The pond net has to be one of the simplest of scientific tools, yet the type of net used will have a major influence on what is caught. This article gives an overview of the main types of net, provides supplier details and some guidance on net care.
Nets can be divided into two main types – hand (or pond) nets used on the end of a pole used in a sweeping motion from a bank or jetty, and trawl (or plankton) nets designed to be dragged through the water on a rope. Hand nets are usually small, supported on a metal frame of 150 to 250 mm opening, with a square or conical net less than 300 mm long and handle up to 2.5 m long; beyond this length is unwieldy. Older hand nets were of round frame and supported a net made from bolting silk (used in silkscreen printing). More modern nets usually have a square frame, making sampling along a flat surface (jetty wall or lake bottom) much easier, and nets are now constructed of nylon with defined mesh size.
A hand net
The size of net mesh will influence what is caught; generally for Cladocera a mesh of 50 µm is recommended to retain the smaller species (e.g. Chydorus). Larger meshes (or holes in the net) will result in these smaller species being missed. Smaller meshes are also available (for nanoplankton) but tend to clog more easily. At the end of the net either a sample tube is screwed in to retain the catch, or a nylon filter that allows water to pass through (easing the ‘sweep’); my personal preference is for a tube (e.g. 30 ml Universal container) which can then be emptied into a larger bottle. End filter arrangements tend to cause more damage to water fleas, and require the filter surface to be washed into a tray or bottle after collection.
Hand nets obviously have very limited reach, and to sample open-water species in larger ponds or lakes a trawl net is necessary. These usually have a circular frame of 250–300 mm opening, a net of up to 1 m length terminating in either a tube or filter, and a drawline of about 10 m. Trawl nets can either be towed through the water from a boat (a canoe works well) or thrown into the water from a bank (some practice is necessary to achieve a good throw!). Again, a mesh of approximately 50 µm is recommended for freshwater species, but for marine Cladocera a slightly wider mesh (e.g. 150–250 µm) is preferable to prevent the mesh clogging too quickly.
A trawl net
For micro-habitats such as puddles, tree boles or bromeliads, no net is necessary; I find that a plastic turkey-baster works well!
Nets can be home made and several designs have been published providing instruction on how to cut a conical net. However, commercial nets are readily available at reasonable price. In the UK, EFE-GB is the main supplier with a wide range of nets, frames and handles; Watkins & Doncaster also provide simpler, traditional-style nets. The EFE-GB nets are robust and will last many years. My own (personal) reservation is that the standard net frames (250 mm) are larger than necessary and very restrictive in small ditches and puddles, and the solid wood handles are much heavier than necessary. My own net was made as a special, using EFE’s children net frame (150 mm opening) with 53 µm mesh net. I replaced the wood handle with three locking sections of net handle from Watkins & Doncaster (actually metal tent poles) and have replaced the net end filter with a 30 ml bottle.
In the USA (and available by mail order), most of the biological supply houses such as Carolina Biological Supplies and Turtox supply nets, although shipping costs outside of the USA may be higher).
Modern nylon nets are much less prone to rotting than the traditional bolting silk, but still should be thoroughly washed in running water and dried completely after use. The risk of transfer of species from one area to another must be prevented by thorough washing and drying, especially the risk in the UK of facilitating the spread of the invasive Dikerogammarus villosus, recently found in Cambridgeshire and South Wales (check the Environment Agency website for details of the Check Clean Dry campaign). Importantly, if using nets and other equipment overseas, equipment should be sterilised both before going and immediately upon return to prevent the transfer of exotic species. I seal my nets in a polythene bag containing kitchen paper soaked in formalin (carcinogen – care!) for a few days to expose the nets to formaldehyde vapour; for those unable (or unwilling) to use formalin, hydrogen peroxide solution can be tried. Well-cared for, a net will last a lifetime of sampling.