楼层:#3 由 wetnose » 10 12月 2013 17:04
Life History
Growth and Reproduction
During late winter or early spring, when water temperatures rise, the maturing adult rainbow trout and steelhead usually seek out the shallow gravel riffles or a suitable clearwater stream. Spawning takes place in the spring from late March through early July as daily water temperatures reach 6 – 90 C. The female uses her tail to prepare a redd, or nest, 4 to 12 inches deep and 10 to 15 inches in diameter. From 200 to 8,000 eggs are deposited in the redd, fertilized by a male, and covered with gravel. Hatching normally takes place from a few weeks to as much as four months after spawning, depending upon the water temperature. A few more weeks may be required for the tiny fry to emerge from the gravel.
Age and growth of rainbow and steelhead trout are controlled by complex interactions of genetics and environmental conditions. The onset of sexual maturity varies markedly between individuals due primarily to such factors as food availability, population density, water temperature, productivity of the aquatic environment, and genetic makeup. In small streams, rainbow trout may spawn at 2 or 3 years of age and males often mature a year before females. Mature steelhead have generally spent 3 years in freshwater before migrating to the ocean and then another 2 years feeding in the ocean before they return to spawn. Once maturity is reached trout may spawn annually or skip a year or two before spawning again. Rainbow trout up to age 11 have been observed spawning.
Among resident rainbow trout, those living in or migrating to large lakes with sockeye salmon runs generally grow faster and larger than fish which remain year round in streams.
When compared to the mundane habits of resident trout, steelhead have a very complicated and diverse life history. Juvenile steelhead smolt and begin their emigration to saltwater during late April through the middle of June. Once in the marine environment the steelhead grow rapidly because of the abundance of food and can grow an inch per month until they return to their natal streams as mature adults. Steelhead will have moved hundreds of miles into open ocean before returning to their natal stream to spawn. Most steelhead return after 2 or 3 years but a few may return after just one year and some not until 4 years. The oldest known-aged steelhead in Alaska is an 11-year fish from the Situk River which spent 5 years rearing in freshwater followed by 6 years in the ocean; the number of times this fished spawned is unknown.
Steelhead are often grouped or classified as to the time of year they return to their home waters or natal stream, i.e., spring, summer or fall. Spring-run steelhead return to streams in Alaska between March and early June while the rare summer-run fish will return to freshwater during July. Fall-run steelhead enter the freshwater systems as adults in August through October and possibly throughout the winter. Regardless of when they return to freshwater, all steelhead spawn during the spring months when daily water temperatures reach 6 – 90 C, usually about mid-April through May to early June.
Summer-run steelhead are relatively rare in Alaska and may be found in only a few select Southeast Alaska systems; annual returns of summer-run fish may vary considerably. Fall-run steelhead is the predominant form along the Gulf of Alaska west of Yakutat, including all systems on Kodiak Island and the Alaskan and Kenai Peninsulas. There are approximately 36 fall-run systems in Southeast Alaska but the spring component of these streams is thought to exceed the fall-run component. With the exception of the large transboundary rivers which originate from headwaters in Canada, the steelhead systems in Southeast Alaska all have a dominant spring-run component, even if a system has a summer or fall-run component.
Steelhead are “iteroparous” and may spawn more than once while Pacific salmon are “semelparous” and spawn only once and then die. The percentage of steelhead which return a second time to spawn typically ranges from 20 to 30% but may be as low 10% or as high as 50%. Generally the larger and older females survive at a higher rate than the smaller and younger ones, and males do not survive spawning as well as females.
The ragged and spent spawners, sometimes called kelts, move slowly downstream to saltwater, and the vibrant spawning colors return to a bright silvery hue as their depleted stores of fats are restored once they resume feeding. Most steelhead that spawn more than once come back annually but some may skip a year before returning to spawn.
While the adult steelhead are returning to their ocean feeding grounds and their spawning wounds are healing, the eggs which were deposited deep in the gravel during the spring quickly develop into alevins or "sac-fry". These tiny fish gradually absorb the yolk sac and work their way to the surface. By mid-summer the fry emerge from the gravel, minus the yolk sac, and seek refuge along stream margins and in protected areas. Mortality of steelhead fry is high as most of them are killed or washed from the stream each year. For those that do survive they may grow to be 2 to 3 inches by the fall before heading into their first winter. These juvenile steelhead typically will spend 2 – 5 years rearing in freshwater before smolting and returning to the ocean.
Given all the variables and complexities of the steelhead life cycle, including the many possible age classes, the possibility of repeat spawning, and the variation in run-timing, perhaps nature has designed the steelhead life history so that a harsh flood, winter, or drought does not severely impact a specific population.
Feeding Ecology
Upon emergence, the small trout fry assemble or school together in groups and seek shelter along the stream margins or protected lake shore, feeding on crustaceans, plant material, and aquatic insects and their larvae. Resident rainbow trout rear in similar habitat for the first two or three years then move into the larger water of lakes and streams and turn more to a diet of fish, salmon carcasses, eggs, and even small mammals. After they smolt and enter the open ocean, steelhead grow to be significantly larger by feeding on more nutritious squid, amphipods and other fish.
Migration
There is little information on the migration of Alaska steelhead away from our coastal waters to the open ocean. Steelhead are most abundant in the Gulf of Alaska and the eastern North Pacific and western Aleutian areas and generally found in the 50 C isotherm in the north and the 150 C zone in the south. When high-seas drift gill-netting was occurring large numbers of steelhead from the Pacific Northwest streams and Alaska were intercepted in this fishery; this indiscriminant fishing is no longer allowed. One anecdotal piece of the puzzle comes from a steelhead tagged in 1989 as it left the Karta River in Southeast Alaska. This fish was recaptured about 3 weeks later by a Japanese research vessel on the high seas of the Pacific Ocean.