Some Effects of Gravel Mixtures on Emergence of Coho Salmon and Steelhead Trout Fry

Eight mixtures of sand and gravel were tested in experimental troughs, to simulate hatching conditions in coho salmon (Oncorhynchus kisutch) and steelhead trout (Salmo gairdneri) redds. Fry were released into perforated, open-ended chambers below the gravel surface. An inverse relationship was found between the quantity of fines and emergent survival. Mean emergent survival for coho salmon ranged from 96% in the control mixture to 8% in 70% sand (less than 3.3 mm diameter). Mean emergent survival for steelhead ranged from 94% to 18%, respectively. Premature emergence of coho fry was related to higher concentrations of fines. These premature fry were smaller, and retained more yolk than fry emerging at normal times.