The population ecology of juvenile Brown trout (Salmo trutta L.) in Loch Leven, Kinross, Scotland

Abstract

This study on the population ecology of juvenile trout in Loch Leven included investigations on the population structure of the nursery streams; the input of juvenile trout into the loch; the distribution, the age and growth and the food of juvenile trout in the loch; and the emigration of juvenile trout from the loch into the outflowing River Leven. The stream population structure was studied by repeated electrofishing on selected sites in the nursery streams while the input of juvenile trout into the loch was monitored by traps set on each of the major tributaries to the loch. The mean highest population density of juvenile trout in the selected stream sites 2 was 1.103/m and this was observed from the electrofishing catches in September 1976. Three year classes of juvenile trout: I, II and III were involved in the downstream migration of juvenile trout from October 1975 to July 1976. Migration of juvenile trout into the loch was associated with increased water discharge caused by the onset of floods while increased stream water temperatures accelerated the pace of migration into the loch in the spring, reaching a peak in April. Among the three fishing methods employed on the loch to study the distribution of juvenile trout, only gill-netting caught juvenile trout in significant numbers in the offshore areas of the loch. From gillnets set along three traverses on the loch, juvenile trout were found to be concentrated in the offshore, above the mud zone of the loch. During the summer period, gillnet captures of juvenile trout were found to increase with increasing gillnet sets away from the shore, reaching a maximum concentration at the deepest part of each traverse. However, during the winter, juvenile trout appeared to be evenly distributed with no clear area of concentration as in the summer. Though there were few areas of overlap, juvenile trout concentrations seem to be separated from the adult trout, perch and pike populations in the loch. Migration of juvenile trout into the loch consisted 55.9%, 37.2% and 6.9% of the first, second and third year group migrants respectively. Fast growth in Loch Leven feeder streams was responsible for the high proportion of early migrants of juvenile trout into the loch. All migrant groups showed a faster growth during their first loch year. Compared with other lochs, growth of juvenile trout in Loch Leven was relatively good and this was linked with the high productivity of the loch and the area of loch habitat relative to that of nursery stream habitat. The main food items of juvenile trout in the loch are chironomids, crustaceans and nematodes and these are abundant in the offshore mud zone of the loch. Emigration of juvenile trout from the loch into the outflowing River Leven extended from November to August with peaks in February and June. This occurred among all age groups of juvenile trout in the loch soon after entry into the loch from the streams or after a few months growth in the loch. The distribution of juvenile trout in the loch is discussed in relation to food, growth, competition and predation.