A very good question.
Every time we go out sampling, we collect replicate samples with a tow net. We go from near the bottom of the port to the surface water of the port. The net is one-half metre in diameter. We use two different nets: one that has 80-micrometre mesh, a very fine mesh; and another one that has 200-micrometre mesh. The reason why you use the larger mesh is because sometimes with a smaller mesh it starts to clog and as you pull the net up to the surface, some organisms can detect the pressure wave in front of the net and swim out of the way. By using a larger mesh net, they get captured.
As I mentioned, we collect from six different areas in the same port. We put all six samples together into one sample, so we're sampling comprehensively.