How to Identify Brook Stickleback

To identify Brook Stickleback (Culaea inconstans), look at these key features.

Physical Characteristics

Body shape

elongated

Coloration

Generally olive-green to brownish on the dorsal side, fading to silvery-white on the ventral side. During breeding season, males develop a striking black coloration, sometimes with iridescent blue or green highlights, and their pelvic fins may turn reddish-orange. Females retain a more subdued coloration.

Scale type

none

Mouth position

superior

Distinctive features

4 to 6 (typically 5) isolated dorsal spines, not connected by a membrane, Absence of bony lateral plates (or very few, rudimentary plates), Slender caudal peduncle, Small, upturned mouth

Dorsal fin

Two dorsal fins: the first consists of 4-6 isolated, sharp spines; the second is soft-rayed with 9-11 rays.

Tail fin

Slightly forked or truncate caudal fin.

Pectoral fin

Well-developed pectoral fins, typically with 9-11 rays.

Distinctive fins

Pelvic fins are thoracic, each consisting of a strong spine and one soft ray. Anal fin has 1 spine and 8-10 soft rays.

Identification Tips

1. 1

   Dorsal Spines

   Count the isolated, sharp spines on the back. Brook Sticklebacks typically have 5 (range 4-6) distinct spines, unlike other stickleback species that have fewer or more, or spines connected by a membrane.

2. 2

   Lateral Plates

   Examine the sides of the body. Brook Sticklebacks lack bony lateral plates, or have only a few rudimentary ones near the caudal peduncle, which distinguishes them from other stickleback species like the Threespine Stickleback (Gasterosteus aculeatus) which has prominent lateral plates.

3. 3

   Body Coloration (Breeding Males)

   During the breeding season, males turn almost entirely black, a distinctive feature not seen in other stickleback species to the same extent.

4. 4

   Size

   Brook Sticklebacks are relatively small, typically reaching a maximum total length of 5-7 cm (2-2.8 inches).

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