A small project was funded by the United States’ Nebraska Soybean Board to develop a new methodology for farms on Lake Volta, Ghana to track changes in the thermocline (Fig 1) and associated dissolved oxygen levels and communicate potential risks to nearby farms by sharing the data through the Chamber of Aquaculture in Ghana. The measuring devices were installed to log hourly measurements of dissolved oxygen and temperature at various depths to detect where the thermocline is and to track the thermocline depth to relate any changes to weather events, rapid cooling of the surface or sudden algae die-offs. Deployment of the data loggers will be altered as data are analyzed to identify the depth where an abrupt change in water temperature occurs.
In temperate zones, lake turnover (when the bottom layers of low-oxygen water rise to mix with the upper layers of more oxygen-rich waters), is well known to be a seasonal event both in spring when ice is melting and in fall when ice is forming. In the tropics, the thermocline tends to be much shallower, and turnover can occur much more frequently and unpredictably, especially in eutrophic waters.
There have been several large-scale fish kills at cage culture operations in the tropics that have been the result of mixing of very low- dissolved oxygen water with surface waters where the cages are located; some of which could have been predicted if changes in the thermocline depth were being monitored. First, the approximate thermocline will be sought by comparing water temperature at surface and bottom. If there is a difference, then the point of the thermocline will be estimated by working upwards to seek the point of a temperature change.
In this presentation we will describe the inexpensive monitoring system used at 2 cage culture sites: one being in the “open lake”, where large cages are used, and water quality is very good for at least the first 8 meters. The second site is at the opening of a bay on the lake where intensive cage culture occurred several years ago but has since been reduced. A combination of dissolved oxygen/temperature loggers and less expensive temperature loggers are being used. Secchi disk readings, weather observations and fish feeding behavior are also being tracked.