ON THE OCCURRENCE OF THE GUPPY 189
per square metre, but it contains few other macro-invertebrates.
Even midge larvae, such as chironomids, normally common in
muds of polluted rivers, are relatively scarce because of the very
small amount of dissolved oxygen occurring in the vicinity of the
mud-water interface for most of the year. On the other hand,
Springtails (Collembola) occur on the surface film and at certain
periods of the year air-breathing mosquito larvae appear. Finally,
a thriving population of invertebrates (including Wandering Snail
Limnaea pereger, leeches Erpobdella octoculata and Haemopsis
sanguisuga, the beetle Haliplus ruficollis and the isopod crustacean
Asellus aquaticus) has been found to exist in the growths of
filamentous green algae (Chlorophyceae) that grow along the
vertical banks of the canal. Also plankton sweeps in the vicinity
of the algae usually produce Cladocerans and cyclopoid Copepods.
In the open water, save for a small strand of Funnel-leaved Pond-
weed, Potamogeton pectinatus, which appeared in a rather
shallow part of the river in 1967, vegetation is absent. The reason
for the lack of plants would seem to be due to factors, such as
restricted light penetration caused by the turbid nature of the
river water, rather than exposure to high temperatures, since
other macrophytes, such as Ceratophyllum demersum, will appear
occasionally at Hackney if suitably placed cracks in the bank
develop. The only fish, other than guppies, that have been found
regularly in any numbers has been the Three-spined Stickleback,
Gasterosteus aculeatus, and it would be interesting to know if any
competition takes place between the two species that could affect
feeding sites. Preliminary observations suggest that the smaller
guppies are feeding predominately in the vicinity of the algal
growths, although older fish often show a preference for the open
water where they are probably relying on mosquito larvae and
surface drift.
Acknowledgements
I am grateful to the Lee Conservancy Catchment Board for
permission to publish this paper; to D. Marlborough of the British
Ichthyological Society for drawing my attention to an earlier
record of Lebistes in the British Isles and to A. C. Wheeler of the
British Museum (Natural History) for examining specimens of
fish collected from the River Lee at Hackney.
References
Dumbill, P. (1967). B.I.S. Monthly Newsletter, March 1967: 1.
Lee Conservancy Catchment Board (1967). Report for the year ended
31st March 1966. London.