When I was ten, I carried a tin can of worms and a battered fishing rod to the wild shores of Brickyard Pond, in the woods behind our subdivision. We caught mostly scrappy sunfish and white perch, with the occasional bass thrown in. There were alewives in some of the brooks, too, and we caught them with nets. As for the pretty trout that came from the hatchery truck, I never caught one. The fish I caught were mostly round, dark green or gray, and mottled like the mud and sand bottom of the pond.
Then one day a friend’s older brother, a real fisherman with a green fishing vest, caught a large brown trout. I couldn’t believe my eyes. The fish, shaped like a torpedo, was a yellowish gold and it had big red spots on its sides. Years later, I caught my first brook trout on a fly rod at Shoal Pond in the White Mountains. Again, I was mesmerized by the intense colors: the yellow and red spots, some with bluish halos, the fins that were bright red with white and black trim.
It all begs the question: why the trout’s fancy colors when so many other fish are dishwater dull?
Part of the answer can be found in the biological record. Trout are part of the salmon family, which diverged from other bony fishes at the end of the Oligocene, about 30 million years ago. This was a time of global cooling, which suited the trout just fine. They were coldwater pioneers, who pushed into higher elevation watersheds to spawn and sometimes reside. Brook trout, our native trout, are part of the char genus (Salvelinus) and among the most cold-tolerant fish, able to live in glacial meltwater and isolated mountain lakes that stay frozen much of the year. As the last glacier melted and receded from the Northeast – about 10,000 years ago – brook trout migrated along the edge of the retreating ice, occupying enormous glacial bays, dammed glacial lakes, and northern rivers and streams.
Where the water is clear and cold, bright colors can be seen. There is an advantage to this. Trout are territorial, and males and females aggressively defend feeding stations. They flash their colors in lateral and frontal threat displays, and if that doesn’t work to push off an intruder, they nip and chase each other to defend their position in a stream.
Color and pigment patterns seem to matter during fall spawning, too. The fins and bellies of brook trout, like the maple leaves above, turn orange in the fall. While shortened day length triggers spawning behavior, it’s the trout’s heightened color that brings on the aggression as males vie for the opportunity to be closest to an egg-laying female. The largest, most brightly colored males will most effectively fend off the peripheral males. These males are not just rivals, they also cannibalize the eggs.
Bright color can be a disadvantage, of course, especially to an animal that must be constantly wary of predation from above. So trout have evolved a two-toned skin. The bright threatening flash of their silvery sides – in rainbow trout the silver is superimposed by a brushwork of red – contrasts with a dark back, engraved with ornate markings called vermiculations. These speckled patterns break up reflected light, merging trout with the gravely substrate below. In moving water, trout are nearly invisible from above.
Environmental factors also influence a trout’s colors. Food supply, for example, can have a major effect. Whether a brook trout has a pale or a pink belly is partly a function of how many crustaceans – small crayfish and shrimp –it eats. Brook trout that spend part of the year in the ocean take on a bluish hue, while trout living in the acidified waters of isolated beaver ponds are often deep yellow, closer to the tannin leachates that turn such waters brown. Chemical pollution may influence trout pigmentation, too. Chemicals found in certain anti-fouling-boat-bottom-paint turn rainbow trout paler. Hybridization adds to the puzzle of fish pigmentation. The High Sierra’s golden trout, arguably the prettiest trout in the world, hybridize with west slope cutthroats to make a fish that fishermen call “cutgolds.” The fact that we’ve been busy introducing trout to waters everywhere since the 1850smuddies the color palette even more.
Looks can be deceiving, and maybe we put too much stock in beauty. But there’s an ecological lesson in a trout’s good looks. These fish are decked out in sky, sunshine, and the multi-hued gravels of the places they call home. They’re a sentinel species: as goes the cold, clean water, so go the trout.