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Recovery Strategy for Cultus Pygmy Sculpin

3. Threats

Given their extremely restricted distribution, Cultus pygmy sculpin are especially vulnerable to local threats.  Much information is lacking on the general biology of the species, which makes a thorough threats assessment difficult.  Nevertheless, it is possible to identify general threats, and these are discussed below.  Quantifying these threats is not possible until more is known about the biology of Cultus pygmy sculpin.

Exotic Species.-- Non-native species cause a range of biological impacts, from subtle to spectacular and globally are the primary driver of biotic change in freshwater systems (Sala et al. 2000).  Types of biological impact include extinction, altered abundance and distribution, altered food webs, changes in community interactions through species additions and deletions, and altered evolutionary trajectories.  When impacts occur to an extreme endemic, they are particularly worrisome since exotic species are exceedingly difficult to control or eradicate once they become established.

The exotic aquatic plant, Eurasian water milfoil (Myriophyllum spicatum L.), was first noted in Cultus Lake in 1977 and has since spread throughout the lake.  By 1991 dense mats of milfoil covered 22 ha of the lake’s 74 ha littoral area (Truelson 1992).  Milfoil has been identified as a significant threat to sockeye, which spawn on beaches of Cultus Lake (COSEWIC 2003).  Water milfoil can also cause changes to native vegetation and macroinvertebrate communities, reduce the area of shallow water habitat, deplete oxygen levels, and restrict fish swimming and forage potential (Keast 1984; Engel 1995).  Milfoil beds provide shelter for juvenile northern pikeminnow (Schubert et al. 2002), a key predator in Cultus Lake, although they also provide cover for potential prey species.

Adult pygmy sculpin forage only in pelagic waters, and it is unclear whether milfoil has affected this habitat or altered predator-prey relationships in Cultus Lake.  Cultus pygmy sculpin likely use deep littoral areas for spawning and incubation, and it is not known if they historically used littoral areas now infested with milfoil.

Other exotic species have been identified as potential concerns, notably black bullhead (Ameiurus melas), brown bullhead (Ameiurus nebulosus), pumpkinseed (Lepomis gibbosus), smallmouth and largemouth bass (Micropterus salmoides and M. dolomieui) and yellow perch (Perca flavescens).  These species have affected other fish species when introduced to new waterbodies and are believed to have the potential to affect abundance and distribution of Cultus pygmy sculpin through predation or competition.  These species do not currently occur in Cultus Lake, but their presence and their general spread through the area are of concern.

Altered Predation Rates.-- Cultus pygmy sculpin are a key prey item for char, and potentially for other species in Cultus Lake.  The majority of sculpin samples collected to date have been from gut contents of char (Ricker 1960).  Other piscivorous species in the lake include coho, coastal cutthroat trout, rainbow trout and northern pikeminnow.  However, these latter species tend to consume fewer Cultus pygmy sculpin than do Dolly Varden (Ricker 1960).  It is assumed that the pygmy sculpin has evolved in the presence of this native predator community.  However, there is a concern that predation rates could be altered through stocking or enhancement (especially of salmonids), changes in habitat, or introduction of non-native piscivorous fish species.  The extent of this threat is difficult to gauge.  Although Cultus Lake was stocked periodically from 1919 to 1987 with various salmonid species (Freshwater Fisheries Society of BC release records), it has not been stocked recently and stocking is not currently under consideration for most species.  Future stocking could potentially result in greater predation or competition pressure, depending on the species.  For example, sockeye may compete with pelagic sculpins for similar foods, and enhancement may increase competition. As part of recovery efforts for Cultus sockeye, supplementation of fry and smolts has been occurring both in the lake and the outlet creek.  Releases in the lake in 2006 exceeded 340,000 fish, but this number is still likely considerably below historic natural production. Likewise, it is conceivable that low abundance of sockeye could result in higher predation rates on sculpins due to altered encounter rates between sockeye juveniles and sculpin and their predators. 

Water Use.-- A water licence query ( Land and Water BC 2006) lists only two water licences on Cultus Lake, with an additional 50 licences on tributaries.  The licences on Cultus Lake sum to 7.14 million m3 · yr-1.  Assuming a lake area of 6.3 km2 this translates into a little more than 1 cm of water elevation.  Water licences on tributaries sum to about 10.83 million m3· yr-1, of which 6.81 million m3 · yr-1 is on inflow and the remainder on outflowing tributaries.  The licensed amount on inflowing tributaries translates into little more than 1 cm in lake level elevation.  There are likely unlicensed water users in the Cultus watershed, though they likely consume less water than licensed users. 

