Welcome to our Fish Factors Mini-Series! Each quarter we will share important stories on factors impacting Upper Columbia salmon and trout!
Written by Tracy Bowerman
Photo: Juvenile salmon just before their transition to the smolt stage by Russ Rickets.
The UCSRB is excited to continue with our Fish Factors Mini-Series that aims to spotlight important stories on factors impacting Upper Columbia salmon and trout.
Last quarter we explored the adaptations and challenges for salmon eggs and larvae. This quarter we consider the journey juvenile salmon take from their home stream out to the ocean.
Young salmon are known as smolts during the transitional stage when they undergo physical changes that prepare them to move from freshwater to the ocean. This stage begins when environmental cues trigger fish to begin their downstream migration. Salmon smolts leave their home streams in the spring when they can be carried downstream tail-first by high flows. During their journey, salmon memorize the smells of the waters they move through, building a mental map that allows them to return several years later to the stream of their birth.
For Upper Columbia salmon and steelhead, the more than 500-mile journey to the ocean holds many dangers. Smolts must swim through seven to nine reservoirs and navigate the associated dams. Smolts are also preyed upon by larger fish, such as native Northern Pikeminnow and non-native smallmouth bass and walleye. Colonial nesting waterbirds, including cormorants, gulls, and terns, also feed on smolts during their migration. Recent estimates suggest that 30-50% of Upper Columbia Steelhead smolts and 10-35% of spring Chinook smolts are consumed by birds between Rock Island Dam and the ocean. Although these birds were found in the region historically, human manipulation of river flows and nesting habitat has influenced the distribution and size of colonies.
During their seaward migration, salmon smolts undergo a remarkable physical transformation. Their bodies gradually elongate and their spots are replaced with silvery sides and a dark back, which makes it harder for predators to see them in the open ocean. Changes occur in their gills and kidneys, which switch from taking in salt ions while in freshwater to actively excreting salt when the fish enter the ocean. These physical changes are completed in the estuary, where the river meets the ocean and fresh water mixes with the salt water, creating a nutrient-rich environment. Smolts need time in the estuary to adjust to new conditions and to feed heavily on the abundant supply of prey, in the form of insects and crustaceans, before moving into open water.