Sediment Deposition within Fry Emergence Traps: A Confounding Factor in Estimating Survival to Emergence

Abstract

We provide evidence that the effects of sediment infiltration and deposition within fry emergence traps can lead to a potential negative bias when estimating survival to emergence (STE) of salmonid fry. Fry emergence traps were placed over 12 redds of brown trout Salmo trutta located in sections of three streams in the upper Clark Fork River system in Montana. The redds were capped in March 1989 and monitored through June 1989. During this period, sediment deposition became noticeable inside 8 of the 12 traps. During the final removal of the traps, substrate core samples were collected from a location inside and outside of each emergence trap. Substrate size gradation analysis indicated that concentrations of fine sediments (≤0.84 mm in diameter) were significantly higher in samples collected inside the traps than outside. These results suggested that the local sediment transport capacity was reduced within the emergence traps, resulting in increased deposition of fines. An understanding of the hydrology, ambient sediment concentrations, and sediment transport capabilities of river systems is important when considering the use of fry emergence traps. Without consideration for this potential confounding factor, STE estimates that involve the use of fry emergence traps may underestimate actual STE values.