anadromous

Guest post by Emily Argo

Each year adult salmon migrate upstream to spawn (lay eggs) putting into motion their offsprings’ journey from hatching to making their way back to the ocean as juveniles. In addition to being a spectacular event to witness, these migrations also play a central role in ecosystem health. Over 100 different species benefit from the nutrients that the salmon provide! In order for a juvenile salmon to successfully complete their journey to the ocean and continue providing benefits to marine and freshwater ecosystems when they return to as adults, the juveniles must undergo a series of changes through a process called smoltification.

Coho salmon are one of many salmon species, their smolt lifestage is pictured here.

Smoltification occurs in anadromous species prior to entering saltwater. For example, as salmon juveniles move from their freshwater nursery habitats to the ocean, they transition from what scientists call parr to a smolt lifestage. During this transition, they change color, losing their parr bars (dark, vertical bars) and defined countershading, making them appear silvery. Their bodies also change to be more streamlined and buoyant. Even their attitudes change! While parr are typically highly territorial in shallow water, this aggression declines as the smolts move into deeper water and form shoals (large groups). During smoltification salmon smolts get a good whiff of the water to imprint on the odor of their natal stream (stream where they were born) in order to identify it during their return spawning migration as adults. Finally, how the salmon breathe (take oxygen out of the water with their gills) also changes as they go from freshwater to saltwater.

Smoltification does not begin at a specific time in all salmon species, but varies depending on temperature, latitude, size, rate of growth, age, and feeding opportunities or a combination of these factors. Since the changes that take place during smoltification are not necessarily beneficial for remaining in freshwater, the changes will reverse (desmoltification) for fish who are unable to migrate to saltwater. This is sometimes the case when there is a barrier to migration, which can cause fish to remain in freshwater throughout their lifecycle. Luckily for these individuals, salmon are equipped to survive in freshwater as long as they need to, but the inability to migrate will limit their ability to provide important nutrients to these freshwater ecosystems on their return migration as adults.

How many species of salmon can you name?