The Food of Arctic Charr in the Presence and Absence of Brown Trout RONALD N. B. CAMPBELL Department of Zoology, University College, Cork Republic of Ireland Received August 1982 Within the British Isles, the Arctic Charr (Salvelinus alpinus (L.) ) almost always co-exists in lochs with other salmonids. All previous research has therefore been concerned with Chan- in competition with other, closely related species. In 1974, however, Charr alone were found in Loch Meant on the eastern side of the Trotternish peninsula of Skye, and in 1975 the food of this allopatric (separately occurring) population was compared, by stomach analysis, with that of a Charr population in Loch Fada on North Uist. The Charr there are in the more usual British position of being sympatric (co-existent) with Brown Trout.* Since 1974 some other lochs have been found that contain Charr alone. (Maitland et al 1982). There has been very little interest in British charr, and most has been concerned with their taxonomy (Friend 1956, 1959). First scientific analysis of their diet gave "minute entomostracea" and "aquatic insects" as their food (Jardine 1834, Baird 1857) and subsequent investigations largely confirmed this (Maitland 1884, Frost 1951) with an exception for the Charr of Loch Borralie in Sutherland which ate Sticklebacks (Gasterosteus aculeatus L.), corixids and insect larvae (results quoted in Darling and Boyd 1969). • This work was presented as a thesis for the honours degree in Zoology of the University of Aberdeen in 1976. Glas. Nat. 20 part 3 (1982) The Lochs Loch MealIt, Skye This is eutrophic 8.2 ha (20.3 acres) in area and its deepest known point is 3m. It is located on top of an exposed sea cliff, in a rock basin 51m above sea level, within a complex of Jurassic and Tertiary dolerite sills. It has three small inflow streams and is drained by a single outflow that runs for only 15m before falling over the sea cliff. It is not therefore accessible to migratory salmonids. Loch Fada, North Uist. This is a typical oligotrophic moorland loch 10m above sea level, 44.25 ha (109.3 acres) in area with a maximum depth of 30m, lying on a platform of Lewisian gneiss. The inflows and out- flows are small and have not yet been accurately mapped, though it is said to be accessible to migratory salmonids. Methods Since the two lochs are of different types, benthos samples (taken with a hand-net from the shores of the lochs) and zoo- plankton samples (taken with a tow-net from a boat) were obtained in addition to fish samples, so that differences in the feeding available to the fish populations could be assessed. All three types of sample were taken from both lochs in spring and autumn. The same broad categories of food were found to be available in both lochs. Netting of Fish Monofilament nylon nets of the "Tundhovn" type were used. These were "bottom set" (i.e. leadline of net bottom lay on the substrate) at the entrances to bays and left overnight. Loch Meallt was netted on the 1 and 4 May 1975 (6 Charr) and on 26 August 1975 (11 Charr). It was also netted on 21 May 1975 by staff of the Department of Agriculture and Fisheries for Scotland Fresh- water Fisheries Laboratory, Pitlochry, who caught 98 Charr of which 77 were given to the author for this study. Loch Fada was netted on 2 and 3 April 1975 (17 Charr, 19 Brown Trout) and on 28 August 1975 (8 Charr, 22 Brown Trout). Analysis of Fish Stomachs Jones and Hynes (1955) discussed the many methods used to record and demonstrate the contents of fish stomachs concluding that, for fish of generalized diet, and with a sufficient sample size, all methods gave much the same results. A problem arises however when, as in this study, some fish have eaten small and numerous food items, such as zooplankton, and others have fed on fewer and much larger items. To accommodate both these diets in the one analysis the "Percentage Occurrence Method" has been used here. This involves: (a) Finding the number of stomachs in which each prey category occurs. (b) Adding together the totals for each category to give a grand total for the sample. (c) Expressing the number of stomachs found for each category as a percentage of the grand total. When more than one category was found in a stomach, a full occurrence point was noted for all of the categories so found. Since the purpose of this study was to compare the basic ecology of the two Charr populations, broad categories of prey items were used that were of very different ecological significance. These categories were: A. Benthos. Larvae of Chironomidae, Coleoptera, Ephemeroptera, Megaloptera, Plecoptera and Trichoptera. Adults of Gammarus, Mollusca and Oligochaeta. B. Mid-water and Surface Dwellers, Adult Insects (aquatic and terrestrial), pupal Chironomidae, Hydracarina and Sticklebacks (Three-spined, Gasterosteus acu- leatus, in Loch Meallt and Ten-spined, Pungitius pungitius (L.), in Loch Fada). C. Zooplankton. These categories are approximately those of Frost and Brown (1967). They were drawn up from the Benthos and Zoo- plankton samples as well as from the fish samples. Not all the types in a category were found in stomachs. 