Abstract
An investigation into the ecology of benthic macro-invertebrates
in earthen ponds subjected to intensive trout (Salmo trutta L.)
culture practices at Howietoun, Central Scotland, was conducted
between May 1984 and January 1986. Soil and water quality, seasonal
changes in benthos, its role in the trout diet and the interaction
between fish and benthos were studied.
Pond benthos mainly comprised 6 major groups including oligcchaeta
(10 species), chironomidae (18 species), mollusca and hirudinea
(2 species each) and an asellid and a sialid species. Oligochaeta
formed 78 to 90% of benthic fauna, dominated by Tubifex tubifex,
Limnodrilus hoffmeisteri, L. udekemianus and Psammoryctides barbatus,
-2 with an average population density of 68,400 - 191,200 worms m
and exhibited peaks in summer and late autumn corresponding to
two major breeding periods. The principal species of chironomidae
were Chironomus spp., Procladius spp. and Prodiamesa olivacea,
-2 with a population density of 5,400 to 14,900 ind. m and forming
7 to 13% of the total benthos with peaks in spring and autumn.
-2 Dry biomass of total benthos varied from 24-59 g m 2 in the cultured
-2 ponds with oligochaetes accounting for 14-49 g m and chironomids
-2 4-7 gm The mean annual dry weight production of total benthos
-2 varied from 130-215gm in the cultured ponds, with oligochaete
-2 production of 94-160 g m and chironomid production of 20.6-33.5 g m
2 In an unstocked control pond the total production was 55 g m
-2 -2 35 8 m of which was accounted for by oligochaetes and 8.C6 g m
by chironomids. Analyses of gut contents of the farmed trout showed
that 12% of the diet by volume consisted of natural food, mainly
benthos. Fish selectively fed on chironomid larvae, mollusca,
Asellus aquaticus and Sialis lutaria. Fish took maximum natural
food in the morning and evening.
Experimental enclosures to exclude fish from selected areas of
the ponds demonstrated that number of species increased outside
the enclosures but, except for chironomids, population density,
biomass and production generally increased inside the enclosure.
The possibility of explaining this result in terms of differential
predation is discussed.