Blooming Waters Lead to Booming Harvests

Figure 3. Dissolved oxygen concentration and zooplankton density in a high success pond and a low success pond from Sea Center Texas in June 2021.

Here at Sea Center Texas, Red Drum and Spotted Seatrout are raised in outdoor grow-out ponds as part of the Texas Parks and Wildlife Department (TPWD) stock enhancement program. The goal of stock enhancement is to supplement existing populations of recreationally important fish to support fisheries management along the Texas coast.

A key step in the hatchery process is growing out or rearing juvenile fish in outdoor ponds. This is a complex task and involves delicately balancing water quality, nutrients, and food availability. Too much or too little of something can influence the success of a pond and the fish it supports. Before a pond is stocked with fish, the primary objective is to ensure an adequate source of food is available for optimum growth and development.

The pond food chain

Recently hatched fish, called fry, eat microscopic animals known as zooplankton. For example, tiny crustaceans called copepods and small soft-bodied animals called rotifers make up a large portion of their diet. Just as with any food chain, zooplankton need a food source as well, and their cuisine of choice? Phytoplankton, microscopic plant-like organisms.

Phytoplankton are a diverse group of organisms that obtain their energy from the sun through a process called photosynthesis, in the same way plants do. There are over 20,000 known species of phytoplankton, largely comprised of microscopic algae called diatoms and dinoflagellates. Just like any other plant phytoplankton require sunlight, carbon dioxide, and nutrients like nitrogen and phosphorus to grow. When phytoplankton rapidly grow and reproduce, a “bloom” happens. When hatchery ponds “bloom” with phytoplankton, there is a subsequent increase in zooplankton, meaning more food for fry. Thus, blooms are important to ensure the survival and health of hatchery fish.

All about timing

Knowing when to stock a pond with fish is important. Too early and there may not be sufficient food available to sustain the entire fish population, leading to large losses. Too late and you miss the window for peak growth and development as the zooplankton population may begin to naturally decline.

Hatchery staff can manipulate conditions to quickly establish an adequate zooplankton population. The first step to accomplish this is to boost the density of phytoplankton by fertilizing the pond. Like fertilizing a garden, carefully calculated amounts of nitrogen and phosphorus are added soon after filling a new pond. Next, plankton abundance is monitored by collecting daily samples and counting them under a microscope.

Even before collecting samples, visible changes are observed in the pond in the first few days. After fertilization, the color of the pond will turn a deep amber brown, indicating a large bloom of phytoplankton. Following the bloom, the pond will turn a more golden-green color coinciding with an influx of zooplankton grazing on the algae. A clear, blue-green pond is an indicator of low density of both types of plankton and is not a good candidate for stocking fry.

Along with color changes, there is noticeable difference in the concentration of dissolved oxygen (DO) in the water as phytoplankton numbers increase. Oxygen, being a product of photosynthesis, increases in concentration throughout the day as phytoplankton absorb sunlight and declines after sunset as photosynthesis comes to a halt.

The two graphs in Figure 3 show data from different Red Drum ponds in June of 2021. You can see an increase in DO after fertilizer was added in both ponds followed by an increase in zooplankton, indicators of a phytoplankton bloom. In the first graph, fish were stocked when the zooplankton density was over 700 organisms per liter and increasing. That pond resulted in 87% survival, which was nearly 400,000 fish! In the second graph, the pond was stocked at less than 200 organisms per liter, which resulted in a lower survival of 10% (about 40,000 fingerlings).

Maintaining the balance

Once a pond is stocked with fish, the zooplankton population will decline as they are consumed by the fry. It is important to keep the population up to supply food for the fish as they continue to grow. Throughout the duration of pond culture, several things are done to keep a balance between plankton numbers, nutrient levels, and oxygen concentration. As the number of zooplankton begin to decline, additional fertilizer is distributed, stimulating a new phytoplankton bloom, which is then followed by an increase in zooplankton for the fish to eat.

However, too much of anything can be a bad thing, and if the phytoplankton bloom is too dense, this can lead to bigger problems such as high pH, which can kill fish, or a depletion of oxygen as the billions of plankton die off and decompose. Even weather patterns can complicate things with cloudy days leading to low phytoplankton growth or conversely, hot sunny days leading to overproduction.

The (not so) sweet smell of success

Of course, all good things must come to an end. Once the fish reach an average length of 35 millimeters, the pond is harvested. It is at this point that we can finally see the results of all our efforts and monitoring. There is nothing more satisfying than to see buckets and buckets full of healthy fingerlings being transferred into a trailer to be released into the bay. Post release, all that is left is to clean up the muck of decomposed plankton and fish waste and start the process all over again.