Culaea inconstans
Brook stickleback
(Also: Common freshwater stickleback; Common stickleback; Five-spined stickleback; Pinfish; Six-spined stickleback; Variable stickleback)

Culaea inconstans is common and abundant in its 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. There are isolated populations in some Canadian drainages as well as in northeastern New Mexico. (Elloran, 2010; Page and Burr, 1991)

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

Culaea inconstans has a deep, compressed body with a typical length between 38 to 61 mm with 4 to 6 dorsal spines. The fish is scaleless, but has small bony plates along a complete lateral line. 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)

Brook stickleback eggs are demersal and adhesive (Winn 1960); the eggs are approximately one millimeter in diameter and are clear to light yellowish in color (Barker 1918). 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 (Barker 1918, Becker 1983, Winn 1960). They are transparent for up to a week after hatching, but soon after develop a golden coloration (Barker 1918). After about 10 to 14 days the yolk sac is fully absorbed, and the larvae are actively feeding with newly developed teeth (Barker 1918). 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 (Becker 1983). Sexual maturity in C. inconstans is reached at one year (Becker 1983, Winn 1960). (Barker, 1918; Becker, 1983; Winn, 1960)

When the photoperiod and temperature requirements are met, adult brook stickleback 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 (Reisman and Cade 1967, Winn 1960). 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 (Barker 1918).

As the male builds the nest he aggressively defends his territory from male and female conspecifics as well as other species like trout, Gambusia and darters (Winn 1960). The male may perform a “lateral display” at his territorial borders to warn potential intruders by slightly undulating his body while extending the ventral and dorsal spines. This display typically leads to fighting and the male will chase intruders from his territory until he crosses into the territory of another and is in turn chased (Reisman and Cade 1967). The territorial aggressions of the male toward an encroaching 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 (Ward and McLennon 2006).

Once the nest is complete, the male performs a “tail flagging” dance to attract gravid females. Dancing was observed on occasion in a laboratory setting and was not necessary for reproduction (Reisman and Cade 1967). 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 counteracting any forward momentum with the pectoral fins. 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 take one of four actions: move toward the nest, remain motionless, drop to the bottom or leave for another territory (Winn 1960). 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 nuptially colored females with a variegated dark and light pattern as mates over gravid plain colored females (McLennon 1995). The male then prods the ventral portion of the female’s caudal peduncle and the female responds by laying her eggs. This male prodding action usually occurs numerous times during the egg laying process. 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 (Becker 1983, Winn 1960). 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 (Winn 1960). (Barker, 1918; Becker, 1983; McLennon, 1995; Reisman and Cade, 1967; Ward and McLennon, 2006; Winn, 1960)

Culaea inconstans are iteroparous and spawn in the spring and early summer typically from mid-April to late June depending on range location. Spawning initiation is influenced by photoperiod requirements of between 14 and 16 hours of sunlight (Reisman and Cade 1967) and also by temperature requirements between 15 and 19°C (Barker 1918, Reisman and Cade 1967, Winn 1960). 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)

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 begins to aerate the eggs by fanning his pectoral fins (Barker 1918, McKenzie 1974, Tomelleri and Eberle 1990, Winn 1960). After 9 to 11 days the eggs will hatch and the male responds by pulling apart the top of the nest to create more space between the organic debris, thereby creating a nursery for the larvae (McKenzie 1974). 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 (Becker 1983, McKenzie 1974). (Barker, 1918; McKenzie, 1974; Tomelleri and Eberle, 1990)

The lifespan of brook stickleback is about three years. They become sexually mature after one year (Becker 1983). (Becker, 1983)

Brook sticklebacks are communal and school 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. The nesting behavior and mating rituals have been studied extensively.

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

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.

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)

Culaea inconstans are predominately carnivorous feeders on aquatic invertebrates, mostly larvae and crustaceans. Fish eggs, from their own species as well as others, and algae may make up a smaller portion of their diet. (Becker, 1983; Winn, 1960)

An aggressive behavior termed “food fighting” determines a feeding hierarchy among the present individuals. 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. (Reisman and Cade, 1967)

Brook stickleback as well as other members of the family Gasterosteidae have lateral dermal plates potentially used for defense against predators. They also have dorsal spines which serve to deter predators by increasing both body size appearance (predator gape limitation) and causing difficulty with handling (inside mouth) and swallowing. Their brown, speckled coloration serves as camouflage in their river-bottom habitats. (Helfman, et al., 2009)

Brook trout (Salvelinus fontinalis), smallmouth bass (Micropterus dolomieu), northern pike (Esox lucius), bowfin (Amia calva), yellow perch (Perca flavescens), largemouth bass (Micropterus salmoides) and walleye (Sander vitreus) are known to prey on C. inconstans. It is also most likely preyed upon by fish-eating birds like kingfishers, herons, terns and mergansers. (Becker, 1983)

The brook stickleback is not utilized as bait and is not economically important (Becker 1983), however it is a carnivore that feeds on aquatic invertebrates and occasionally algae, and is itself an important prey species for many of predators in both riverine and lacustrine systems. It provides an important trophic transfer of energy from primary producers to the larger, economically important predators within systems. (Becker, 1983)

Culaea inconstans is an important food source for many commercially important species of sport fish such as yellow perch, brook trout, and smallmouth bass, and although they are not direct food sources for humans it is important to maintain healthy prey populations.

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 its aggressive territoriality.

In Wisconsin, these fish are being used for mosquito control because they feed on the larvae and can often survive in suboptimal habitats. (Becker, 1983)

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

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)