Smolt Trap General Information

Courtesy of Mid Puget Sound Fisheries Enhancement Group

What is a smolt?

"Smolt" is one of the life-stages of a juvenile salmon. This life stage occurs when the juvenile salmon begins its migration from freshwater to the estuary and adjusts to living in saltwater. Different Pacific Salmon species spend different amounts of time rearing in freshwater. Coho salmon, the focus of our study, spend one to two years rearing in freshwater after they emerge from the gravel as fry. Coho reach about 50­100 millimeters in size before they smolt and begin migration to the estuary. The timing of this movement correlates with spring freshets (high water flow from snow­melt).

What is a smolt trap?

A smolt trap is a standardized method of quantifying how many fish are moving through a water system. There are several different types of smolt traps. The objective of the study and the size of the system determine the type of smolt trap to use. On big rivers and streams, "screw traps" are often used to catch fish moving downstream. These traps only capture a portion of the fish in the river because the trap does not collect fish from the entire channel.

The trap below is a full-spanning v-shaped weir that is modeled after traps developed in the 1970s by Washington Department of Fish and Wildlife. This method is commonly used to evaluate juvenile coho use of winter rearing habitat, which is the purpose of this project. The weir allows water to flow downstream but all fish over a certain size are forced to swim into a livebox where they wait until someone collects them. The captured fish are then identified and measured before they are released downstream to continue their migration.