Moving between fresh and saltwater environments would kill the vast majority of fish species on this planet, yet salmonids have developed a strategy known as smoltification to deal with this physiological challenge. A buildup of the enzyme ATPase in gill tissues has been linked to migratory behavior and signals that juvenile salmonids are entering the smolt stage. This enzyme allows them to maintain osmotic balance when entering salty sea water, and if levels drop too low they will not survive the transition. ATPase levels are tightly linked to water temperature and research suggests the threshold for maintaining elevated levels is a surprisingly cool 13°C (55°F). Interestingly, fish that reared in cold water are able to maintain elevated ATPase levels longer than fish reared in warm water providing them with a longer window to complete their transition. Photoperiod also plays into this equation as migratory behavior significantly decreases by the summer solstice in mid-June. Smolts generally begin their migration in early spring and are aided by late winter rains and high flows. For coastal fish this is a relatively easy task as they may only have to swim a few miles. Inland summer steelhead face additional challenges on their many hundreds of miles long journey and any hindrance can mean they do not reach the ocean within their optimal physiological window, likely resulting in death. This has implications for climate change which is predicted to increase temperatures in headwater streams and in watersheds with large impoundments which increase surface water temps and slow juvenile migrations.