Vermont, October 10th 2011. The Connecticut River – the boundary between Vermont and New Hampshire – is often referred to as a great river, a distinction based on its diverse four state watershed and its key role in the development of New England. It’s the region’s largest river, for sure – its deep, serpentine channel visible from space. Anglers who know the river and its watershed would agree that it’s great; after all, it’s not everywhere that one can catch cold water species like brook trout and warm water species like largemouth bass in the same day of fishing on the same river.

It is the particular ancient and modern history of the Connecticut River that created this wonderful mix of fish. The river that we know today got its start about 10,000 years ago, when the last glacier that was crushing our region under one-mile-thick ice began to retreat. Once the glacier melted, fish migrated back into the Connecticut. The returning species of brook trout, darters, anadromous fish, and pickerel either swam their way into this reopened habitat or were transported by birds as eggs or small fish. Scientists believe species are still finding their way back into the watershed 10,000 years later.

This natural migration, however, does not account for the diversity of fish species found in the watershed. Recent arrivals that were introduced by humans include bass, rainbow and brown trout, walleye, and pike. Bass were introduced after the railroads and steamboats allowed for fast shipment of fish or eggs – intentionally or by accident – from the Lake Champlain, Great Lakes, or Mississippi watersheds. The brown trout was deliberately introduced from Europe in the late 1800s.

In 1873, Livingston Stone of Charlestown, New Hampshire developed a railroad aquarium that transported fish, an act that led to cross-continental distribution of species like the rainbow trout. Rainbow trout subsequently became the species of choice for many fish hatcheries on account of their ability to thrive in a wide range of habitats. Most people mistakenly believe rainbow trout are native because they’re so ubiquitous throughout our watershed.

Habitat and water quality influence where different species of fish live in the watershed, but the key factor is water temperature. The Connecticut River runs south from latitude 45.20 degrees at the Canadian border (plant temperature zone 3b) to 42.45 degrees at the Massachusetts border (plant temperature zone 5a). As the river flows south, it drops from an altitude of 2,670 feet above sea level at Fourth Connecticut Lake to 240 feet above sea level at Brattleboro, Vermont. Combined, these factors represent more than a 20 degree shift upwards in atmospheric temperature, and thereby, the ambient river temperature.

Human activity has increased this natural temperature change. Reservoirs formed by the 11 hydroelectric and 3 water storage dams open the water to the heat of the sun, and if you add heated summertime water from impervious surfaces (roads and roofs), direct industrial hot water discharges, and now climate change, you get a hotter river. Today there is substantially more warm water habitat than there was in the past.

From a fish’s perspective, the true issue is not temperature, but respiration. The warmer the water the less dissolved oxygen it holds. Brook trout demand high dissolved oxygen levels, so they are known as a cold-water species; bass make do with less oxygen and are thus at home in warmer water.

The makeup of the resident fish population changes as the Connecticut flows south; cold-water trout give way to cool water smallmouth bass and pickerel, then warm-water species like walleye and largemouth bass. Tributaries follow the same basic rule: upstream reaches might be trout waters, the lower reaches close to the confluence with the Connecticut might hold cool- or warm-water species, depending on how far south in the watershed the tributary is located. Start the day up high in the tributary for trout, and then fish downstream near the confluence for bass.

The cool- and warm-water species end up upstream in the tributaries because they move in order to stay at their preferred temperature, and also because they can’t spawn in the deep, silt-bottomed Connecticut. Most species prefer to spawn in clear, boney streams where the water depth is 12 to 36 inches. Once spawning is completed, the fish stay put until the water temperature drops in the fall, at which time they’ll head back to the Connecticut River for the winter.

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David Deen is River Steward for the Connecticut River Watershed Council.