Salmonid egg membranes are composed of a dense layer of proteinaceous fibres, interspersed with micropores. While external factors influence oxygen supply and demand, the protective membrane that surrounds the embryo can impede diffusion and exacerbate hypoxic stress. Therefore, different populations of Atlantic salmon in the UK are likely to experience different levels of oxygen stress during the incubation stage of their life cycle. While factors such as fine sediment load, groundwater input and temperature vary significantly within river systems, field data show that this variation is much greater among river systems that host different Atlantic salmon populations in the UK. In addition, oxygen demand is greater at higher temperatures, so vulnerability to oxygen stress is likely to be greatest in warmer conditions. The rate of supply to the boundary layer is greatest when the interstitial velocity and oxygen concentration of water flowing past the eggs is high, so is influenced by factors such as fine sediment concentration in redds and groundwater upwelling which reduce stream bed permeability and oxygen concentration respectively. In the case of Atlantic salmon ( Salmo salar), oxygen is supplied to the embryo from a thin film of water surrounding the egg known as the boundary layer. Īlthough the nature of oxygen delivery to eggs of oviparous organisms varies enormously depending on the life cycle and incubation conditions of the species in question, it can become limiting to survival and post-hatch fitness when embryonic demand outstrips supply. However, this action can increase the susceptibility of eggs to microbial and fungal infection, siltation and hypoxia. By burying their eggs in nests in riverbed gravels known as redds, the vulnerability of salmonid ova to predation is reduced.
While the nature of hazards faced by oviparous organisms varies throughout the animal kingdom, benthic spawning teleosts such as salmonids frequently suffer mortality through predation, fungal and bacterial colonization, siltation and hypoxia. Their immobile nature means eggs cannot escape these stressors, so the incubation environment determines the nature of the hazards they face. Consequently, stock enhancement techniques such as supportive breeding that relieve incubation stress could erode structural adaptations.ĭuring the incubation stage of their life cycle, oviparous organisms are exposed to a range of stressors that can play a crucial role in shaping individual fitness and population dynamics. Variation in egg membrane structure influences low oxygen tolerance of Atlantic salmon embryos and could represent adaptation to low oxygen stress. In addition, membrane porosity was lower than previously reported indicating that oxygen requirements during incubation have been underestimated, so models such as the mass transfer theory that predict incubation success could currently overestimate ova survival. Furthermore, comparison of membranes of eggs that survived laboratory controlled low-oxygen conditions compared to those that died suggested that ova with less permeable membranes were more susceptible to hypoxia-induced mortality. Membrane thickness, porosity and permeability to dissolved oxygen varied among populations. Using electron microscopy, the membrane structure of eggs obtained from five UK Atlantic salmon ( Salmo salar) populations is described. Therefore, if membrane architecture influences diffusion rate to the embryo, selection for more permeable membranes could occur in oxygen-stressed environments.
Published field data indicate that oxygen stress experienced by salmonid eggs can vary widely among populations. Therefore, the structure of egg membranes could affect the rate at which embryos obtain oxygen from their surroundings. Oxygen supply to the salmonid egg surface can be limited by external factors such as sedimentation and groundwater upwelling, while the egg membrane itself can impede diffusion from the egg surface to the embryo.
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