Estimation of the number of brown trout Salmo trutta (L.) in Loch Leven, Kinross, Scotland

The numbers of adult trout in Loch Leven were estimated in April each year 1968–71 by tagging fish caught by seine net, and estimating the proportion of the total stock tagged from examination of the angling catch during June-August. The vulnerability of tagged fish before June and after August was higher than that of untagged fish. Tag losses, estimated by double marking, were 2.15 % over a whole angling season. The reporting rate of tag recaptures varied within and between years from 0.43 to 0.71. Differential mortality of tagged and untagged fish was unimportant, except in two subsidiary experiments when fish were tagged in June and August, when handling losses reached 2.7%. With adjustment for these measured errors, the stock of trout beginning their third or more years in the loch in April, fell from 126 665 in 1968 to 52 337 in 1971.