How to Calculate the Depth of Your Plankton Net

Plankton nets are used by many scientists in marine biology and other fields to collect plankton from bodies of water for research. Often the depth at which the plankton were collected is crucial data. Professionals usually have access to digital devices that record how deep the net is during the collecting, however there are many people using plankton nets, such as student, who do not have access to this technology but still need to know the depth of the plankton net. This article describes how to calculate the depth of ones plankton net without expensive equipment.


  1. 1
    Measure the length of your plankton net line
    • Before the collecting trip, measure the total length of your plankton net line.
    • During the trip, either measure how much line you put out when you put the plankton net in the water OR after the net is all the way in the water, measure the length of line not in the water. If you measured the length of the out of water line, subtract that length from the total length of the plankton net line. (Record this number!)
  2. 2
    Assemble the Protractor
    • Cut around 15 centimeters of line. The length does not affect the depth calculation.
    • Tie the fishing weight to one end of the fishing line you cut.
    • Get your protractor. Hold it so the 0 degree line is horizontal. If you have a semi circle protractor, make sure the flat side is facing up.
    • Using tape attach the fishing line to the center point of the protractor (where the 0 degree line meets the 90 degree line). If there is a small hole in the center point, string a couple centimeters through the hole and tape the fishing line to the protractor on the other side. If there is no hole in the center point, tape the top of the fishing line just above the center point so the fishing line hangs down. Make sure that the bottom of the tape is flush with the 0 degree line, and the top of the tape is above the 0 degree line.
  3. 3
    Measure the Angle of the Plankton Net Line
    • While the plankton net is in the water collecting plankton, find a part of the plankton net to measure. Make sure the line does not change angle between your spot and the plankton (i.e. does not hang off the side or back of boat). If the plankton net line is attached to a crane, find a part is arms reach. If the plankton net line is being hung off the side or the back of the boat by hand, find the part of line right after it goes off the side towards the water.
    • Arrange the modified protractor so that the plankton net line matches up with the 0 degree line of the modified protractor.
    • Look and see where the fishing line crosses the protractor. (Record this number!) If the line is moving because of wave action, then use the median angle. (The angle in the middle of the swaying of the fishing line)
  4. 4
    Calculate the Depth of the Plankton net
    • Get your numbers ready, the length of the plankton line underwater (L) measured in step 1 and the angle of the plankton net line (A) measured in step 3.
    • Using a calculator do the following operation: Cosine (A) X (L). The result will be the depth of the plankton net in the unit you measured the plankton net line in.


  • Doing measurements on a moving boat can be dangerous due to wave action. Always wear a life jacket and secure your footing before attempting this.

Things You'll Need

  • Fishing line (15 cm)
  • Fishing weight
  • Calculator with trigonometry functions
  • Tape
  • Cheap protractor

Article Info

Categories: Mathematics | Biology