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COSEWIC assessment and update status report on the Enos Lake stickleback species pair Gasterosteus spp. in Canada

Summary

COSEWIC previously recognized the stickleback pair in Enos Lake as threatened (McPhail 1985. 1988).  The population size of the limnetic form may be lower than that for benthic sticklebacks, because of proportionally less limnetic habitat compared to that for benthics, making limnetics increasingly susceptible to extinction.  Further surveys are required to determine if that is the case.  However, there is less habitat available to the limnetics and that has been impacted by the increased turbidity which is apparently leading to a breakdown in isolating mechanisms.  The recent increase in hybridization implies that their genetic integrity will soon be greatly altered and compromised.  Enos Lake represents one of five BC lakes where species pairs evolved, each lake independently supporting parallel post-Pleistocene evolution.  Each is not evolutionarily synonymous with the other.

Changes in water quality and the introduction of an apparently exotic crayfish were cited by Schluter (pers. comm) as being possible causes.  As there are no known indigenous species of crayfish on Vancouver Island, Pacifastatus leniusculus is presumed to be the species in Enos Lake (Premek 1998).  Schluter also suggested habitat disturbance by crayfish may have altered water colour and interfered with female selection for males during reproduction.  Within Enos Lake, morphological and genetic factors differentiating benthic and limnetic populations may be lost due to introgression, to eventually produce a genetically homogeneous population.