258
THE ESSEX NATURALIST
is plentiful. An examination of the stomach contents of several
of the Stone Loach caught in the Sweep Transect showed
chironomid larvae (a), simulid larvae (f), Gammarus (vf),
Ephemeroptera and Coleoptera larvae (r). Smyly (1955) in his
excellent paper on the Stone Loach has also found its chief food
to be chironomids and gammarids, these two groups forming the
main food supply for all seasons of the year. This is further
confirmed by Hartley (1940). Furthermore, like Erpobdella
octoculata the Stone Loach can tolerate a very wide range of
environmental conditions. To quote Smyly (1955):—"It has
been found in soft and hard waters, in still and flowing waters,
under ice and in water at 21°." The presence of Erpobdella
octoculata and Nemacheilus may be regarded as indicative of an
organic pollution state and they could be termed secondary indica-
tors of pollution.
The presence of a large number of chironomids in a riffle and
almost entire absence of ephemeropterans may be regarded as an
indication of pollution since in an unpolluted riffle the reverse is
the case.
It is known that Gammarus will quickly die if the oxygen
content falls below 30 % saturation and it is, therefore, a sensitive
indicator of the amount of available oxygen. The absence of
Gammarus indicates a gross pollution state and its presence in
large numbers shews that a river is well aerated. Another possible
explanation for a large population of gammarids will be found in
the food-web discussion.
The effect of organic pollution on the flora and fauna of rivers
has been extensively studied both in this country and in many
Fig. 7. The effects of an organic polluting discharge