How to Identify a Brook Trout (Salvelinus fontinalis)

• Spot pattern: Red spots with bluish halos dot the body.
• Fin edge color: The front edges of the pectoral fins (on the sides and bottom of the fish) are white.
• Tail shape: The tail is nearly square.
• Body markings: An elongated, compressed body, a squarish tail, and worm-like markings on the back and head.
• Fin edge color: The lower fins are red, with a distinctive white leading edge followed by a black border.
• Spot pattern: Overall, light spots appear on a dark background — the opposite of the rainbow trout and cutthroat trout, which have dark spots on a lighter background.

The brook trout's lower fins have a distinct white leading edge, which the brown trout's fins completely lack; the brook trout shows light spots and blue-haloed red spots on a dark background, while the brown trout shows large dark spots with pale halos on a lighter background. See the Brown Trout page for its full mark list.

The brook trout has light spots on a dark background, the opposite of the rainbow trout's dark spots on a lighter background; the brook trout's tail is nearly square, while the rainbow trout's tail is forked and heavily spotted. See the Rainbow Trout page for its full mark list.

Prefers well-oxygenated cold water, about 53°F or cooler, with poor tolerance for high temperatures. Native to northeastern North America, from the east coast to Minnesota, north to Hudson Bay and the North Atlantic coast; now introduced to western North America and many other regions worldwide.

Lay the fish flat on its side in even light and photograph the whole body first, then the details below. Spread the tail fin so its shape (forked, rounded or square) is clear. Raise the dorsal and anal fins fully; ray counts and fin edges only read when the fin is spread. Shoot the side of the body before the fish dries; spots and stripes fade fast out of water. The app reads the photo and names the likely species; use the marks on this page to confirm it.

Size and daily limits for this species differ by state and by water, and some waters are catch-and-release only. Check the current regulations for where you are fishing before keeping any fish; if you are unsure of the species, release it.