Global Short Term Trend: Relatively stable (=10% change)
Comments: Overall, extent of occurrence, area of occupancy, number of subpopulations, and population size probably are relatively stable or declining at a rate of less than 10% over 10 years or three generations.
Global Long Term Trend: Increase of 10-25% to decline of 30%
Comments: This species is apparently still abundant throughout range, but some unique, isolated populations have declined to very low levels requiring protection (e.g., G. a. williamsoni and species pairs in B.C.). Other populations have increased or were created as the result of accidental or purposeful introductions to habitats where this species was not native; increasing population size and distribution in the upper Great Lakes area is an example of growing non-indigenous populations in the U.S. (Moyle1976b, Fuller 2005).
Several studies report evidence of considerable fluctuation in population size from year to year (Greenbank and Nelson 1959, Wootton and Smith 2000). A 26-year study (1972-1998) of a Wales population showed cyclic changes in abundance with a period of about 6 years, around a generally declining trend (Wootton and Smith 2000).