If asked to draw a trout, most people would probably draw something that looked like a rainbow trout. With a bright blushy stripe along its silvery sides and dark spots from head to tail, the rainbow epitomizes the trout family. Despite its familiarity, the rainbow trout is not a native, having been introduced to New Hampshire in 1878 and Vermont eight years later.
At that point, after so many of our rivers had been dammed for commerce, the Atlantic salmon was no longer finding its way back to local rivers and the brook trout had been relegated to free-flowing headwater streams. Rainbows and brown trout were brought here to fill the fishless gaps in the larger, slower portions of rivers. Generally, they needed to be replenished each year because their new home – warm and with relatively little oxygen – proved hospitable for only one season.
But in some rivers in Vermont and New Hampshire, the rainbow has managed to hold over and to reproduce, and it has done so with a strategy unlike that of any of the native salmonids: lake trout, arctic char, brook trout, and Atlantic salmon. Instead of spawning in the fall and having the eggs overwinter in gravel waiting for the water to warm, the rainbow spawns in the spring, generally from March through May.
The signal that it’s time to spawn is given by rising water levels from snowmelt and lengthening hours of sunlight, causing rainbows to ascend rivers to smaller tributaries. Don Miller, a lake biologist for New Hampshire Fish & Game said, “You’d be surprised, but a stream that is only 4 or 5 feet wide could have a spawning pair in it. Males are very aggressive at this point, so often there’s only one pair.”
The female finds a spot at the tail of a pool in a streambed made of fine gravel. She turns on her side and uses her tail as a fan to make a depression in the gravel. Digging this redd, which is generally 4 to 12 inches deep and 10 to 15 inches wide, can take a few days, and when she’s done, she’ll deposit 400 to 3,000 eggs. A male will join her and release his milt, fertilizing the eggs. She will cover the redd back over with gravel, and, depending on water temperature, incubation will take from four to seven weeks.
Eggs hatch into alevins, which remain in the gravel for two weeks while they continue to live off their yolk sacs. Once they’re out in the stream, the inch-long fry begin to feed on zooplankton and other small things.
Rich Kirn, fisheries biologist for Vermont Fish & Wildlife, said that the spring runoff gives rainbows access to streams they couldn’t reach at other seasons, and that sometimes they even choose intermittent streams. “The fry drift out prior to drying up, so some streams that dry up still have biological significance,” Kirn said.
It might seem that this spring strategy would give them an advantage over the native brook trout, whose eggs must survive an entire winter’s worth of ice scouring and wildly fluctuating water levels. But the brook trout has survived in these rivers for thousands of years, partly by having learned to spawn near upwellings of spring water, which is warmer than the rest of the river, so they get a head start hatching. In addition, they can better handle acidic water, which is the bane of rainbow’s continued existence.
Particularly in New Hampshire, with its generally more acidic streams, the rainbow has trouble. They cannot reproduce in streams with pH of less than 6.0. Don Miller said, “The Connecticut River valley is better because it has sweeter soils, but in general, most of New Hampshire is not well suited for rainbow reproduction.”
In fact, pH is so important that, farther east in New Hampshire, where the bedrock is solid granite, two of the best streams for rainbow reproduction are dependent on the application of lime. Not in the water, mind you. One of the rivers is fed by brooks that flow out of a ski area, the other flows through a golf course. Both get enough lime to accommodate the rainbow.
Acid precipitation continues to be a problem in the Northeast, so reproducing populations of rainbows will continue to be confined to their present waters. And even there, they face a further obstacle: low, warm water. Instead of cold springs and a snowpack that lasts into July feeding the Rocky Mountain streams these rainbows call home, our streams experience extreme low water most summers. Drought conditions of the last couple of summers have brought water temperatures into the high 70s and low 80s, which can prove lethal to rainbows. This introduced species is holding its own in cold, swift, low-acid water – if it can find it.