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

Abstract

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.

Factors Influencing Brook Trout (Salvelinus frontinalis) Egg Survival and Development in Streams Influenced by Agriculture

“…Fudge et al (2008) determined that sediment during laboratory incubation did not affect oxygen delivery to the incubating eggs, rather it restricted the movement of the fry and precluded emergence. Another laboratory assessment by Franssen et al (2012) had similar findings, where substrate with a high proportion of fines precluded the emergence of brook trout fry. Witzel and MacCrimmon (1983) reported an increase in survival from 2-96% in substrates between 60-20% sand mixtures due to reduced entrapment.…”

“…Although the present study could not establish a threshold for fine sediment, other studies like Levasseur et al (2006) have established levels at which survival of salmonids eggs is significantly decreased. A laboratory assessment of brook trout by Franssen et al (2012) found low survival in substrate when the percent finer than 2 mm was above 22%. Hausle and Coble (1976) found that survival to emergence of brook trout declined when the amount of fines <2 mm was over 20%.…”

Effects of fine sediment, hyporheic flow, and spawning site characteristics on survival and development of bull trout embryos

“…The potential for high interstitial velocities and strong rates of VHG to help mitigate embryo mortality in moderate levels of fine sediment has been suggested previously . However, in a recent laboratory study, Franssen et al (2012) did not observe a significant effect of VHG on survival to emergence of brook trout across a range of substrate compositions. In natural streams, there is substantial spatial and temporal variation in sediment sorting, hyporheic flow paths, and the strength of VHG, all of which could potentially explain differences in the observed effect of VHG on survival between field and laboratory settings.…”

“…Mortality at the larval stage can also occur as a result of entombment within the incubation environment (Franssen et al 2012), and entombment may have been one reason why fine sediment in the incubation capsule was the best predictor of STE. For example, fine sediment may have impeded fry emergence from the two incubation capsules in which we found large numbers of live fry when the capsules were removed from redds.…”

“…Lack of dissolved oxygen (DO) can lead to mortality via asphyxiation and can have sublethal effects, such as reduced larval size and fitness Roussel 2007). Fine sediment can also directly affect larval survival by blocking pathways through the gravel matrix, leading to entombment (Sear et al 2008;Franssen et al 2012). In some locations, ambient levels of DO can be greatly reduced in hyporheic zones with strong groundwater influence, resulting in embryo mortality and impaired larval development even when fine sediment is limited Youngson et al 2004).…”