Marine Invertebrate Larvae in Hong Kong Waters
Marine decapod crustacean larvae are typically collected using one of the following methods: plankton net tow, plankton pump and light trap. Some sampling methods allow for depth-stratified sampling (e.g., multiple open and closing nets and plankton pump), while others are depth-integrated.
Plankton Tow
The number and composition of organisms collected varies with the net design, net mouth size, mesh size, and towing speed.
In general, a finer mesh (e.g., 63 microns or smaller) is used for collecting phytoplankton while coarser meshes (220 microns – 0.5 mm) are used for studies of zooplankton, including marine invertebrate larvae, as well as fish eggs and fish larvae.
Tow speeds typically range from 1 – 2.5 knots depending on the mesh size and can affect the net's filtering efficiency. A flowmeter is usually attached to the net mouth to measure the volume of water filtered over the distance towed.
A good, classic reference can be found here: http://unesdoc.unesco.org/images/0007/000715/071517eo.pdf
Plankton Pump
Submersible pumps, or diaphragm pumps, can be used to draw water from a given depth. It is important to consider whether the sample volume will be passed through the pump's internal mechanisms and risk the zooplankton being damaged.
Pump speed varies depending on the power of the pump and the sampling depth (i.e. overhead pressure). Sieving, either with mesh or nets, is often used to concentrate the organisms from the water sample.
Light Traps
Many marine invertebrate larvae and other zooplankton are attracted to light (i.e. are positively phototactic) and would swim into traps that are fitted with an artificial light source.
The color and intensity of light used, the design of the trap aperture, and the deployment schedule (e.g. full moon, new moon) can affect the trap's effectiveness.
You can also make a simple light trap using recycled drinks bottles and glow sticks to sample zooplankton.
An unsorted sample of mixed zooplankton from a plankton net tow.
Studying plankton can be physically tough work and seasickness is an occupational hazard.
The Bongo net is washed down with seawater once it is hauled on board to collect the plankton in the cod-end.
Best Practice Methods for Collecting Marine Crustacean Larvae
Please note that a scientific permit approved by Agriculture, Fisheries and Conservation Department (AFCD), HKSAR Government is required for towing a plankton net from a vessel in Hong Kong, which must also be a licenced fishing vessel.