Comments: On a range-wide scale, no major threats are known.
Locally, threats include human impacts on spawning habitat and water quality, stocking or other introduction of non-native, predatory fishes, including introductions of certain forms of G. aculeatus itself.
Craig (1984) noted that large-scale industrial and petroleum extraction development in the Beaufort Sea could cause direct mortality as a result of intake of juvenile fishes with seawater and indirectly harm populations by altering coastal habitat, including water circulation patterns.
The California Department of Fish and Game (CDFG) recognized that increasing development along and recreational uses of the Santa Clara River were threats to the existence of the endangered G. a. williamsoni population (CDFG 1974).
In British Columbia, beaver activity has resulted in fluctuating water levels that have reduced access to some spawning sites used by the giant stickleback (currently recognized as G. aculeatus, but considered by some scientists to be a separate species, this fish is black in color and more than twice as long as threespine stickleback; occurs only in Mayer Lake, Queen Charlotte Islands; see sources in Rubidge 2000, Species at Risk Canada 2004). Increasing human recreational use of lake habitat also threatens this population (Species at Risk Canada 2004).
In Alaska and elsewhere, non-native species such as the northern pike (Esox lucius) and stocked salmonids may threaten sticklebacks through predation and competition for juvenile food resources (Hatfield 2001a, Hatfield and Ptolemy 2001, Foster et al. 2003, Wood 2003).
Hybridization between different forms of G. aculeatus threatens the unique genetic characteristics of specific populations recognized as rare or divergent; extensive hybridization between the native G. a. williamsoni (unarmored) and introduced G. a. microcephalus (armored) forms in California contributed to declines in the now endangered G. a. williamsoni (CDFG 1974, Moyle 1976b, Fuller 2005).