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Brook stickleback

Culaea inconstans

What do they look like?

Brook stickleback have deep, compressed bodies with a typical length between 38 to 61 mm with 4 to 6 dorsal spines on their backs. The fish is scaleless, but has small bony plates in a line along the sides of their bodies. The anal fin has one spine and 9 to 10 fin rays; the spineless dorsal fin has 9 to 11 rays. The pelvic fins each have one fin ray and one well developed spine while the pectoral fins are spineless and have 9 to 11 rays. Tiny, sharp teeth are found on both jaws with the lower jaw extending beyond the upper jaw.

Both non-breeding males and females are olive in color on the back and sides with white or pale green speckling or with wavy, pale vertical lines. They may have a pale stripe along the side. The belly and ventral portion of the head tend to be silver-white to light green. Breeding males have black or dark green bodies and fins that may be tinged copper or red. (Becker, 1983; Page and Burr, 1991)

  • Sexual Dimorphism
  • sexes alike
  • sexes colored or patterned differently
  • Range length
    38 to 61 mm
    1.50 to 2.40 in
  • Average length
    60 mm
    2.36 in

Where do they live?

Brook stickleback are native to North America and occur across southern Canada and northern United States. Brook stickleback are common and abundant in their native range which includes Arctic and Atlantic drainages from Nova Scotia to the northern territories of Canada, Great Lakes-Mississippi River basins from southern Ohio to New Brunswick, Canada and west to Montana and eastern British Columbia. They are common in Michigan. There are isolated populations in some Canadian drainages as well as in northeastern New Mexico. (Elloran, 2010; Page and Burr, 1991)

What kind of habitat do they need?

Brook stickleback typically inhabit the shallow edges of cool, clear lakes and ponds with moderate to dense vegetation cover. These fish also favor pools and backwaters of cool to cold creeks and small rivers with mild to moderate current and sand, gravel, or muddy river bottoms. Temperature tolerance ranges from 4°C to 18°C with the lethal high temperature threshold, determined by experiment, of 31°C and a low temperature threshold of 0 to -2°C. (Becker, 1983; Elloran, 2010; Page and Burr, 1991; Reisman and Cade, 1967; Tomelleri and Eberle, 1990)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams

How do they grow?

Brook stickleback lay their eggs underwater, on the bottom of lakes or stream beds. The eggs are approximately one millimeter in diameter and are clear to light yellowish in color. They are deposited in groups of approximately 100 inside nests built by males and the eggs hatch within 8 to 11 days depending on water temperature. The larvae are about five millimeters in length with a sizable yolk sac and without fully developed fins. The larvae stay in the nest or nursery for up to two days until their fins are developed and they are able to swim well. They are transparent for up to a week after hatching, but soon after develop a golden coloration. After about 10 to 14 days the yolk sac is fully absorbed, and the larvae are actively feeding with newly developed teeth. Age 0 fish have been found to be 26 to 38 mm. Age 1 fish range from 40 to 58 mm and age 2 fish are typically around 60 mm. Like most fish, brook stickleback will continue to grow throughout their lives. Brook stickleback can reproduce when they are one year old. (Barker, 1918; Becker, 1983; Winn, 1960)

How do they reproduce?

Brook stickleback can sense changes in day length and water temperature and when these conditions are right, adult fish will move from deeper water into the warmer, shallow waters along the shoreline where vegetation is plentiful. The males establish territories and begin nest construction; they also change coloration from the non-mating olive to dark green or black. They typically construct the nest on a vertical piece of grass or a stick by using materials such as algae, plant fibers, dead leaves and small twigs. The organic materials are held together by a white secretion from the kidneys and other related organs. The nests are initially built with one opening.

As the male builds the nest he aggressively defends his territory from male and female brook stickleback as well as other species like trout and darters. The male may perform a “lateral display” at his territorial borders to warn intruders by slightly wiggling his body while raising this ventral and dorsal spines. This display typically leads to fighting and the male will chase intruders from his territory. The territorial aggressions of the male towards other fish are very high during nest building. Once the nest is built aggressiveness remains high, but is more intense toward males than toward potential mates. Aggressive behavior diminishes toward all intruders when egg nurturing is required. Aggressiveness toward all intruders then becomes more prominent once the larvae have hatched and are free swimming.

Once the nest is complete, the male performs a “tail flagging” dance to attract females. To begin the dance, the male swims towards a female and stops in front of her with his head down and his whitish tail held high above his back. He then waves the tail back and forth. Once the dance is performed the male moves toward the nest with the same movement.

