The primary issue with an earthen pond is that you’re constantly fighting the environment. Water seeps into the soil, currents are slowed by mud, and you have almost no say in where the water actually goes. But a liner puts you at the helm. A liner’s smooth surface lets water move exactly how you want it to, so your pond becomes a powerful tool for managing your hatchery.
The practical benefits of this level of control are significant. When you control the flow, the water does the work for you, so fresh, oxygenated water gets to all your fish, not just the ones near the inlet. More importantly, you can use that same flow to continuously sweep away dissolved metabolic wastes before they can harm the alevin.
This chapter explores the modern engineering principles used to create these ideal, controlled environments. To fully appreciate why this level of control is so critical, however, it’s helpful to first examine the cascade of problems presented by the traditional alternative: unlined earthen ponds.
The Earthen Pond Problem
While sometimes used for less sensitive stages, such as holding broodstock, using a traditional earthen pond for the delicate hatchery phase presents a cascade of problems. The issue is created by a complete lack of control over the culture environment.
No True Biosecurity
The soil itself acts as a natural, permanent reservoir for a host of bacteria, fungi, and parasites. This makes effective biosecurity a practical impossibility, as there is a constant, uncontrollable pathway for pathogens to enter the system.
Unpredictable Water Chemistry
An unlined pond is always a reactive environment. Unwanted minerals or pollutants present in the soil can leach into the water, altering its chemistry and pH in unpredictable ways. This makes the precise management required for the hatchery stage nearly impossible.
Physical Dangers
The rough, uneven surface of a mud bottom can cause physical abrasion to delicate alevin, while constant water movement can erode the pond banks, degrading the structure and clouding the water.
Impossible to Clean
An earthen pond can never be truly sterilized between cycles. Waste from the previous cycle becomes embedded in the soil, where it can decay, release toxic compounds, and harbor disease.
Proof from the Field
The problems associated with unlined earthen larval ponds are well documented. Traditional earthen ponds used in Southeast Asia’s shrimp aquaculture industry, for example, have historically been plagued by rampant disease outbreaks and poor water quality, which are directly linked to the soil. Similarly, smallholder farmers in regions like Kenya have faced numerous setbacks in earthen ponds, from stock loss due to predation and floods to the general lack of environmental control, often leading them to abandon their farms. These real-world challenges underscore why modern aquaculture has moved toward more controlled systems.
Letting the Water Do the Work in Larval Tanks
To overcome the dangers of an earthen pond, modern hatcheries move newly hatched alevin into a highly controlled environment: the indoor larval tank. This is where the fish are at their most vulnerable life stage— weak swimmers, still attached to their yolk sac for nutrition, and exquisitely sensitive to their environment. Establishing an ideal environment in larval tanks requires managing two key sources of physical stress: the water current and the tank surface itself.
Establishing an ideal water flow for this delicate stage is a specialized task. Its singular purpose is to provide life support with minimal physical disturbance. An ideal flow slowly and gently delivers a constant supply of fresh, oxygenated water while flushing away metabolic wastes without tiring the fish or causing physical stress.
This is where the tank’s surface texture becomes a critical factor, and liners become an essential component. The perfectly smooth surface of a geomembrane liner eliminates physical abrasion to the alevin’s soft bodies and yolk sacs—a common source of injury and secondary infections even in concrete or rough-walled tanks. The inert, non-porous surface also provides the highest level of biosecurity by protecting the fish from environmental threats when their immune systems are least developed.
Integrating Aeration
Alevin are raised in such high densities that dissolved oxygen levels can drop quickly to levels where supplemental aeration is required. The challenge is to add oxygen without creating turbulence that could harm the fragile fish.
Hatcheries solve this problem by using fine-pore air diffusers, also known as “air stones.” Unlike coarse bubblers that create violent currents, these devices produce a gentle curtain of micro-bubbles, creating gentle circulation through a process called an “airlift.” As the column of bubbles rises, it draws water up with it, creating a slow, rolling, vertical current throughout the tank. Diffusers are placed so their airlift effect aids and reinforces the primary flow from the water inlet. Together, they form a single, stable circulation pattern that oxygenates all corners of the tank without creating hotspots of turbulence that could stress the alevin.
When integrating these systems, protecting the liner is a high priority. Aeration equipment, such as diffusers and tubing manifolds, should never rest directly on the liner itself. Best practice requires placing them on smooth, pre-cast concrete pads or other non-abrasive platforms to prevent the equipment’s weight and subtle vibrations from scuffing or abrading the liner over time.
How a Liner Makes It All Possible
These engineering principles—gentle flow and aeration in a non-abrasive environment—all depend on a single component: the liner. The smooth, predictable, and inert surface of a high-performance geomembrane makes these advanced techniques possible.
A liner minimizes friction, providing the necessary hydraulic control to reliably maintain slow, gentle currents. Its perfectly smooth surface eliminates the risk of physical abrasion that is always present in crowded, rough-walled tanks. Finally, a liner’s impermeable, non-porous qualities create a biosecure container that can be completely sterilized between cycles. For concrete and bolted steel tanks, a high-performance liner is essential for creating a safe, non-abrasive, and biosecure environment for the hatchery’s most vulnerable life stage.
A Note From the Trenches
Before introducing a single alevin into a new tank, test your water flow. A tiny pinch of a non-toxic, food-grade dye added to the water inlet will allow you to visually trace the currents. This simple test is the best way to confirm that your gentle circulation is efficiently reaching all corners and that there are no “dead zones” where water could stagnate.
Looking Ahead
A well-engineered larval tank provides the physical foundation for a healthy crop. However, the hardware is only half the equation; to be effective, physical control must be paired with rigorous operational practices. In the next chapter, we will expand from engineering the tank to engineering the entire facility for biosecurity, creating a defensible space against pathogens.