100 % Spring Autumn Allopatric Charr . ■ Sympatric Charr 0 0 Sympatric Trout 0 • Figure 1. Percentage occurrences of the three categories of prey items in the allopatric Charr of Loch Meant and in the sympatric Charr and Trout of Loch Fada in spring and autumn. Results Stomach analysis of each sample (spring and autumn) of each of the three populations under investigation (Loch Meallt Charr, Loch Fada Charr, Loch Fada Trout) gave a percentage occurrence figure for each of the three broad dietary categories used. These figures could therefore be used to plot the position of their sample on a three-axis diagram. This has been done in Figure 1, which shows: A) The very strong dependence on zooplankton of the sympatric Charr of Loch Fada, in both seasons. B) The equally strong concentration of both the sympatric Trout of Loch Fada and the allopatric Charr of Loch Meallt on bottom feeding in spring. C) The shift of both these spring bottom feeding populations to more surface and mid-water feeding in autumn. This is pro- bably connected with the reduction in amount of bottom fauna available to fish in late summer after the hatching of insect larvae through the season. This shift is less marked in Loch Meallt, a richer loch, where there is a large supply of permanent bottom fauna (Gammarus, Molluscs, etc.). Discussion These results show that Charr co-existing with Trout are zoo- plankton feeders, but that Charr in a loch by themselves are mainly bottom feeders, as are Trout. Trout and Charr together in a loch therefore would be in competition, apparently severe enough to displace the Charr into another niche altogether; that of zoo- plankton feeding. The results presented here for the sympatric Charr are in agreement with the modern studies of such Charr elsewhere (in Windermere, Frost 1951, 1977). The allopatric Charr results have no British comparison as yet, but such comparisons are available in Scandinavian work. Nilsson, in a series of papers (1955, 1960, 1961, 1962, 1964, 1965, 1969), examined the relationship of Brown Trout and Arctic Charr in lochs. He found that allopatric Charr were bottom feeders — only in one monthly sample were zooplankton found to be their main food, and in the same sampling he also found zoo- plankton to occasionally dominate the food of Trout — but that sympatric Charr only bottom-fed in spring, becoming increasingly dependent on zooplankton through the year. In his 1960 paper, Nilsson explained these findings by showing that in spring in Scandinavia there is a superabundance of bottom food in lakes, accumulated under the winter ice and by the reduced feeding of fish at low temperatures, and on this abundance Charr and Trout feed freely. As this bottom food abundance declines, however, the Trout become increasingly aggressive and territorial, splitting up the littoral areas amongst themselves. The less-aggressive and non-territorial Charr are therefore gradually excluded from these bottom-food producing areas and must take to pelagic zooplankton feeding in the deeper waters. There is an obvious climatic difference between Scandinavia and the Hebrides, in that lochs in the latter seldom, if ever, freeze. A spring abundance of bottom food accumulated under winter ice cannot therefore occur, nor does fish feeding stop in winter in Scotland (Campbell 1954). It is probable therefore that in Loch Fada (and other Scottish Charr lochs) the Chan are restricted to zooplankton all the year round. This would explain why sympatric Charr in Scotland are such small and valueless fish whereas in Scandinavia they are of size large enough to be of sporting and commercial value. Steinbock (1950) found that Charr fed solely on zooplankton had their growth stunted. The one Scottish exception to such a rule may be Loch Borralie where the Charr have a varied, non-zooplankton diet despite the presence of Trout. Campbell (1961) noted that spawning for Trout in this loch was very poor, and that consequently the Trout population was sparse. Such a restriction of Trout numbers would allow the Charr to stay on the littoral zones and bottom feed. Acknowledgments I should like to acknowledge with gratitude the permission given to me to net the lochs in this study; to the Rt. Hon. The Earl Granville for Loch Fada, and to the Department of Agricul- ture and Fisheries for Scotland for Loch Meallt. References BAIRD, W.