A female entering the territory of a male is attacked and will do one of four actions: move toward the nest, remain motionless, drop to the bottom or leave for another territory. If the female moves toward the nest ahead of the male she will be attacked and chased out of his territory. If the female remains motionless or drops to the bottom, the male may move toward the opening of the nest. The female will then enter the nest headfirst with her tail sticking out the entry hole. Males are more likely to choose females that have a dark and light pattern over females with plain coloring. The male then prods the female's tail and the female responds by laying her eggs. When the female finishes laying her eggs she swims out the back of the nest creating a second hole and is attacked by the male. Once the female leaves the area, the male quickly swims through the nest to fertilize the eggs. The male may or may not repair the hole created by the female. More than one female may lay her eggs within the nest and on occasion males may actually build and maintain more than one nest. (Barker, 1918; Becker, 1983; McLennon, 1995; Reisman and Cade, 1967; Ward and McLennon, 2006; Winn, 1960)

Brook stickleback spawn in the spring and early summer every year. This typically occurs from mid-April to late June depending on range location. Spawning initiation is influenced by day length of between 14 and 16 hours of sunlight and also by water temperatures between 15 and 19°C. Females lay between 92 and 182 eggs per clutch which hatch after 7 to 11 days. Both male and female brook stickleback are able to reproduce at 1 year of age. (Barker, 1918; Reisman and Cade, 1967; Winn, 1960)

  • How often does reproduction occur?
    Brook stickleback breed once a year.
  • Breeding season
    Brook stickleback breed from mid-April to late June depending on range location.
  • Range number of offspring
    92 to 182
  • Range time to hatching
    7 to 11 days
  • Average time to hatching
    10 days
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Average age at sexual or reproductive maturity (male)
    1 years

Once eggs are fertilized the male pushes them to the sides of the nest and down into the nest material. He then takes a position in front of the nest entrance and provides oxygen to the eggs by fanning his pectoral fins. After 9 to 11 days the eggs will hatch and the male responds by pulling apart the top of the nest to create a nursery for the larvae. The larvae float up to the top of the nursery sometimes escaping and making their way to the surface of the water. The adult male gathers the escapees in his mouth and spits them back into the nursery. He continues to retrieve larvae for a day or two until the rate at which they escape is faster than the rate that he can retrieve them. At this time he either abandons the larvae or eats them. (Barker, 1918; McKenzie, 1974; Tomelleri and Eberle, 1990)

  • Parental Investment
  • male parental care
  • pre-fertilization
    • provisioning
    • protecting
      • male
      • female
  • pre-hatching/birth
    • provisioning
      • male
    • protecting
      • male
  • pre-weaning/fledging
    • protecting
      • male
  • pre-independence
    • protecting
      • male

How long do they live?

The lifespan of brook stickleback in the wild is about three years. (Becker, 1983)

  • Average lifespan
    Status: wild
    3 years

How do they behave?

Brook sticklebacks are social and school together with other fish in the non-breeding season. Downstream migrations have been observed from mid-June with peaks in January in the upper Great Lakes.

During the breeding season, the males are highly territorial. They will aggressively chase out or attack any fish that gets near their nests. When a female nears the nest, males will either chase them away or begin a courtship dance. Courtship dances include fin fanning, and body and tail swaying.

The brook stickleback will burrow into silty river bottom, sometimes remaining buried for 30 minutes. The fish will burrow at times in search of food. (Becker, 1983)

Home Range

Little is known regarding the home range size of brook stickleback. Males become very aggressive during the breeding season and will chase out or attack any intruders that come near his nest. Males likely have small territories that consist of the immediate area surrounding the nest.

How do they communicate with each other?

As do most fish, brook stickleback perceive chemical, tactile, visual and acoustic signals. Males use body movements to visually communicate the boundaries of their territory to other fish. Males also use body movements to communicate interest to potential mates. When a male is interested in mating with a female, he will perform a "dance" by waving his body or fanning his fins. Though little is known about the methods, brook sticklebacks are sensitive to changes in day length and water temperature that signal the change of season and stimulate migration. (Barker, 1918; Becker, 1983; McLennon, 1995; Reisman and Cade, 1967; Winn, 1960)

What do they eat?

Brook stickleback are predominately carnivorous feeders on aquatic invertebrates, mostly insect larvae and crustaceans. Fish eggs, from their own species as well as others, and algae may make up a smaller portion of their diet.

Occasionally, a group of fish come together to eat. A large food item is held in the mouth and shaken vigorously. This food item is then passed mouth to mouth and shared among the group for a type of communal feeding. (Becker, 1983; Reisman and Cade, 1967)

  • Animal Foods
  • eggs
  • insects
  • aquatic crustaceans
  • Plant Foods
  • algae

What eats them and how do they avoid being eaten?

