angler holding a very large lake trout

Bull trout identification guide

A guide to the field identification of bull trout, brook trout, and their hybrids.

Directions:

This guide is used to identify bull trout, brook trout, and their hybrids that are at least 125 mm in length. The guide uses only color patterns in the dorsal fin to distinguish these three kinds of fish. Therefore, observers should disregard all other features and focus only on the dorsal fin when making identifications. It may be easier to see the color patterns in the dorsal fin if the fish is submerged in water.

Bull trout

Dorsal fin lacks light or dark markings. Some fish may have a few light markings along the base of the dorsal fin.

Bull Trout ID
Note the light markings along the base of the dorsal fin.

Brook trout

Dorsal fin has solid black markings with distinct edges.

Brook Trout ID

Hybrid

Dorsal fin has light or dark markings but dark markings are not solid black and lack distinct edges. In some fish less than about 175 mm the markings may be very faint.

Hybrid Trout ID
Note the dark colored bands, which are not solid black and lack distinct edges.

This guide was prepared in a joint effort between the USDA Forest Service and Idaho Department of Fish and Game. For more information about this guide please contact Bart Gamett at bgamett@fs.fed.us or (208) 588-3420 or Matt Campbell at mcampbell@idfg.idaho.gov or (208) 939-6713.

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