salmonid kin, steelhead are iteroparous (able to spawn several times, each time separated by months) and make several spawning trips between fresh and salt water, although fewer than 10 percent of native spawning adults survive from one spawning to another. The survival rate for introduced populations in the Great Lakes is as high as 70 percent. As young steelhead transition from freshwater to saltwater, a process called "smoltification" occurs where the trout undergoes physiological changes to allow it to survive in seawater. There are genetic differences between freshwater and steelhead populations that may account for the smoltification in steelhead. Juvenile steelhead may remain in the river for one to three years before smolting and migrating to sea. Individual steelhead populations leave the ocean and migrate into their freshwater spawning tributaries at different times of the year. Two general forms exist—"summer-run steelhead" and "winter-run steelhead". Summer-run fish leave the ocean between May and October before their reproductive organs are fully mature. They mature in freshwater while en route to spawning grounds where they spawn in the spring. Summer-run fish generally spawn in longer, more inland rivers such as the Columbia River. Winter-run fish are ready to spawn when they leave the ocean, typically between November and April, and spawn shortly after returning to fresh water. Winter-run fish generally spawn in shorter, coastal rivers typically found along the Olympic Peninsula and British Columbia coastline, and summer-run fish are found in some shorter, coastal streams. Once steelhead enter riverine systems and reach suitable spawning grounds, they spawn just like resident freshwater rainbow trout. Growth and oxidative stress on mitochondria During periods of rapid growth and aging, trout display high levels of metabolic activity. High metabolic activity has been correlated with increased levels of oxidative stress and decreased machinery repair in rainbow trout. During high oxidative stress, the mitochondria are the most important organelle contributing to tissue damage because of their role in metabolism and production of reactive oxygen species. In a study done by Almaida-Pagàn et al., researchers identified changes to the heart and brain mitochondrial membrane phospholipid composition in rainbow trout due to the differing levels of oxidative stress each organ faced during a high-stress time such as rapid growth and development. Stock rainbow trout of ages 1, 2, and 4 years had their heart and brain mitochondria isolated and analyzed for fatty acid composition. The tissues showed an overall similar percentage