When you add fertilizer to your garden, you add nutrients like nitrogen and phosphorus. These materials enhance the soil that grows plants you want to harvest. When you are fertilizing lakes, it’s a little more complicated, but the idea is the same.
One of the first efforts of the Cook Inlet Aquaculture Association (CIAA) to enhance salmon fisheries started with reviewing all the freshwater lakes that drain into Cook Inlet. This effort produced an inventory of each lake’s capability of providing good conditions to rear juvenile sockeye salmon.
Sockeyes spend up to three years of their early life in these lakes. At that point, they migrate to the ocean, where they grow rapidly into adults before returning to the same lakes and streams to spawn. CIAA wanted to know what limited the production of immature sockeye salmon in the lakes and if CIAA could enhance production.
Lake Production in a Nutshell
There are two ways to describe lake production.
- Primary production is the conversion of inorganic nutrients into organic substances using an energy source like the sun. In lakes, small single-celled plants collectively called phytoplankton are often the dominant primary producers. Phytoplankton forms the base of the food chain.
- Secondary production is the rate other life forms incorporate organic substances. In Alaska’s salmon producing lakes, small crustaceans called zooplankton eat phytoplankton. Juvenile sockeye salmon, in turn, eat these zooplankton.
The objective of lake fertilization is to enhance immature salmon survival and growth by providing more food for the zooplankton that feed the immature fish. To do this CIAA needed to know what was limiting phytoplankton in the salmon rearing lakes.
What Phytoplankton Need
The growth of phytoplankton typically depends on these three factors:
- Light provides the energy phytoplankton need to live and grow. It can limit phytoplankton production in bog stained or turbid lakes where adequate light is only available in a thin layer near the surface.
- Nutrients are the inorganic chemicals that phytoplankton need for growth. Through a process called photosynthesis, phytoplankton and other aquatic plants convert these inorganic chemicals into organic substances. In most lakes the key nutrients are nitrogen and phosphorus, but carbon and other chemicals also play a role
- Temperature affects the rate plants grow. In cold glacial lakes phytoplankton growth may be slow.
Returning to Lakes for More Measurements
CIAA worked with the Alaska Department of Fish and Game (ADF&G) to measure the light, temperature and the nutrient concentrations in the salmon rearing lakes. Several of these lakes were clear with good light penetration. They were not influenced by summer glacial melt, and nutrient concentrations were low. CIAA and ADF&G considered these lakes to be good candidates for salmon enhancement through lake fertilization.
CIAA and ADF&G returned to those lakes for additional measurements of the zooplankton population. They also studied the growth rate of the immature salmon as well as the ratio of nitrogen and phosphorus concentrations. For phytoplankton to thrive, there has to be a perfect balance of phosphorus and nitrogen.
Finally, researchers measured phaeophytin, a pigment found in phytoplankton. Phaeophytin helps scientists understand how well phytoplankton are synthesizing sunlight.
The Four Original Lake Fertilization Winners
Based on these studies, CIAA identified four lakes for enhancement through lake fertilization: Packers, Bear, Leisure, and Chenik. CIAA began to fertilize Packers and Bear Lakes while ADF&G fertilized Leisure and Chenik Lakes.
Each year, CIAA identifies the amount of fertilizer and the ratio of phosphorus and nitrogen it will apply to each lake. CIAA sprays liquid fertilizer onto the surface of the lakes from a boat about once a week throughout the active growing season from mid-June to late-August. CIAA then conducts routine monitoring of the lakes to make sure it meets salmon enhancement objectives and to adjust fertilizer applications.
Today’s Lake Fertilization Status
Today, CIAA still manages salmon enhancement in these four lakes, although only two are still being fertilized.
- Bear Lake — Enhanced through lake fertilization and sockeye stocking since 1990. CIAA has boosted growth rate and number of immature salmon produced without any negative changes to the lake.
- Packers Lake — Enhanced through lake fertilization and the stocking of salmon fry by CIAA for several years. Based on routine monitoring of the lake, CIAA determined the lake fertilization program was not meeting enhancement objectives and discontinued it. CIAA also stopped stocking salmon fry several years later when harvest management plans changed and natural lake production increased.
- Leisure Lake — A barriered lake with no natural salmon population. Leisure Lake provides good rearing conditions for immature fish and is stocked annually with salmon fry. CIAA uses lake fertilization to maintain the rearing conditions. ADF&G originally stocked and fertilized the lake, but CIAA now conducts the Leisure Lake salmon fisheries enhancement program.
- Chenik Lake — Enhanced through lake fertilization and the stocking of salmon fry by ADF&G for several years. CIAA installed jump pools to assist adult salmon returning to the lake to spawn and reproduce. CIAA identified a viral disease in Chenik Lake sockeye and discontinued both the lake fertilization and stocking programs. With the aid of the jump pools salmon production has improved and there are no plans to resume the lake fertilization or the stocking enhancement program.
Lake fertilization can be good for salmon, but it requires regular monitoring. Lakes are dynamic and the amount and type of nutrient applied must be adjusted each year.
To find out more about the lakes we fertilize see CIAA’s 2021 Annual Report.