The ecology of benthic macro-invertebrates in earthen trout ponds at Howietoun, central Scotland

Abstract

An Investigation Into the ecology of benthic macro-invertebrates in earthen ponds subjected to intensive trout (Salmo trutta L.) culture practices at Howletoun, 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 Oligochaeta. (10 species), Chironomldae (18 species), Molluscs 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, with an average population density of 68,400 - 191,200 worms mˉ², 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, with a population density of 5,400 to 14,900 ind. mˉ² and forming 7 to 13% of the total benthos with peaks in spring and autumn. Dry biomass of total benthos varied from 24-59 g mˉ² in the cultured ponds with oligochaetes accounting for 14-49 g mˉ² and chironomids 4-7 g mˉ². The mean annual dry weight production of total benthos varied from 130-215 g mˉ² in the cultured ponds, with oligochaete production of 94-160 g mˉ² and chironomid production of 20.6-33.5 g mˉ².

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 Oligochaeta (10 species), Chironomidae (I8 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, with an average population density of 68,400 – 191,200 worms mˉ², 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, with a population density of 5,400 to 14,900 ind. mˉ² and forming 7 to 13% of the total benthos with peaks in spring and autumn.

Dry biomass of total benthos varied from 24-59 g mˉ² in the cultured ponds with oligochaetes accounting for 14-49 g mˉ² and chironomids 4-7 g mˉ². The mean annual dry weight production of total benthos varied from 130-215 g mˉ² in the cultured ponds, with oligochaete production of 94-160 g mˉ² and chironomid production of 20.6-33.5 g mˉ².

In an unstocked control pond the total production was 55 g mˉ², 35 g mˉ² of which was accounted for by oligochaetes and 8.06 g mˉ² 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.