We investigated the potential of dual-frequency identification sonar (DIDSON) deployed from a drifting cataraft for estimating abundance in rivers of Brown Trout Salmo trutta larger than 20 cm. We compared triplicate trout density estimates made by DIDSON with drift-diving density estimates in three reaches of a clear-water river in New Zealand. DIDSON density estimates were much lower (»22% of drift-dive estimates, range D 7-33%) and less precise than drift-dive estimates (DIDSON CV D 0.13-0.47; drift diving CV D 0.15-0.17). Variation in detecting fish in the DIDSON field survey contributed substantially more (95%) to DIDSON count variability than did fish detection in the image files. Highest precision with DIDSON was achieved in the reach with the least shallow habitat and most uniform channel. Fewer person-hours were required to undertake the field component of DIDSON surveys than the drift dives (5 versus 8.3 h), but the substantial time spent on image review (3.3 h) made DIDSON surveys 34% more costly than drift dives in terms of overall effort. Despite observed shortcomings, cataraft-mounted DIDSON has utility as a noninvasive survey method for estimating abundance of large (>20-cm) fish, particularly in situations where turbidity is too high for visual counting methods to be effective.

The post-spawning movements and distribution of 15 brown trout (Salmo trutta Linnaeus 1758) and 21 rainbow trout (Oncorhynchus mykiss Richardson, 1836) were monitored weekly between August 1996 and March 1997 in the hydro-electric reservoir, Lake Otamangakau, New Zealand, using radio-telemetry. The most frequently used areas were the vicinity of the main inflow and the channel running through the main body of the lake. After spawning, brown trout regained rapidly their usual habitat. Rainbow trout were using the inlet intensively before recolonising the main channel of the lake. Brown trout made greater use of the weedy parts of the lake and littoral margins, had a more confined home range, and were less mobile than rainbow trout. Rainbow trout occupied the main body of the lake by day, and moved nearer the surface, and closer to the margins, at night. High lake levels allowed brown trout to exploit wetland margins of the lake.