recovery strategy.  The threat may be assessed and addressed at future stages of recovery planning for Vancouver lamprey.