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In the northern regions, most sea cucumber farming is conducted in open-air tidal zones and reef-building ponds. Many farmers believe that during winter, low temperatures reduce the need for management, leading to neglect and significant losses of sea cucumbers. Over recent years, it has become clear that large-scale disease outbreaks in farmed sea cucumbers typically occur at similar times. In Shandong, this period usually falls between mid-February and early May, with the highest incidence concentrated between March and April.
During this time, a key environmental change is the gradual recovery of air and water temperatures after the cold winter. However, the rate at which these temperatures rise differs—air temperature increases more rapidly, while water temperature rises slowly, creating a lag effect. When the air temperature exceeds the water temperature, a thermal stratification occurs in the pond, with warmer, less dense water sitting on top and preventing mixing with the colder, denser water below. Measurements show a temperature difference of about 1 to 2 degrees between the surface and depths of 1 to 3 meters.
This stratification prevents the natural convection that would normally circulate oxygen throughout the water column. As a result, the bottom layer becomes "dead water," lacking sufficient dissolved oxygen despite the upper layers being well-oxygenated. Sea cucumbers, which live near the bottom, are severely affected by this lack of oxygen. Additionally, their metabolic activities and the decomposition of organic matter consume large amounts of oxygen, which is not replenished in time. Testing shows that dissolved oxygen levels in the bottom water often fall below 3 mg/L.
Under hypoxic or anoxic conditions, sea cucumbers experience reduced metabolic activity, and their circulatory, nervous, digestive, and respiratory systems suffer. Their ability to withstand stress and resist diseases declines significantly. At the same time, anaerobic bacteria thrive, breaking down organic matter and producing toxic substances like ammonia and hydrogen sulfide, further harming the sea cucumbers. As water quality deteriorates, the sea cucumbers become weak, making them more susceptible to bacterial infections and other pathogens.
To prevent such issues, farmers should maximize water exchange in the ponds, especially during high tides when fresh water can be introduced. It's also important to minimize daily temperature fluctuations and extend the duration of water flow as temperatures rise. Regular monitoring of the pond is essential—divers should be sent to check the sea cucumbers’ activity at the bottom if necessary. If any individuals show signs of illness, they should be removed immediately and disinfected to prevent the spread of disease. Proactive management is crucial to maintaining healthy sea cucumber populations and reducing losses.