Unarmored Threespine Stickleback

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Unarmored Threespine Stickleback

Gasterosteus aculeatus williamsoni

ListedOctober 13, 1970
FamilyGasterosteidae (Stickleback)
DescriptionSmall, olive-brown fish with prominent dorsal spines.
HabitatVegetated, slow-flowing, shallow streams.
FoodInsects, larvae, and snails.
ReproductionSpawns for three months.
ThreatsUrbanization, hybridization, predation.


One of three subspecies of threespine stickleback in North America, the unarmored threespine stickleback, Gasterosteus aculeatus williamsoni, grows to a length of about 2.4 in (6 cm). Its streamlined body is olive-brown to dark green above and light yellow, white, or silvery below. The head, and sometimes the back, of breeding males turns bright red and they may exhibit some blue coloration. Most sticklebacks are heavily plated on the sides, but this subspecies is "zero plated" or unarmored. It is called "threespine" because two spiny projections replace the first dorsal fin, and a third spine protrudes in front of the rear back fin.


The unarmored threespine stickleback is an opportunistic feeder, subsisting mostly on insects, insect larvae, snails, and some organic matter. It is more active during the day and probably completes most of its feeding cycle during daylight hours. Nesting males do not feed as often.

It reproduces throughout the year with a peak of activity corresponding to warm water temperature and minimal flood conditions. The male establishes a territory and constructs a nest from aquatic vegetation on the river bottom. Once the nest has been completed, the male attempts to attract females with a zig-zag "dance." A receptive female follows the male to the entrance of the nest and enters. He vibrates his snout against her exposed tail to stimulate her to release eggs. Then he drives her from the nest and fertilizes the eggs. This courtship ritual may take up to an hour to complete. He may then attempt to attract other females to the same nest for up to 24 hours, when he begins his parental duties. After the female lays eggs in the nest, the male fans the eggs for 24 hours to aerate them and aggressively drives away predators through combat. Under normal conditions eggs take about five days to hatch and males guard the fry for another six days. If the fry stray too far from the nest, the father captures them in his mouth and returns them to safety. After hatching, fry remain in the nest until large enough to fend for themselves. They will feed from the yolk of the large eggs for a couple of days and then begin foraging for food. Individuals live for one year.

Females may remain sexually active for up to three months and may be able to spawn every fifth day during this period, producing as many as 1,000 eggs per season. The length of her sexual activity depends upon favorable water conditions and availability of food. The male reproductive cycle requires two days for building the nest and attracting females, five days for the eggs to brood, and six days to guard the fry. In cooler waters this cycle takes up to 18 days. The availability of suitable nesting sites, rather than a paucity of males, seems to be the limiting factor for reproduction.


Optimum stickleback habitat consists of clean, clear-flowing streams in deeper pools where there is a slow, steady current. They do not occur in pools with no flow or in swifter flows. In stronger currents, adults shelter behind obstructions, particularly vegetation. They are most abundant in small impoundments, behind obstacles, and in riffles with vascular plants and filamentous algae. Breeding has been documented only in still waters and impoundments. Fry aggregate in the shallow margins of impoundments where vegetation provides protection from predators. Juveniles congregate in backwaters, hidden among aquatic plants. This fish is not found in waters that are even slightly turbid.


Threespine sticklebacks are common throughout much of North America. The unarmored threespine stickleback, however, was found only in southern California. At one time, its range included the portions of the Los Angeles, San Gabriel, and Santa Ana Rivers, which now pass directly through metropolitan Los Angeles (Los Angeles County). It also occurred in the headwaters of the Santa Clara River (northern Los Angeles County), and the Santa Maria River and San Antonio Creek (Santa Barbara County).

At present, the unarmored threespine stickleback survives in the headwaters of the Santa Clara River near the towns of Acton and Saugus, and in San Antonio Creek near Lompoc. A transplanted population is thought to survive in Honda Creek on the Vandenburg Air Force Base Reservation. A remnant population of the fish may have been located in Shay Creek (San Bernardino County).


This stickleback has been extirpated from the Los Angeles basin by urbanization. Groundwater pumping, water diversion, stream channelization, and degraded water quality have combined to permanently eliminate most of its historic habitat. Where it survives in San Antonio Creek, the unarmored threespine stickleback has interbred with another subspecies (G. a. microcephalus ), which was accidentally introduced into the river system. If interbreeding continues, the unarmored threespine stickleback will lose its unique characteristics. Populations in the upper Santa Clara River have survived because the mountainous region is largely undeveloped, and natural barriers have prevented hybridization. In these headwaters, however, sticklebacks have been forced to compete for food and breeding sites with the introduced mosquitofish. Sometime in the 1970s, the African clawed frog (Xenopus laevis ), which was a popular pet until prohibited, became established in a Santa Clara River tributary. This carnivorous frog is regarded as a threat to all native fishes. The U.S. Fish and Wildlife Service (FWS) has undertaken a project to eliminate the frog from streams in the region. In Southern California habitats, the stickleback young and some adults are preyed upon by aquatic insects, garter snakes, herons, and belted kingfishers. Some birds that prey on the stickleback carry tapeworms that destroy the fish's reproductive potential and cope-pods that cause fatal infections.

Conservation and Recovery

The FWS continues its research into the biology and ecology of the unarmored threespine stickleback and will attempt to reestablish additional populations within the historic range. Because several habitat streams are in danger of being dried up by groundwater pumping, the FWS has implemented a strategy to supply emergency water to these streams or to salvage fish populations if necessary. A major goal of the FWS Recovery Plan is to control the many non-native fishes and pests that abound in the watershed. Without some effort to remove these exotics, the native unarmored three-spine stickleback will be unable to return to formerly inhabited streams.


U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121


Irwin, J. F., and D. L. Soltz. 1982. "The Distribution and Natural History of the Unarmored Three-spine Stickleback in San Antonio Creek, California". Report. U.S. Fish and Wildlife Service, Sacramento.

U.S. Fish and Wildlife Service. 1985. "Revised Un-armored Threespine Stickleback Recovery Plan." U.S. Fish and Wildlife Service, Portland.