Asphyxiation and entombment mechanisms in fines rich spawning substrates: experimental evidence with brook trout (Salvelinus fontinalis) embryos

We investigated the distinct physical controls causing entombment and asphyxiation, the key mechanisms influencing salmonid egg-to-emergence (EtE) survival. Entombment occurs when sediment blocks the interstitial pathways (macropores) that larvae use to emerge from the streambed, while asphyxiation is related to low oxygen flux, which is a function of interstitial flow velocity. EtE survival has been related to substrate composition and flow velocity. However, in streambed sediments these variables are correlated, and few studies have examined the sensitivity of EtE survival to changes in velocity and oxygen flux at fixed substrate composition. EtE survival has not yet been directly related to the size and density of macropores. We incubated brook trout (Salvelinus fontinalis) embryos in artificial redds with different sediment compositions and hydraulic gradients to examine independently the effects of substrate composition, macropore geometry, and flow velocity on EtE survival, emergence timing, and fry condition. In situ measurements of macropore size were obtained using a computed tomography scanner. Despite high oxygen concentrations, we observed that entombment or blockage effects caused high embryo mortality in fines-rich substrates with few large macropores, and triggered early emergence of rare survivors. These outcomes could not be mitigated by increased flow velocity and oxygen flux to the egg pocket.