A favourite subject
One of the most interesting subjects for study is pond life, and
here the word pond is meant to include all types of freshwater
habitat whether lake, reservoir, pool, ditch or puddle. Dipping
an empty jar in the pond will give you plenty of water to study
but the results can be disappointing because the catch is not
concentrated enough bearing in mind that you only use a drop on a
slide.
Using a special net
So how to concentrate a catch. This needs a pond net, about
6" diameter tapering to a little over an inch, with
removable plastic jar or tube attached at the narrow end in which
the catch is caught, but pond and plankton nets obtainable from
biological supply houses are expensive, from 20 up to over 50. In
the previous article, Douglas Laycock explains how to construct a
simple handle and frame to hold a pond net.
There are many fascinating specimens to find in pond etc. water but a fine net is required, around 50 to 70 microns square mesh. Nets supplied by many biological supply houses have a mesh of around 100 microns (0.1 mm), with a filter net over the jar. A 100 microns mesh net will be fine for collecting daphnia but you will lose a lot of your catch of smaller creatures through the side of the net. The filter net over the small tube needs to have a mesh larger than the creatures you are trying to catch, but sufficiently fine to keep out those you do not want. Nets used for wine making, which have a mesh of around 400 micron, are of no use whatsoever for pond nets, the mesh being far too wide.
Very fine net can be purchased but it is expensive. If you know anyone at a silk screen printers any of their off cuts or even used cast away pieces of silk would be ideal, these having an extremely fine mesh of around 60 to 70 microns. With a pond net you can walk along a canal bank dragging the net in the water, or you can stay in one spot and sweep the net back and forth provided you change the depth of the sweep. You must always keep the net moving forwards, and if you are sweeping at one spot, then in a figure of eight. Once put in the water you must keep the net moving, otherwise your catch in the net may well escape.
Technique
The jar will of course fill as soon as you put the net in the
water and now matter how dense the water is with creatures you
hope to catch or for how long you sweep with the net the same
water with its catch will remain in the attached jar and you will
get few extra specimens in it. So what is the purpose of all this
sweeping? The net will build up a good quantity of specimens
which are not in the jar because it filled with the first
immersion in the water, but they cannot escape because of the
force of the water through the net so long as you keep it moving.
What is wanted of course is to get a concentration of the catch in the jar and this is accom-plished by lifting the net out of the water. As the water drains through the net the creatures in it can only drop as the water level recedes until those which are not too large to do so, i.e.,, those you want, drop through the lower filter net and you will have a rich concentration in the jar. It is important, therefore, as recommended by Mr. E. D. Hollowday in his excellent article "Some Hints and Tips on the Collecting and Handling of Monogonontid Rotifera" in the Quekett Journal "Microscopy" 35, January-June 1985, pages 208-220, to maintain the full extension of the net cone until the very moment it is lifted out of the water. This is because the pressure of the water passing through the net forces the creatures against the side of the net. Again to quote Mr. Hollowday, protozoa and small algae etc. only enter the tube with the loss of the last 50 to 100 ml of water.
When the water has fully drained from the net remove the jar and rinse the inside of the net in the water to let the concentration of unwanted creatures clinging to the inside of the net swim to freedom. It is the rule that we do not take more than we need and that we do not destroy life, no matter how small, any more than is necessary.
Warning on
river, canal, reservoir danger zones
Two warnings were given by River and Health Authorities during
the summer of 1994. The first is of the rat bug danger, causing
Weils Disease, which is spread by harmful bacteria in rat urine,
found not only in sewers but also in many still waters. It can
enter the body through open cuts and abrasions and, a warning to
young people who take a dip in rivers and mill ponds during hot
weather, through the eyes, ears, nose and mouth. Symptoms are
said to be like a dose of mild 'flu which usually clears up after
two or three weeks, but the disease is dangerous and in extreme
cases can cause kidney or liver failure leading to death. So when
fishing or pond dipping cover any cuts or abrasions on the hands
with a waterproof plaster and wash your hands before handling
food. Do not forget, of course, that these same precautions also
apply when handling the pond water back home.
The second warning is against touching or
brushing against the dangerous giant hogweed plant often found
growing up to nine feet tall on river and canal banks. It is a
beautiful plant brought over from Russia to London's Kew Gardens
a hundred years ago but which has since spread around the
country. Touching it with bare hands or even through thin
clothing such as a shirt or blouse can cause severe dermatitis,
skin inflammation which usually develops within 24 hours, and
sometimes painful blisters. The condition is made worse if the
affected part is exposed to sunlight. In severe causes a
condition like a chemical burn may arise which will leave
permanent brown patches after healing. The National Rivers
Authority is carrying out a five year spraying programme in an
endeavour to eradicate the dangerous plant. Eating any of the
plant can result in severe damage to the lips and mouth.
Note: This article was written by Roy Winsby and was published in the newsletter of The Manchester Microscopical Society - Newsletter 30. It has been reproduced here through the kind permission of Roy Winsby.
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