Observations on the Perch Population of Ullswater

Abstract

Differences in the population dynamics of a fish species are often observed when widely differing places and environments are compared. The present investigation was undertaken to explore the reasons for known differences between the populations of perch (Perca fluviatilis L.) in Ullswater and Windermere which are two limnologically similar lakes only 16 km (10 miles) apart in the English Lake District. Windermere is the largest of the fifteen major lakes in the district and Ullswater the second with a total area of 8-9 kM2, a length of 11 -8 km and mean breadth of 869 m (Mill 1895). The average depth is 25-3 m, maximum depth 62-5 m and total drainage area 230-5 km2. The area of shallow water less than 7-5 m, i.e. the area of spawning and summer feeding ground available to the perch, is 3-82 km2 or 43 % of the total, compared to Windermere where the area of shallow water is 4-82 km2 or 33% of the total. The present water level is 145 m above sea level. Pearsall (1949) classifies the English lakes as a series from those that have rocky shores and are unproductive to those with shores more or less silt-covered that are relatively productive. Ullswater and Windermere are both towards the middle of this series, but Ullswater is a little less productive than the north basin of Windermere; in neither lake is the hypolimnion deoxygenated in summer. Apart from the perch, the fish species known to be present in Ullswater are salmon (Salmo salar L.), small numbers of which come up to spawn from the River Eden, trout (S. trutta L.), schelly (Coregonus wartmanni Dottrens), eel (Anguilla anguilla L.), minnow (Phoxinus phoxinus L.), stickleback (Gasterosteus aculeatus L.), bullhead (Cottus gobio L.), loach (Nemachilus barbatula L.) and lampreys (Lampetra planeri Bloch and L. fluviatilis L.). The trout occur in large numbers but are small in size (Swynnerton & Worthington 1939). Certain owners used to exercise netting rights on Ullswater, and Bush (letter of 7 April 1948) has given details of the fish caught during the previous 50 years by his family. An annual average of 113 kg of trout of an approximate mean weight of 170 g were seined and gill netted, and no change in average size was noted over the period. In mid-June each yearthe catches of trout declined and perch began to be caughtsometimes as many as 100 per night. The size of perch selected by such a gill net would not normally be less than 20 cm (Kipling 1963) and would therefore be comparable to the size of adult perch caught in Ullswater today. Char (Salvelinus sp.) originally occurred ii Ullswater but from about 1820 became rare (Watson 1899), and are now extinct, perhaps owing to the partial destruction and pollution of their spawning ground by washings from lead mines at Glenridding on one of the afferent becks. The main difference in fish fauna between Ullswater and most other lakes in the district including Windermere is the absence of pike (Esox lucius L.). In Windermere pike are numerous and appear to be important as predators on the perch (Frost 1954). This provides one of the main possible causes for differences between the population dynamics of perch in the two lakes and was a stimulant to the investigation.