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Freshwater skirts: An aquaculture estuary for sockeye

A freshwater skirt helps juvenile salmon make the transition from freshwater to saltwater. Learn how CIAA uses this important tool.

by | April 18, 2023

A freshwater skirt at Tutka Bay Lagoon Hatchery. CIAA

In a previous article, we talked about a freshwater lensing bag. CIAA’s hatcheries use this tool to create a suitable artificial adult sockeye spawning environment in the ocean. We can also use another aquaculture tool known as a freshwater skirt.

Just like the lensing bag, the name implies its purpose. A freshwater skirt may sound like a new high fashion trend, but its purpose is to simulate the natural flow of saltwater and freshwater.

Let’s take a closer look at how we can use a skirt to aid in the rearing and saltwater acclimation of sockeye smolt. 

What is a freshwater skirt?

A freshwater skirt is made of the same hypalon material seen in our freshwater lensing bag.  You may have encountered it before as it is the same material whitewater rafts and inflatable dinghies are fabricated from. 

The difference between a skirt and a dinghy is that the skirt does not have a bottom. It is more of a curtain about six feet tall. We deploy a skirt around our rearing pens wrapped around the outside perimeter of the rearing net with the skirt tied to the net at the water line. This creates a layer within that six-foot depth that will hold freshwater.

How does this work? Saltwater is denser than freshwater. Allowing freshwater supplied to the pen to float above the saltwater below it.  The skirt creates a barrier keeping the saltwater out and containing the freshwater. Freshwater is continually added, which displaces the saltwater inside of the skirt and pushes it out of the skirt’s open bottom.  Using the skirt reduces water turnover.  

How does a skirt help with aquaculture?

Why would we want a freshwater environment in saltwater rearing pens? Before sockeye smolt begin their journey to the sea, their lives are entirely in freshwater. Before they leave Trail Lakes Hatchery, the fish must pass a saltwater challenge, ensuring they are ready for the drastic life cycle change and have developed physiologically to be able to live in a saltwater environment.

When dealing with large numbers such as the Trail Lakes to Tutka sockeye program, we expect to see salmon ranging across the developmental spectrum. Even though most fish are ready for the change to salt, some may not be.

When the fish are transferred to the pens, there will be a small layer of freshwater they can hold in until ready to drop below that line into pure saltwater. 

The skirt functions much like an estuary in nature—an area where freshwater and saltwater meet. Young salmon spend time in estuaries adapting to their new environment. 

We will remove the skirt once the fish are fully transitioned to their saltwater life stage. The freshwater skirt will typically be deployed for a week and then removed. The smolt will continue to be held and fed for two to three more weeks and released at night on an outgoing tide cycle. 

By using the freshwater lens we can create an environment suitable for all developmental stages seen in our sockeye smolt, further allowing us to provide our salmon resource.

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