Brook stickleback as well as other members of the family Gasterosteidae have bony plates in a line along their sides that are potentially used for defense against predators. They also have spines on their backs which serve to deter predators by increasing both body size appearance (intimidating predators) and making it difficult for a predator to catch and swallow. Their brown, speckled coloration serves as camouflage in their river-bottom habitats.

Brook trout, smallmouth bass, northern pike, bowfin, yellow perch, largemouth bass and walleye are known to prey on brook stickleback. They are also most likely preyed upon by fish-eating birds like kingfishers, herons, terns and mergansers. (Becker, 1983; Helfman, et al., 2009)

  • These animal colors help protect them
  • cryptic

What roles do they have in the ecosystem?

The brook stickleback is not used as bait and humans do not often catch and eat this species. However, it is a carnivore that feeds on aquatic insects and occasionally algae, and is an important prey species for many predators in river and lake ecosystems. (Becker, 1983)

Commensal or parasitic species (or larger taxonomic groups) that use this species as a host

Do they cause problems?

There are no known negative impacts of brook stickleback on humans.

How do they interact with us?

Brook stickleback are an important food source for many commercially important species of sport fish such as yellow perch, brook trout, and smallmouth bass. The presence of brook stickleback supports fish species that are economically important to humans.

These fish are also common in the aquarium trade, however, they have been known to kill other small fishes kept with it in a tank due to their aggressive territoriality.

In Wisconsin, these fish are being used for mosquito control because they feed on the aquatic mosquito larvae. (Becker, 1983)

  • Ways that people benefit from these animals:
  • controls pest population

Are they endangered?

Although the International Union for Conservation of Nature and Natural Resources (IUCN) lists these fish as least concern, they are of special concern at the state level in parts of their range. These fish are considered vulnerable in Pennsylvania, New York, Nebraska, and Nova Scotia. Threats to brook stickleback include many human activities such as pollution, river siltation, and deforestation. Although these fish have a relatively high tolerance to low water quality, too many environmental pollutants may irradiate these fish from native habitats.

They have been accidentally introduced through bait buckets in the states of New Mexico, Utah, Washington, Kentucky, and Connecticut. Though these are small, isolated exotic populations, some scientists are concerned that their aggressive territorial behavior may harm native fish species. (Pennsylvania Natural Heritage Program, 2008)


Julie Howard (author), Northern Michigan University, Rachelle Sterling (editor), Special Projects, Jill Leonard (editor), Northern Michigan University, Renee Mulcrone (editor), Special Projects.


Barker, E. 1918. The Brook Stickleback. The Scientific Monthly, 6/6: 526-529.

Becker, C. 1983. Fishes of Wisconsin. Madison, WI: University of Wisconsin Press.

Elloran, C. 2010. "Fish Base" (On-line). Accessed December 08, 2010 at

Helfman, G., B. Collette, D. Facey, B. Bowen. 2009. The Diveristy of Fishes. West Sussex, United Kingdom: Wiley-Blackwell.

McKenzie, J. 1974. The parental behaviour of the male brook stickleback Culaea inconstans (Kirtland). Canadian Journal of Zoology, 52: 649-665.

McLennon, D. 1995. Male mate choice based upon female nuptial coloration in the brook stickleback, Culaea inconstans (Kirtland). Animal Behavior, 50: 213-221.

Page, L., B. Burr. 1991. A Peterson Field Guide to Freshwater Fishes of North American north of Mexico. Boston, MA: Houghton Mifflin Company.

Pennsylvania Natural Heritage Program, 2008. "Brook stickleback (Culaea inconstans), freshwater species of concern" (On-line). Fact sheet. Accessed December 09, 2010 at

Reisman, H., T. Cade. 1967. Physiological and behavioral aspects of reproduction in brook stickleback, Culaea inconstans. American Midland Naturalist, 77: 257-295.

Tomelleri, J., M. Eberle. 1990. Fishes of the Central United States. Lawrence, KS: University Press of Kansas.

Ward, J., D. McLennon. 2006. Changes in agonistic, courtship and parental displays of the male brook sticklback, Culaea inconstans, across the breeding cycle. Behaviour, 143/1: 33-56.

Winn, H. 1960. Biology of the brook stickleback, Eucalia inconstans (Kirtland). American Midland Naturalist, 63/2: 424-438.

University of Michigan Museum of ZoologyNational Science Foundation

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Howard, J. 2011. "Culaea inconstans" (On-line), Animal Diversity Web. Accessed April 16, 2024 at

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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