Redd Counting for Monitoring Salmonids in Finnish Inland Waters

Abstract

During recent years, systematic redd counting has been used as a monitoring method of wild Brown Trout Salmo trutta spawning stocks, both resident and lake-migrating, in the Lake District in Southern Finland. Counting is done by wading and viewing with an aqua-scope, and is related to channel microhabitat measurements, to estimation of gravel origin, to regression between redd length and female length, to regression between female length and egg number in redd, and to parr density. This produces valuable information about spawning environment, spawning stocks, and stock-recruitment ratio for management of stream-spawning fishes and river channels. Number of trout redds and of female spawners was mostly 10−30 (length of each stream sampled 200−500 m), and in maximum about 100 redds/individuals, per stream in Kymijoki watercourse. Average redd length was 1.71 m, water depth 57 cm, and water current velocity in three cm above bottom surface 27 cm/s. Most common dominant gravel size was 64−128 mm in pot and 32−64 mm in tail. Both natural and artificial gravel were important as spawning grounds, but gravel carried in with buckets by volunteers produced much more redds per gravel volume than gravel deposited with an excavator. Most often, redds were situated in small pools or holes inside riffle sections, but also in upstream edges of riffles in some larger rapids. About 70 % of redds were situated no more than 50 cm from a shelter, like a stone or a channel bank. In Kymijoki streams, mean fork length of female spawners was 38 cm, estimated from redd tail lengths. In other Finnish and Swedish spawning rivers of lake-migrating Brown Trout, average redd length was 2.5−3.0 m, and average female length 45−52 cm. In one Kymijoki stream, parr density was linearly correlated with egg density, estimated from redd number and redd tail lengths.