Automatic measurement of rotifer Brachionus plicatilis densities in first feeding tanks

Rotifers are an important live food in the culture of marine fish, but the process of measuring rotifer culture densities is time consuming. This is especially true at low densities such as those applied in first feeding tanks. A particle counter for making automatic measurements of rotifer densities has been designed. The instrument automatically extracts samples, and relies on a digital camera and image processing to measure the rotifer density. Due to its autonomous nature, the instrument is suited for use as a component in a process monitoring and control system. The rotifer counter design is presented, and the statistical properties of the measurement derived. The accuracy achieved in practical countings is then investigated in a series of test counts. To assess the quality of measurements achieved in an actual first feeding tank with samples extracted from a single location, the rotifer counter is used in an experiment studying rotifer dynamics in a continuously diluted tank. The results indicate that the rotifers are approximately evenly distributed in the water column, and that one needs to consider rotifers attaching to the tank wall to be able to predict rotifer densities under these conditions. The experiment gives an example of the considerable potential for experimental work assisted by the automated rotifer counter.