Native to the West Coast and introduced to Vermont in the late 1800s, Rainbow Trout (Oncorhynchus mykiss) are the most habitat-sensitive of the trout species and are stocked and bred extensively in state fisheries. Rainbow Trout will live in high- to moderate-gradient streams and rivers, as well as cold lakes. They are very sensitive to water pH levels and do not do well in acidic conditions.
The Rainbow Trout inhabits moderate- to high-gradient cold-water streams with swift riffles and deep, clear pools, often overlapping portions of upstream Brook Trout habitat and downstream brown trout habitat. They are also well adapted to deep, cold-water lakes within certain temperature limits.
In streams, Rainbow Trout prefer water temperatures similar to those favored by Brown Trout, from 54°F to 66°F, while lake-dwellers select waters between 45°F and 64°F. The maximum tolerable water temperature is 77°F, but some populations may be able to withstand temperatures in the low eighties for short periods of time.
Rainbow Trout are more sensitive than other salmonids to very high or low pH levels, especially acidic conditions. A pH range of 6.5 to 8.0 is considered optimal and adults can tolerate levels from 5.5 to 9.0. Natural reproduction is not successful in waters with pH less than 6.
Optimal physical stream habitat for Rainbow Trout consists of the following:
- A relatively stable flow of clear, cold water;
- A rocky bottom in riffle-run areas, with no silt;
- Having as many pools as there are riffles;
- Areas of slow, deep water and abundant in-stream cover; and,
- Stable banks with a lot of plant growth.
Adult Rainbow Trout use the large, and often the deepest, parts of the water pools, especially during low summer flows and winter freezing. Both adults and juveniles also use "pocket water" (slack, slow water in otherwise fast-flowing riffles) in riffles, often found behind or under large rocks or woody debris. They will all use undercut banks, overhanging vegetation, rocks, pool depth, water turbulence, and woody debris as a cover to protect them from predators.
Life expectancy of Rainbow Trout is highly variable over its range but is generally three to five years, sometimes longer in lake populations. Growth rates in Vermont streams are similar to those of Brook Trout, but they tend to grow larger due to their greater life span. They generally reach 4 to 6 inches after two years, 6 to 9 inches by their third year, and 8 to12 inches by their fourth year. As with the other species of trout, rainbow trout populations in lakes grow faster: four-year-old fish attain lengths of 13 to 17 inches or more.
Rainbow trout in streams generally become mature during their second or third year, whereas lake fish tend to mature later. They spawn almost exclusively in streams. Unlike brook trout and brown trout, most rainbow trout spawn in the spring (usually March through May in Vermont) triggered by rising spring flows and warmer water temperatures. Selective breeding in hatcheries has produced strains that spawn in the fall months or other times of the year.
When the female is ready to reproduce, she digs a small hole, usually in the fine gravel at the tail of a pool. Water temperatures for incubation range from 45°F to 54°F. The eggs will hatch in 28 to 49 days. Fry emerge from the gravel about two weeks after hatching and congregate in schools in the calm areas near the edges of the stream channel. After several weeks, the fry grow more territorial and the schools disperse. By the end of their first year, juvenile Rainbow Trout move into the more swiftly flowing riffle areas.
Rainbow Trout consume a wide variety of foods, depending on availability. Stream populations tend to prefer drifting aquatic and terrestrial insects while lake populations may feed more on microscopic animals and bottom-dwelling organisms such as worms, crustaceans, aquatic insect larvae, mussels, clams, and crawfish. They will shift more to smaller fish as they reach about 12 inches in length.
In Vermont, over 180 lakes and ponds and 3,800 miles of streams and rivers are managed by the Vermont Fish & Wildlife Department for one or more trout species. The Department must decide whether or not to stock an area, where to set length and creel limits and gear restrictions, and when and where to allow or not allow fishing by anglers.
The general areas the state considers in management are:
- Habitat capacity, or quality and quantity of existing fish habitat in the water;
- Fishing pressure, or how heavily people fish the area;
- The productivity or food base of the stream, river or lake;
- The present species of fish that are managed in the body of water;
- Whether natural reproduction of the trout species would be supported;
- Timing and duration of spawning runs; and,
- Public input.
Steps the Vermont Fish & Wildlife Department make in managing trout in Vermont start with monitoring and evaluating the existing trout population. Biologists take care to protect the selected habitat of the trout and partner with others to implement restoration efforts according to their evaluations. After evaluating the stream, river or lake, they stock trout if needed.
Stocking is determined with one of their three techniques: "put-grow-take," "put-take," or "species recovery" (specific for Atlantic salmon).
- Put-take: Catchable-size trout (greater than 6" and often 8-10" long) are stocked in areas where fishing pressure is high, but habitat does not support sufficient natural reproduction or growth of young fish to meet fishing demands. Put-take stocks are removed by fishing usually within one season. The fish that are not caught, rarely survive to the following season. This method of stocking is used primarily in rivers and streams.
- Put-grow-take: Smaller-sized fish stocked in spring to "grow" to catchable size before being caught. Often used in ponds and lakes where fish can survive the winter and where adequate food is available for fish survival. This technique is usually used to maintain populations where spawning habitat is lacking.
- Species Recovery: Stocking of fingerlings with the goal of reestablishing the trout species in a particular body of water.
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Rainbow Trout were also introduced into Vermont waters beginning in the late 1800s. Over the years they have become an important component of the state's fisheries in both river and lake habitats. These fisheries exist today as a result of natural reproduction, stocking, or a combination of the two. However, while brown trout eventually established wild populations throughout much of the state, wild Rainbow Trout populations are noticeably absent from the large watersheds of southeastern Vermont.
Even though Rainbow Trout have been, and continue to be, stocked extensively in this region, these waters characteristically have low alkalinity, a condition that does not support rainbow trout reproduction. Consequently Rainbow Trout fisheries in the southeast region are dependent on stocking.
Notable exceptions include numerous small, lowland watersheds draining into the Connecticut River. It is believed that these small watersheds provide spawning habitat for the Connecticut River population. Typically these small spawning streams have drainage areas less than 20 square miles.
Lake populations of Rainbow Trout in the Northeast Kingdom are the only areas in Vermont that are supported entirely or in part by natural reproduction. As noted for stream populations, alkalinity is also a factor limiting the distribution of wild populations in Vermont lakes and ponds.
The inability of many of the state's lake and pond populations to be supported to some extent by natural reproduction can also be attributed to the lack of suitable spawning habitat. Lake-residing fish must have access to streams offering suitable spawning and nursery habitat in order to maintain a wild population. In the absence of quality spawning streams, most lake populations must be maintained by stocking.