The CPR is a self-contained automatic sampler. It is towed behind the ship at normal ship speed and can operate in nearly all sea conditions.
As the CPR is towed along, water and zooplankton enter a small (1.25 cm2) aperture in the nose cone. This expands into a wider collecting tunnel, slowing down the water flow. The plankton are trapped between two bands of mesh silk (6 m long x 15 cm wide), and loaded in a removable cassette. The silk and plankton ‘sandwich’ is wound onto a take-up spool inside a formalin preserving chamber. All this is driven by passing water turning an external propeller.
The sheets of silk advance at a fixed rate of 1 cm per nautical mile travelled. Each tow represents a 450 nautical mile track of continuous sampling.
Back in the laboratory, each set of silks is unrolled and cut into sections. Each section represents 5 nautical miles. The caught plankton can then be identified and counted under the microscope.