January 27, 2010

Technical Question of the Month: Why do we use a plumb line?


Reef Check Tropical — Why do we use a plumb line?

In a Reef Check survey on a coral reef, three separate surveys are carried out – fish, invertebrates and substrate. The latter focuses on determining the percentage “cover” of sand, rock, rubble, live and dead coral and other substrate types. During the substrate survey, the type of substrate is recorded every 0.5m along the transect line to provide an estimate of cover. Often the transect line is suspended above the seabed and typically will swing back and forth with the waves and current. By dropping a weighted string called a plumb line at each 0.5m mark along the transect line, the survey personnel minimize any observer bias that could occur if the data collector chose a particular spot to collect the data. The data collector can simply line up the plumb line with the half meter mark and drop it – even without looking — then look to see where it landed. The plumb line also speeds up the data collection process so is a very helpful tool. A Reef Check plumb line can be constructed using a 1.5m long cotton or nylon string with a wrist loop at one end and a 0.5cm nut at the other.