Once the alevin has absorbed its yolk sac, it is called a fry, and it begins its life as a juvenile. It is about an inch long at this stage. When it grows to two inches, it is called a parr or fingerling. It will grow almost three inches in its first year, and it will add close to two more inches in its second year.
The juveniles of migratory bull trout leave their natal tributaries when they are two or three years old and about four-and-half to six inches long. Those that remain in tributary streams for the rest of their lives—resident bull trout—become sexually mature when they are about three or four years old and eight to twelve inches long. Resident bull trout mature a year or two earlier than migratory bull trout. Migrants do not become adults until they reach five to seven years.
Bull trout fry are relatively secretive. They emerge from the gravel in early spring (mid-April to mid-May), and their favorite habitat is the shallow edges of streams, especially in areas of large, loose gravel and cobble where they use the spaces between the rocks to hide. There is some evidence that newly-emerged fry do not fill their swim bladders until about three weeks after emergence. Instead, they stick close to the bottom and spend much of their time in the gravel. The delay in filling the swimbladder is probably an adaptation that prevents them from being transported downstream until they are large enough to take up a feeding site. They tend to be most abundant in side-channels and small pools with slow moving water and are often associated with rocks and woody debris that slow the water and provide cover. They seem to stay in these areas at least until late fall (October).
As they grow, their color lightens, they develop their characteristic light lateral spots and spend less time on the bottom. They still remain, however, in the lower 25% of the water column. They are not overtly aggressive, but they seem to prefer certain holding sites, consistently returning to them after feeding forays. If another individual takes up the holding site while the original owner is on a feeding foray, the intruder usually is evicted when the original owner moves immediately upstream of the site and then drifts back into the space held by the intruder. Occasionally the original owner will approach the intruder head-on and, more rarely, may nip the intruder.
Fry eat mostly aquatic insects (off the bottom or in drift) and only rarely catch terrestrial insects. By mid-September, they have gained 1.5 inches in length. The major predators of bull trout fry include rainbow trout, brook trout, and larger bull trout. Ducks and great blue herons also prey on bull trout fry.
The degree of homing (spawning adults returning to their place of birth) is probably variable in bull trout (most likely related to stream size and stability). To the extent that it does exist, it is probably during their time as fry that bull trout imprint on their natal streams. Detecting odors and other stimuli from their surroundings prepares them for the eventual return to the spawning grounds.
Once the alevin has absorbed its yolk sac, it is called a fry, and it begins its life as a juvenile. It is about an inch long at this stage. When it grows to two inches, it is called a parr or fingerling. It will grow almost three inches in its first year, and it will add close to two more inches in its second year.
The juveniles of migratory bull trout leave their natal tributaries when they are two or three years old and about four-and-half to six inches long. Those that remain in tributary streams for the rest of their lives—resident bull trout—become sexually mature when they are about three or four years old and eight to twelve inches long. Resident bull trout mature a year or two earlier than migratory bull trout. Migrants do not become adults until they reach five to seven years.
Bull trout fry are relatively secretive. They emerge from the gravel in early spring (mid-April to mid-May), and their favorite habitat is the shallow edges of streams, especially in areas of large, loose gravel and cobble where they use the spaces between the rocks to hide. There is some evidence that newly-emerged fry do not fill their swim bladders until about three weeks after emergence. Instead, they stick close to the bottom and spend much of their time in the gravel. The delay in filling the swimbladder is probably an adaptation that prevents them from being transported downstream until they are large enough to take up a feeding site. They tend to be most abundant in side-channels and small pools with slow moving water and are often associated with rocks and woody debris that slow the water and provide cover. They seem to stay in these areas at least until late fall (October).
As they grow, their color lightens, they develop their characteristic light lateral spots and spend less time on the bottom. They still remain, however, in the lower 25% of the water column. They are not overtly aggressive, but they seem to prefer certain holding sites, consistently returning to them after feeding forays. If another individual takes up the holding site while the original owner is on a feeding foray, the intruder usually is evicted when the original owner moves immediately upstream of the site and then drifts back into the space held by the intruder. Occasionally the original owner will approach the intruder head-on and, more rarely, may nip the intruder.
Fry eat mostly aquatic insects (off the bottom or in drift) and only rarely catch terrestrial insects. By mid-September, they have gained 1.5 inches in length. The major predators of bull trout fry include rainbow trout, brook trout, and larger bull trout. Ducks and great blue herons also prey on bull trout fry.
The degree of homing (spawning adults returning to their place of birth) is probably variable in bull trout (most likely related to stream size and stability). To the extent that it does exist, it is probably during their time as fry that bull trout imprint on their natal streams. Detecting odors and other stimuli from their surroundings prepares them for the eventual return to the spawning grounds.
Once the alevin has absorbed its yolk sac, it is called a fry, and it begins its life as a juvenile. It is about an inch long at this stage. When it grows to two inches, it is called a parr or fingerling. It will grow almost three inches in its first year, and it will add close to two more inches in its second year.
The juveniles of migratory bull trout leave their natal tributaries when they are two or three years old and about four-and-half to six inches long. Those that remain in tributary streams for the rest of their lives—resident bull trout—become sexually mature when they are about three or four years old and eight to twelve inches long. Resident bull trout mature a year or two earlier than migratory bull trout. Migrants do not become adults until they reach five to seven years.
Bull trout fry are relatively secretive. They emerge from the gravel in early spring (mid-April to mid-May), and their favorite habitat is the shallow edges of streams, especially in areas of large, loose gravel and cobble where they use the spaces between the rocks to hide. There is some evidence that newly-emerged fry do not fill their swim bladders until about three weeks after emergence. Instead, they stick close to the bottom and spend much of their time in the gravel. The delay in filling the swimbladder is probably an adaptation that prevents them from being transported downstream until they are large enough to take up a feeding site. They tend to be most abundant in side-channels and small pools with slow moving water and are often associated with rocks and woody debris that slow the water and provide cover. They seem to stay in these areas at least until late fall (October).
As they grow, their color lightens, they develop their characteristic light lateral spots and spend less time on the bottom. They still remain, however, in the lower 25% of the water column. They are not overtly aggressive, but they seem to prefer certain holding sites, consistently returning to them after feeding forays. If another individual takes up the holding site while the original owner is on a feeding foray, the intruder usually is evicted when the original owner moves immediately upstream of the site and then drifts back into the space held by the intruder. Occasionally the original owner will approach the intruder head-on and, more rarely, may nip the intruder.
Fry eat mostly aquatic insects (off the bottom or in drift) and only rarely catch terrestrial insects. By mid-September, they have gained 1.5 inches in length. The major predators of bull trout fry include rainbow trout, brook trout, and larger bull trout. Ducks and great blue herons also prey on bull trout fry.
The degree of homing (spawning adults returning to their place of birth) is probably variable in bull trout (most likely related to stream size and stability). To the extent that it does exist, it is probably during their time as fry that bull trout imprint on their natal streams. Detecting odors and other stimuli from their surroundings prepares them for the eventual return to the spawning grounds.
