Cultus pygmy sculpin (Cottus sp.) recovery strategy: chapter 3

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2. Description of Needs of the Species

2.1 Ecological Role and Limiting Factors

Ecological Role.-- After hatching, Cultus pygmy sculpin apparently exist solely as a limnetic form, foraging for zooplankton in the pelagic areas of the lake.  They are common items in the diet of char, with as many as 100 Cultus pygmy sculpin occurring in the stomach of a single predator (Ricker 1960).  They are a less common diet item of other fish species, but have been found in the stomachs of cutthroat trout and coho salmon (Ricker 1960). 

Limiting Factors.-- The environmental factors that limit abundance of Cultus pygmy sculpin have not been well-studied.  We assume that populations are affected by competition, predation, habitat quantity and quality, and food availability though the relative effect of each is not known.  It is evident that to persist over the long term, all species require sufficient rearing and spawning habitat, a healthy food base, and predation or exploitation rates that are less than the replacement rate of the population.

2.2 Habitat Needs

Cultus pygmy sculpin is found only in Cultus Lake, a small coastal lake that is part of the Vedder-Chilliwack drainage in the lower Fraser Valley (Figure 2).  Cultus Lake has a surface area of 6.3 km², a drainage basin of 65 km², maximum depth of 41.8 m, mean depth of 32 m and elevation of 47 m (Province of BC 2006).   The lake is steep-sided and has a littoral area (defined here as the area where sufficient light penetrates to the bottom to support photosynthesis) of only 12% of total surface area (COSEWIC 2003).  Cultus Lake is monomictic, with strong thermal stratification in the warmer months, and wind- and temperature-driven vertical mixing in the cooler months.  Surface temperatures exceed 20˚ C in summer, whereas temperatures below the thermocline average less than 7˚ C in the fall (COSEWIC 2003).  Water clarity is fairly high, with average Secchi depths of 10 to 11 m.  Lake water is alkaline, well-buffered, with high conductivity, total dissolved solids and nutrient levels (COSEWIC 2003). 

Relative to other coastal lakes in British Columbia, Cultus Lake has a high primary production rate and zooplankton are abundant (COSEWIC 2003).  Food levels in Cultus Lake for juvenile sockeye (a planktivore) have been described as “exceptional” (COSEWIC 2003).  Numerous other fish species are present in Cultus Lake (Province of BC 2006) including char (bull trout and/or Dolly Varden), chinook salmon, chum salmon, coho salmon, coastal cutthroat trout, kokanee, lake whitefish, largescale sucker, longnose dace, mountain whitefish, northern pikeminnow, peamouth chub, pink salmon, prickly sculpin, rainbow trout, redside shiner, sockeye salmon, steelhead, threespine stickleback, and western brook lamprey.

Spawning habitat requirements of Cultus pygmy sculpin are not known in detail.  It is assumed that the species uses deep littoral areas for spawning, but natural spawning has not been observed.  With the likely exception of spawning and incubation, Cultus pygmy sculpin appear to spend the majority of time in the limnetic zone of the lake where they forage on plankton.  The species likely depends on productive plankton resources, but precise habitat requirements are not known.