Gasterosteus aculeatus

Complex patterns of variation make taxonomic treatment difficult. This stickleback may be considered a species complex with many unique and reproductively isolated populations, subspecies or species.

Populations exist that are strictly marine, anadromous, and freshwater resident. The marine and anadromous forms have given rise to diverse resident phenotypes. Subspecies have been recognized in the past, but current scientific discussion of this species complex recognizes multiple distinct species within evolutionary radiations; current genetic research is underway to determine relationships between evolutionary groups and species before names can be assigned (Hatfield 2001a, 2001b). Lateral plate morphs of resident freshwater forms are recognized as lows (i.e., lateral plates on anterior parts of the fish only), partials (i.e., lateral plates on anterior and posterior ends of the fish with a gap between), and completes (i.e., lateral plates in a continuous row anterior to posterior). In a rare form, plates are entirely absent; G. a. williamsoni is an endangered plateless form exhibiting reduction in pelvic structure, and only occurs in drainages in southern California. Lows from the Pacific coast of North America have been called G. a. microcephalus, Pacific coast completes are G. a. aculeatus, and Penczak (1964, in Wootton 1976) designated lows from Iceland as G. a. islandicus. A plateless form occurring in Shay Creek, San Bernardino County, California, has been identified as G. a. santaeannae (or santa-annae) but is currently recognized as synonymous with G. a. williamsoni (Ross 1973, Moyle et al. 1989).

Studies of allozyme variation (Haglund et al. 1992) and mitochondrial DNA sequences (Orti et al. 1994) in Asian, North American, and European populations recognized two primary clades: (1) European, North American, and some Japanese samples, which could be divided into an (1a) Atlantic basin clade comprising the eastern North American and European populations, and a (1b) basal Pacific basin assemblage comprising western North American and some Japanese populations; and (2) a divergent group of Japanese populations. The divergent Japanese clade deserves further study and possible taxonomic recognition.

Sympatric species pairs bearing "limnetic" and "benthic" life histories and distinct morphologies have evolved in several British Columbian lake systems (Thompson et al 1997, Hatfield 2001, Hatfield and Ptolemy 2001). Some populations of these are endangered or already extinct (Wood 2003).

Several low-lying lakes and streams in the Cook Inlet area contain rare and evolutionarily divergent populations of G. aculeatus including three populations polymorphic for lateral plate morphs, several populations polymorphic for pelvic armor morphs, one lake containing 2 freshwater morphs of the species (a benthic and a limnetic feeder), and one lake containing both anadromous and resident freshwater forms of the species (von Hippel, pers. comm.). Bell and Orti (1994) viewed divergent populations in freshwater habitats around Cook Inlet as parts of an endemic radiation warranting special consideration for conservation as a unit.

Widespread, abundant, secure.

Range encompasses coastal waters of Eurasia, Iceland, Greenland, eastern Asia, and North America. In North America, this fish ranges from Alaska to Baja California on the west coast, from Baffin Island and the west side of Hudson Bay to Chesapeake Bay, Virginia, along east coast, and it occurs also in inland areas (including Lake Ontario) along both coasts. Sometimes it occurs in the open ocean. The species has been introduced and is established in certain areas of California, Massachusetts, and the Great Lakes (lakes Huron, Michigan, Erie, and Superior) (Fuller et al. 1999, Stephenson and Momot 2000). It also has been introduced in Europe, Iceland, Greenland, and the Pacific coast of Asia (Page and Burr 2011).

This species is represented by a very large number of occurrences (subpopulations).

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).