This brief review of water licences suggests that the majority of the annual water level fluctuation in Cultus Lake is due to evapotranspiration and variable timing of inflows and outflows.  Water use associated with agriculture, industry and domestic use has impacted tributaries (COSEWIC 2003), but Cultus pygmy sculpin are a limnetic species and do not apparently use habitats in the tributaries.  The threats to Cultus pygmy sculpin posed by water consumption is therefore deemed to be minor.  Adding strength to this conclusion is the fact that water use was not identified as a threat to Cultus sockeye, another pelagic fish species in the lake.  However, since C. aleuticus spawn in tributaries there is some possibility that pygmy sculpin also use tributary streams for spawning.  If this is the case, flows in the tributaries may be important. This highlights the importance of identifying spawning habitat for Cultus pygmy sculpin.

Water Quality.-- Water quality issues have not been raised as a severe threat to this point, although there are several concerns.  Point and non-point source pollution has the capacity to affect water quality and to degrade aquatic habitat.  Poorly-performing septic systems, inputs from agriculture and domestic fertilizers, sedimentation from land-based activities, and poor groundwater quality have been identified as concerns and have the potential to degrade lake water to some degree (Schubert et al. 2002).  Inputs from water-oriented recreation have also been raised as a concern (see below).  Water quality was assessed as excellent in 1996 (MWLAP 1996), and comparisons of recent and historic limnological information suggest that the lake’s pelagic habitat has changed little over the last 65 years (COSEWIC 2003).  It is unclear if existing levels of pollution negatively affect pygmy sculpin, but they are not identified as a substantial concern at this time. This threat may require additional assessment in the future as related information becomes available.

Water-oriented Recreation.-- Cultus Lake is one of the most heavily used lakes in BC, particularly through the summer months.  The dominant activities on the lake are power boating and swimming, but virtually all water activities are popular.  The shoreline has been affected by the construction of wharves and piers, mainly in the Lindell Beach area (COSEWIC 2003).  In addition, aquatic and shoreline vegetation have been removed and sand added to create a beach in the Sweltzer Creek area (COSEWIC 2003).  Watercraft can be a source of pollutants, through gas and oil spills.  Movement of boats among watersheds is the suspected source of introduced Eurasian milfoil (COSEWIC 2003), and is potentially a vector for other organisms, including disease organisms. 

The various activities associated with water-oriented recreation have primarily altered shallow littoral areas, but it is unclear how such changes affect the pelagic habitat used by Cultus pygmy sculpin.  Comparisons of recent and historic limnological information suggest that the lake’s pelagic habitat has changed little over the last 65 years (COSEWIC 2003).  Water-oriented recreation is not identified as a substantial threat to Cultus pygmy sculpin at this time.

Recreational fishing for any species is not common on Cultus Lake, although in recent years there have been annual derbies to cull Northern pikeminnow (COSEWIC 2003).  Despite low effort, angler activity is a concern because anglers are a common vector of introduction of exotic fish.  For example, Schade and Bonar (2005) found that one in four fish sampled in the 12 western US states were non-native, and the most common non-natives had been introduced for angling. Another potential means of introducing exotic species is through the use of fish or aquatic invertebrates as bait for fishing in lakes; however, this is illegal in lakes in British Columbia, thereby mitigating this threat.

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 water storage capacity in headwaters) or indirectly (e.g., changes to water quality through introduction of pollutants).  The Cultus Lake watershed has undergone development associated with recreational, residential, agricultural forestry, and industrial land uses.  Ninety-two percent of the shoreline has been set aside for recreation purposes and occurs either within Cultus Lake Provincial Park or Cultus Lake Municipal Park, including camping and day use areas and three large swimming areas (Schubert et al. 2002).  An active gravel mine is operating on Parmenter Road and a proposed expansion involving a rock quarry and gravel crushing and screening operation is planned in the vicinity of Hatchery Creek (Schubert et al. 2002).  Agriculture has played a significant role in the local economy in the past, however, no agriculture occurs adjacent to the lake.  Logging occurs in the upper reaches of the Frosst watershed (Schubert et al. 2002), however, no logging has occurred in the immediate vicinity of the lake since 1946 and none is planned (Balanced Environmental Services Inc. 2004).  Residential development is restricted to three small areas around the lake (Schubert et al. 2002).  In summary, several land-based activities have the potential to increase sediment and nutrient loads to Cultus Lake or directly affect littoral habitat, but it is unclear if specific impacts have occurred to pelagic habitats occupied by Cultus pygmy sculpin and water quality has been assessed as excellent (see section above on water quality).

Climate Change.-- Scientific evidence clearly indicates that the 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 Cultus pygmy sculpin abundance and distribution.  This threat is of concern; however, 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 Cultus pygmy sculpin.