Artificial steps mitigate the effect of fine sediment on the survival of brown trout embryos in a heavily modified river

Understanding the factors that determine successful salmonid embryo incubation in the many structurally modified river systems of the Northern Hemisphere is crucial for maintaining healthy salmonid populations. In this context, the joint impact of fine sediment accumulation together with anthropogenic river modifications on salmonid embryo survival has been rarely investigated. We investigated brown trout embryo survival to hatch (STH) together with ten physicochemical, hydraulic and morphological parameters in artificial brown trout redds in a heavily modified stream (i.e. channelized, artificial log steps) in central Switzerland. We were interested to understand whether (i) STH is more sensitive to the timing and duration of low oxygen rather than a mean oxygen concentration, (ii) STH was negatively affected by increased fine sediment deposition decreasing redd gravel permeability, (iii) higher water levels, causing fine sediment resuspension, benefit STH, (iv) STH was negatively affected by organic content in the redds and especially (v) hydraulic gradients related to redd scale bed-form and/or the artificial step structure benefit embryo STH, and hence could mitigate the negative impact of fine sediment and/or organic content. Up to 50%, brown trout embryos survived with interstitial oxygen exceeding 3 mg L−1. Embryos endured up to 6 days ≤ 1 mg L−1 but were more sensitive to oxygen depletion close to hatch. Therefore, timing and duration of low oxygen were important for embryo STH, and hence, oxygen dynamics need to be considered when assessing in redd conditions for salmonid STH. Partial least squares regression identified the horizontal hydraulic gradient, Fredle index, distance to artificial log steps upstream and amount of accumulated fine sediment as influential predictors for embryo STH. The water level above the redd and total organic carbon content in the redd were not influential. Among the identified influential predictors, 70.9% of the variation in STH could be explained by a logistic regression model containing redd distance to the next upstream step (26.4%, P = 0.004), Fredle index (27.2%,P = 0.003) and horizontal hydraulic gradient (10.1%, P = 0.04). In the logistic regression, the amount of accumulated fine sediment (P = 0.75), field seasons (P = 0.93) and field sites (P = 0.66) was non-significant. In summary, brown trout STH was sensitive to redd gravel permeability, which was measured as Fredle index and affected by fine sediment deposition. At the same time, hydraulic gradients related to artificial log steps, which enhanced hyporheic exchange, benefited embryo STH, and hence mitigated fine sediment impact. This result can be probably transferred to other surface water-dominated river systems with good hyporheic water quality. To what extent it can be transferred to river systems with other hydraulic boundary conditions remains to be evaluated. Altogether, our results clearly indicate that the impact of fine sediment on salmonid incubation success needs to be understood in the hydrological and morphological context of the particular river system.