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Aquarium of the Pacific - Online Learning Center - Species Print Sheet

Conservation Status:  Endangered

Aquatic SpeciesUnarmored Threespine Stickleback

Gasterosteus aculeatus williamsoni Bony Fishes, marine

Unarmored Threespine Stickleback | Courtesy of California EPA
Unarmored Threespine Stickleback - popup
Courtesy of California EPA

Species In-Depth | Print full entry

At the Aquarium

The unarmored threespine stickleback is illustrated on The Wave’s mosaic tile mural, Rios de la Vida (Rivers of Life). The fountain, mural and accompanying graphics illustrate the story of our Los Angeles and San Gabriel Rivers. Although not on exhibit in the Aquarium, this fish is included in our website animal database to expand on the information touched on in The Wave Fountain exhibit.

Geographic Distribution

Limited mostly to the northwestern area of Los Angeles County, one small area in Santa Barbara County, and a small, isolated, introduced population in San Felipe Creek in San Diego County. Once common throughout the Los Angeles, California basin.


Although most species of stickleback can adapt to salt, brackish, or fresh water, unarmored threespine sticklebacks appear to be limited to fresh water. They require clear, flowing, well-oxygenated water with associated pools and eddies of quiet water and areas of dense vegetation or debris to provide adequate cover and food supply.

Physical Characteristics

Threespine sticklebacks are laterally compressed, fusiform shaped fish with three sharp, erectile spines at the forward part of the dorsal fin. The pelvic fin usually consists of a spine on each side and one fin ray. Modified scales form a single row of armor-like plates along the length of the body.

Sticklebacks have black to dark olive backs with silvery white sides and white to gold bellies. At spawning time males develop a red throat and belly, the usually clear fins take on a green hue, and the eyes become blue. Females develop a light red or pink throat and belly.


This subspecies may reach a length of 58 mm (2.3 in), but most are less than 50 mm (2 in). Females tend to be slightly larger than males.


Sticklebacks are voracious omnivores and feed on a variety of food including some algae and invertebrates such as insects, snails, small crustaceans, and some types of small worms. Males sometimes feed on stickleback eggs that are not properly guarded. Because they are visual feeders, they require relatively clear water in order to see their food source


A male first establishes a selected territory that he vigorously protects. He then excavates a small depression in the sand in which he builds a nest out of fragments of aquatic plants, strands of algae, twigs, and debris. After cementing the materials together with mucus threads spun from his kidneys, he burrows through it to make an exit and entrance and weights down the nest with pebbles. While building the nest he aggressively defends it against other males and not-as-yet welcome females. Courtship begins when the nest is ready.

Next he searches for a receptive female. When a female carrying eggs enters his territory the male swims near her in a zigzag dancing movement. If the female is attracted, she follows him to his nest where she deposits her eggs. The male immediately fertilizes them and drives the female away. Over the course of the breeding season, the male may entice several other females into his nest.

The male protects the eggs and the fry from predators. During the incubation period he fans the eggs with his pectoral fins to keep them clean and to provide a fresh supply of oxygenated water. Incubation time varies with temperature but is generally 6-8 days. After hatching the fry stay in the nest for a few days getting nutrition from their attached yolk sac. When they start to leave the nest area in search of food, the male continues to protect them, sometimes taking them into his mouth to return them to the nest. When dispersed from the nest they feed in the cover of dense vegetation.


Male sticklebacks are very territorial when breeding and actively protect their nests, eggs, and young, using their spines as weapons in offense and defense. Confronted by intruders or predators, they assume a threatening posture, and rush forward with raised dorsal spines and a wide, gaping mouth. If the intruder is still not intimidated, the stickleback becomes very still, points its head down, and jerks its body as if to bore a hole in the ground below. This display seems to be effective in driving intruders away. In off-spawning seasons they are much less territorial and may even show schooling tendencies.


Sticklebacks are one of the most studied fishes by ichthyologists and behaviorists. Some researchers believe that the two lineages of marine and freshwater sticklebacks are an example of evolution in action since records indicate that some of their adaptations have occurred in only the past 10 years. This species is quite habitat specific and does not do well in other than their preferred surroundings.


Most of these fish have about a one year life span. They do not normally survive to a second spawning season, but an occasional individual may survive for three years.


Unarmored threespine sticklebacks were once common in the watersheds of the Santa Clara, Los Angeles, San Gabriel, and Santa Ana rivers. They were reported as abundant in the Los Angeles basin. By 1942 they were considered to be extinct, but some small populations were later discovered. In 1970 the U. S. Fish and Wildlife Service listed the species as endangered. In 1980 a critical habitat area for the species was proposed to include parts of San Antonio Creek on Vandenberg Air Force Base in Santa Barbara County, and parts of the Santa Clara River, San Francisquito, and Soledad Canyons in Los Angeles County.

These fish have natural predators including water birds, several species of mostly introduced fishes, and bull frogs. The greatest threat to their continued existence is humans. Urban sprawl, accompanying habitat degradation, agricultural expansion, inadequate water supply, and chemical water pollution have all been major contributors to the close extinction of this species.

Special Notes

Although the unarmored threespine stickleback has some of the characteristics of threespine sticklebacks, it is noticeably different from other subspecies in several ways. The most important are that the dorsal and pelvic spines are substantially shorter and most specimens either completely lack or have very few scales or lateral plates, thus the name “unarmored.”