Alastair Thorne1, Alisdair MacDonald2, Joseph L. Thorley1 and Alastair Johnstone2
1Fisheries Research Services, Freshwater Laboratory, Faskally, Pitlochry, Perthshire, PH16 5LB Scotland, UK. Tel: 0044 1796 472060; Fax: 0044 1796 473523; E-mail firstname.lastname@example.org
2Fisheries Research Services, Marine Laboratory, Aberdeen
The main aim of the study is to investigate the movements of ferox (large piscivorous brown trout) in their natural habitat. Observations on the movements of tagged ferox have been made in Loch Garry, Perthshire, during three separate study periods in 1999, 2001 and 2002.
During the first study period (June 1999), the horizontal movements of two ferox (3.0 kg and 4.9 kg) were tracked using radio and acoustic tags. Both tagged ferox made large-scale movements throughout the loch during daylight hours and did not appear to be territorial. Their movements became more restricted and closer to shore during the hours of darkness. In the second study period (September 2001), a data storage tag was used to record the vertical movements of a 2.8 kg ferox. Throughout the two-week study period, the tagged fish spent most time in the top two metres of the loch, interspersed by short dives to a maximum depth of 15 m.
Two data storage tags have been recovered so far from the third study period (May to August 2002). Analysis of the data for a 3.7 kg ferox shows that in May and June the fish spent most time between 5 and 8 m, whereas in July and August it tended to remain in the upper two metres. Throughout the study period the ferox undertook daytime dives to 12m which increased in frequency as the season progressed. The second tagged ferox of 3.4 kg was largely inactive during May to July at a range of depths from 1 to 30 m. In August, it became more active, tending to remain between 4 and 7 m but undertaking short daytime dives to a maximum of 10 m.
We hypothesis that the daytime dives were excursions by the ferox to feed on Arctic char (Salvelinus alpinus) the main prey species which are known to make substantial diurnal and seasonal vertical migrations.