Study of plankton in relation to cage and pond culture of trout in Scotland

Abstract

An investigation of phytoplankton and zooplankton, together with physico-chemical parameters of water quality and periphyton, was undertaken from November 1980 to December 1981 at a large cage fish farm for rainbow trout in Loch Fad, Isle of Bute, and from February to December 1981 in earth ponds used for culturing brown trout at Howietoun Farm, Central Scotland.

Thermal stratification in Loch Fad was not observed. In Loch Fad pH was lower at the cage site than at a control station at the other end of the Loch, while at Howietoun, pH decreased as the water passed through the ponds. These effects were ascribed to the respiratory effects of the fish and the decomposition of fish wastes and metabolites. Inorganic nutrient concentrations were higher at the cage site than at the control station in Loch Fad, while at Howietoun they also increased from the inlet water to the outlet of the farm. This increase in nutrient concentrations was related to increased phytoplankton density at the cage site at Loch Fad and in the outlet water of Howietoun Farm.

A bloom of Microcystis aeruginosa was observed in Loch Fad where it formed 92% to 99% of the total phytoplankton and reached densities of 5 X 10® cells/1. At Howietoun, Cyanophyceae were also dominant from April to November, the most dominant species being Oscillatoria agardhi.

The zooplankton population at both Loch Fad and Howietoun displayed maxima during summer when the phytoplankton population was also maximum. Cladocera and Copepoda in Loch Fad were lower in number at the cage site than at the control, which might be due to predation by cage and wild fishes. In Howietoun all groups of zooplankton increased as the water passed through the farm. Common zooplankton species in Loch Fad were Bosmina longirostris, Daphnia longispina, Cyclops abyssorum and Diaptomus gracilis; while in Howietoun Farm common species were Bosmina longirostris, Cyclops vicinus and Diaptomus laciniatus.

Periphyton collected by using submerged wooden blocks was more dense at the cage site than at the control in Loch Fad. It was also more dense at the outlet than at the inlet at Howietoun. In Loch Fad, Baci11ariophyceae dominated the periphyton in contrast to the observed dominance in the phytoplankton of Microcystis aeruginosa, which was absent in the periphyton. At Howietoun Cyanophyceae, especially Oscillatoria agardhi were dominant from April to October.