Brown trout were tagged with sonic tags in Airthrey Loch, Stirling, and Loch Leven, Kinross. The results were expressed as mean hourly and maximum specific swimming speeds, centres of activity and home ranges. The results indicated that tagging effects lasted for up to 48 hours after release. There were seasonal changes in the levels of activity with a peak in March. Home ranges tended to be widest at dusk and dawn. Mean hourly specific swimming speeds were very low (0·14 BL/sec). The trout did not move randomly throughout either loch, and in Loch Leven remained within 1·4 km of the release point.

The minimum energy requirements of trout and perch populations in Loch Leven, Kinross, were calculated for the year 1970–71. Use was made of laboratory data relating to oxygen consumption and ammonium nitrogen (NH3–N) output. Estimates of the specific dynamic action (SDA) of food material were made at 5°, 10° and 15°C and used to predict periods of growth. It was found that optimum growth would take place at 10°C, as opposed to 15°C, and that negative somatic production in the overwintering trout population was due to the energy content of the food intake being less than the minimum energy requirements of metabolism.

This paper gives results of a survey of the parasites of trout, perch, pike, stickleback and minnow carried out at Loch Leven between April 1967 and March 1972. Twenty-three species of parasites (Protozoa, Cestoda, Trematoda, Nematoda, Acanthocephala and Mollusca) were recorded from trout, 12 from perch, 11 from pike, 12 from sticklebacks and three from minnows. Details of the incidence and intensity of infestation of trout and perch by the parasites, except the Protozoa, are given. The parasites of trout are divided into loch and stream types.

A large water surface, suitable islands for nesting, extensive shallows, rich adjoining farmland and comparative peace help to make Loch Leven internationally important for waterfowl. It has long held the largest concentration of breeding ducks in Britain, consisting in 1966–72 mainly of tufted duck (500–600 pairs) and mallard (400–450 pairs), but also of gadwall (20–30 pairs), wigeon (25–30 pairs), shelduck (5–18 pairs), shoveller (up to 10 pairs) and teal (up to 10 pairs). In winter, it was important for mallard, teal, pochard and in some years for shoveler, and held good numbers of wigeon and goldeneye. These various species differed in the proportion of their food which they obtained from the loch itself. The loch also acted as a winter roost for up to 5000 greylag geese, up to 12 500 pinkfeet and up to 430 whooper swans, all of which fed entirely in nearby fields. The numbers of most species underwent regular seasonal fluctuations, with peaks and troughs in the same months each year; but different species reached peak numbers in different months. In general, waterfowl numbers were greatest in autumn, when the loch held up to 20 000 birds.

The status of several species at the loch has changed markedly during the last 100 years. Some such changes were linked with general changes in the ranges and status of the species concerned, but other (more recent) ones with the reduction in macrophytes following eutrophication.

The tufted duck was chosen as the subject of an energy study at Loch Leven because it is a diving duck which feeds mainly from within the loch and it is present throughout the year, about 500 to 600 pairs nest annually.

Using data from population counts, breeding biology and feeding studies and the analysis of body and egg composition an energy budget for the population has been computed. The values for respiration, R, are calculated on the basis that ‘existence energy’ requirements are 2·5 × Standard Metabolic Rate. Values for rejecta assume an assimilation efficiency of 75 per cent.

The figures have been presented on a monthly basis as there are wide fluctuations in population size, and therefore energy demand during the year and between years. In 1970 very large numbers of tufted ducks were present on the loch during autumn. The additional demands of these birds over 4 months represent an increase in the annual energy requirements of almost 50 per cent.

Chironomid larvae account for 60 per cent of the food of the tufted ducks at Loch Leven; other important food items are caddis larvae and molluscs (mainly Valvata spp.).

Expressed on a /m2 basis and over a full year the energy demands of the tufted ducks represent a small proportion of the total energy available at Loch Leven. However, water depth and the depth of the food in the substrate reduce the actual availability of the food and probably reduce the usable feeding area to 20 per cent of the total loch area.

Twelve species of trematode are described from 70 gulls of 5 species examined at Loch Leven. Observations on these trematodes are given with additional morphological details. The environmental origins of the parasites are discussed and related to the habits of the hosts.

The International Biological Programme provided resources and enabled the work of a considerable number of researchers to be focused on physical, chemical and biological studies on Loch Leven, Kinross. In this paper the results are summarised with the help of an energy flow diagram and relationships between different trophic levels discussed. Major gaps in our knowledge, which have still to be worked on before all the relationships between different trophic levels can be well understood, have been highlighted. The causes of changes which have been recorded at Loch Leven and the unusual variability in the ecosystem are discussed and hypotheses presented. Profitable lines for future research are suggested, particularly in relation to the management of the loch as a National Nature Reserve and a brown trout fishery.