Typically monitoring of marine cultivation systems is done through labour intensive and intrusive field surveys. With the introduction of new sustainable marine production chains such as the cultivation of seaweeds, economically feasible solutions for production and monitoring should also be included in management processes. New technologies have led to a more indirect form of monitoring marine systems, which is more efficient. Remote sensing is such a new technology which was previously mainly used to find large aggregations of fish or underwater topography. This report investigates the use of sonar for a quantitative and effective means of monitoring seaweed cultivation. The sonar was able to detect seaweeds and distinguish between the cultivated and filamentous seaweeds growing on the lines. However difficulties where found due to the movement of both the boat due to waves, and the cultivation lines. Which made it difficult to analyse the sonar video on board. Later snapshots of the video were made which showed a clearer picture. The predicted length and amount of seaweed was compared with the actually harvested seaweed. The actual number of seaweeds was higher than the sonar could see due to the overlapping of the plants, this also increased the maximum length as predicted by sonar. Sonar showed the larger plants most clearly, however the advantage of hydro acoustics is that there are no turbulence interference as is often the case with optical monitoring techniques. Using sonar provides a good estimate of the maximum length of the cultivated plants and once validated could be a high potential monitoring system.