Home » News » Lensing bags: A freshwater oasis in the sea for sockeye

Lensing bags: A freshwater oasis in the sea for sockeye

Lensing bags give sockeye salmon a freshwater space to mature separated from surrounding salt water. Learn how these unusual tools work.

by | December 2, 2022

The lensing bag at Tutka Bay Lagoon Hatchery
Sockeye salmon swimming in the Tutka Bay Lagoon freshwater lensing bag. Alex Walczyk, CIAA

Guest post by Riley Waterman, Tutka Bay Lagoon Hatchery Manager

A lensing bag is like a swimming pool of freshwater suspended in the ocean. The Tutka Bay Lagoon Hatchery uses this tool as a critical stage for sockeye hatched by the Cook Inlet Aquaculture Association (CIAA).

Rest assured, our sockeye don’t take any breaks by the pool, from the moment we add them, they begin the process of sexually maturing. That’s why we hold these fish in the bag for about a month until their eggs are viable for egg take. 

How does a lensing bag work? 

At Tutka Bay Lagoon Hatchery, we use gravity to feed fresh water from the hatchery through underground pipes to the lagoon to fill the bag at a rate of more than 650 gallons per minute. As the bag fills, it creates a current similar to what you’d see in a natural creek. 

Because freshwater is less dense than saltwater, this allows the bag to “float” above the saltwater. The natural current of the water in the bag allows it to spiral down to the bottom and out a drain plug. As they mature, we continually refresh the water supply of these adult sockeye.   

Why is a lensing bag important for sockeye aquaculture?  

Unlike pink and chum salmon, sockeye require a freshwater component to their lifecycle. They spawn in freshwater lakes and streams, and require a significant source of fresh water to reach maturity. Tutka Bay Lagoon Hatchery provides a source of freshwater to hold sockeye for maturation by diverting water from Tutka Creek through the hatchery to the lensing bag at the net pen complex in the lagoon. By using the lens, we create an environment suitable for sockeye to spawn.  

A lensing bag allows hatchery operators to spend the summer collecting fish that will be used as brood stock. Placing these fish in the bag allows us to hold them until they are ready to spawn. This helps us ensure that we will have the precise number of fish required to meet our egg goal during the time of egg take. 

Where do the eggs from these fish go?  

All salmon have an incredible journey and these fish are no exception. Once the eggs have been taken by the Tutka hatchery crew, a floatplane will fly the eggs to Trail Lakes Hatchery to be raised and released into lakes in lower Cook Inlet.

These fish are the source of the China Poot dip net fishery. Without the lens, the China Poot dip net fishery would not be possible.  A percentage of these fish will be raised to smolt at Trail Lakes and brought back to the Tutka hatchery to serve as future brood to start the whole process over again. 

The use of a lens bag is uncommon in aquaculture, and there has been a learning curve to making it function optimally. Through trial and error over previous seasons we have found a properly-sized bag allows for a greater water turnover rate and results in a much higher survival. This is one of those cases where less truly is more. A smaller bag has allowed us to reach our egg take goal for the last two seasons. 

The lensing bag is an important piece of aquaculture equipment at Tutka Bay Lagoon Hatchery, and with it, we can help provide our great salmon resource.

You May Also Like

Upcoming Events

See Full Calendar →


Follow CIAA's monthly newsletter to learn more about local aquaculture.

We promise we’ll never spam! Take a look at our Privacy Policy for more info.

Follow CIAA on Facebook