Redd Site Microhabitat Utilization and Quantitative Models for Wild Large Brown Trout in Three Contrasting Boreal Rivers

Abstract

AbstractRiverine spawning areas are important but restricted source habitats for large brown trout Salmo trutta (≥40 cm total length). In hydrophysically different reaches within three contrasting boreal rivers, redds larger than 100 cm in length were located by direct underwater observation in 1998 and 2001 (n = 157 redds; range = 10–60 redds per reach and year) and were characterized based on 29 habitat variables. The microhabitat of utilized redd sites (total redd length: mean ± SD = 206 ± 83 cm, range = 100–615 cm) was typified by a mean total water depth of 103 ± 39 cm with a wide optimum (range = 23–215 cm) and a mean water velocity of 47 ± 32 cm/s (range = 2–124 cm/s). Mean velocity measured at 5 cm above the substrate was 29 ± 20 cm/s (range = 0–82 cm/s). Mean substrate particle size was 7 ± 6 cm and demonstrated a narrower optimum (range = 2–37 cm) than the other variables. Substantial differences among the three rivers, particularly in water depth and velocity measures, largely reflected size and gradient differences and indicated some flexibility in spawning microhabitat utilization. The use of substrate particle size was less variable. Multivariable analysis indicated that water velocity (except velocity measured just above the substrate) and water depth explained most of the variation. Depths and substrates utilized were larger than previously reported, probably reflecting the in situ habitat availability in the relatively large study rivers.