Redd-Site Selection by Brook Trout and Brown Trout in Southwestern Ontario Streams

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Abstract
Redd-substrate composition, water velocity, depth and other environmental variables associated with red-site selection and spawning by brook trout Salvelinus fontinalis and brown trout Salmo trutta in southwestern Ontario streams were examined. Sympatric and allopatric populations spawned in similar ranges of specific conductance (225-810 .mu.mhos/cm), pH (7.0-8.2), dissolved oxygen (> 83% saturation) and stream gradient (0.2-2.3%). Brook trout spawned exclusively in areas of groundwater seepage, typically near headwaters where streamflow did not exceed 177 l/s. Brown trout spawned in a wider range of flows (21-600 l/s) and utilized locations with and without groundwater seepage. Spawning by brook trout usually began by the 2nd week of Oct., by brown trout a week later. Brook trout spawning periods lasted 3-5 wk; those of brown trout, 2-4 wk. In sympatric populations, an overlap in spawning time occurred for up to 3 wk. Reuse of redds was mostly intraspecific, although interspecific reuse of brook trout redds by smaller brown trout did occur, particularly below barriers to upstream movement. Mean water depth over redds selected by brook (24.0 cm) and brown trout (25.5 cm) were similar (P > 0.05). Mean stream velocities were significantly (P < 0.001) slower at brook trout (17.6 cm/s) than at brown trout redds (46.7 cm/s). Average geometric mean sediment size of brook trout redds was significantly smaller than that of brown trout redds (5.7 mm vs. 6.9 mm; P < 0.02), but less well sorted. Redd-site preference by brook trout for areas of groundwater seepage and by brown trout for faster water velocities and coarser substrates minimized species interactions during spawning. Larger body size of mature brown trout (18.0-54.5 cm fork length) than of mature brook trout (8.4-29.0 cm) was probably a factor in the brown trout''s ability to utilize faster currents where coarser gravels were found.