Role of gravel substrate on ova survival and alevin emergence of rainbow trout, Salmo gairdneri

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Abstract
The role of gravel size of unigranular diameters 2, 4, 8, 16 and 26.5 mm on ova survival and subsequent emergence of rainbow trout alevins is examined using a vertical-flow incubation apparatus. Survival to emergence, time of emergence and alevin condition at emergence were significantly influenced (P .ltoreq. 0.01) by gravel size. Mean percent survival to emergence increased with gravel size from a minimum of 1% in the 2 mm gravel to a maximum of 76% in the 26.5 mm gravel. Survival of control ova from gravel-free incubator was 88% to swim-up stage. Differences in percent survival were most significant within the 2-8 mm range. Poor survival of trout alevins in the finer gravels (2-4 mm) was the result of insufficient void space (entrapment). Days to 1st (43-58 days after ova burial) and 50% emergence (49-62 days after ova burial) also increased with gravel size. At a mean incubation temperature of 10.6.degree. C, the total emergence days was greatest (40 days) from the 8 mm gravel and occurred prematurely on day 37 after ova burial in 2 mm gravel. Alevin length and weight varied directly with gravel size, ranging from 2.17-2.39 cm and 11.9-126.8 mg, respectively. Larger alevins, which emerged later from coarser gravels had the least yolk reserve (.hivin.X26.5 mm = 4%) and the lowest KD values (.hivin.X26.5 mm = 2.10), where KD is a developmental condition factor calculated from the ratio of wet weight to standard length. Premature emergence of free embryos and shortening of the alevin emergence period in 2.0 mm gravel is identified as a stress response.