The Brown Trout of a Moorland Fishpond

Abstract

1. A biological survey of Three Dubs Tarn, in the English Lake District, has been made. The flora and invertebrate fauna have been described by Macan (1949), the food, age and growth of the brown trout (Salmo trutta L.), the only fish present, are described from material collected from 1936 to 1947. 2. The tarn has an area of 4 acres, and is 700 ft. above sea-level. Its water is poor in electrolytes, and has a low calcium content. Bonemeal was added in 1937, 1939, 1941 and 1944. 3. The trout range in length from 7.5 to 28.7 cm., the majority being 21.8 cm. and weighing 126 g. In age they range from 1 to 8 years; they enter the tarn, from the beck, in their 1st year. The mean survival rate for fish of all ages is 39%. 4. From March to September the food consists mainly of the larvae, nymphs and pupae of aquatic insects; Pisidium and surface terrestrial organisms are frequently eaten. Comparison of the food and fauna shows that in general the fish eat what is available and abundant, but there is evidence of preference and avoidance. The trout feed as frequently on weed-living organisms as on mud-living ones, although the weed fauna is poorer than that of the mud. 5. The general growth-pattern, based on all data 1936-47, shows that growth was fastest during the first 2 years of life, slower in the 3rd year and declined steadily thereafter. 6. With the different years (1932-46) there was considerable variation in the length attained by an age-group, but no one year-class had grown better throughout its existence than any other. The values of K indicate that the `condition' of the trout, of all ages, is good; this is true of periods both before and after addition of bonemeal. 7. The effect of bonemeal on growth cannot be isolated from that of other factors, although instances of significantly increased growth-rate coincident with the bonemeal additions were found; but such improvement was not sustained. 8. Comparison of the growth of trout in Three Dubs Tarn with that in two waters of similar chemical character, Windermere and Yew Tree Tarn, suggest that differences in food and spatial conditions account for the differences in size attained by the fish.