Ichthyoplankton Sampling Methods

There are two methods that researchers use to collect plankton at sea. The first is to tow a net through the water to catch the plankton, and the second is to pump the water aboard the vessel, and sort the plankton from it.

I. Plankton tows - The objective of a plankton tow is to obtain a quantitative sample of the zooplankton occupying the water column from a desired depth at the time of the tow. We collect plankton from a research vessel using three main types of net tows:

 1. PairoVET tow

A PairoVET is a small, fine mesh net that is lowered into the water to 70 meters when the ship is stopped and towed vertically to the surface. It is used primarily to sample planktonic fish eggs.

A Bongo tow is named because the nets look like bongo drums. The net is towed obliquely through the water while the ship is underway from 212 meters to the surface, effectively sampling the layer of water that nearly all the ichthyoplankton resides in.

3. Manta tow

A manta tow is different in that the net is dragged just along the surface of the water while the ship is underway. Some larvae are specialized to live at the surface, such as mahi-mahi, grunion, and flying fish.

When the nets are brought back on board the research vessel, a scientist uses a hose to rinse the net, flushing the plankton to the very bottom, or cod end of the net. The contents from one side of the bongo tow are then rinsed into a glass jar, and a 5% buffered formalin solution is added to preserve the sample. The plankton from the other side of the bongo tow is preserved in 95% ethanol, which is a better preservative for genetic analyses and for ageing studies. All the samples are then brought back to laboratory for sorting and identification.

II. CUFES - In addition to net tows, plankton is collected while the research vessel is moving using a Continuous Underway Fish Egg Sampler, or CUFES. Water is pumped aboard the vessel from 3 m depth at 640 liters/min. The water is sent through a concentrator where it passes through a net, and the plankton is diverted to a collector. While CUFES is running, a data logger is recording the date, time, and position for each sample as well as other environmental data from the ship's sensors (e.g. wind speed, direction, SST).