Embryo survival and alevin emergence of brook charr, Salvelinus fontinalis and brown trout, Salmo trutta, relative to redd gravel composition

Effects of three homogeneous gravels (2.7, 6.2 and 9.2 mm in diameter) and five heterogeneous gravel mixtures (with 0, 20, 40, 60 and 80% sand; geometric mean diameters = 14.2, 10.1, 7.2, 5.1, 3.7 mm, respectively) on embryo survival and subsequent emergence of brook charr (Salvelinus fontinalis) and brown trout (Salmo trutta) were examined in the laboratory using vertical flow incubators. Differences in survival to emergence, temporal components of emergence and developmental stage of emergents are significant [Formula: see text] among gravel types within and between species. Alevin survival, time interval to first and 50% emergence and duration of emergence period vary directly with gravel size and inversely with sand concentration. Survivals of 0 to 20% occurred in unigranular gravels 6.2 mm or finer and in multitextured gravels with 60% or more sand and rates of 60 to 96% were found in 9.2-mm gravel and gravels with 20% or less sand. Emergent survival increased from 14 to 79% in gravels of 6.2 to 9.2 mm and from 2 to 96% in sand concentrations between 60 and 20% because of reduced entrapment. Period of emergence was longest in 9.2-mm gravel (mean for charr, 381 day degrees; mean for trout, 423 day degrees) and in 0% sand–gravel mixture (mean for charr, 232 day degrees; mean for trout, 179 day degrees). Premature emergence of alevins over a shortened emergence period in finer gravels is identified as a stress response. Larger gravel and lower sand concentrations produced the largest and most advanced alevins at emergence. Ecological implications of the results are examined.