A University of Southampton study suggests that the membrane of salmon eggs may evolve to cope with reduced oxygen levels in rivers, thereby helping their embryos to incubate successfully

The research, funded by the Environment Agency and published in the journal  has found that differences in the structure of the thin film surrounding a salmon embryo affects its ability to absorb dissolved oxygen from river water

Incubation of salmon eggs and rearing of alevins: Natural temperature fluctuations and their influence on hatchery requirements
The eggs and alevins of Atlantic salmon (Salmo salar L.) are normally reared in Scotland using surface water in open flow systems. The literature indicated that incubation temperatures should generally be in the range 4°–11°C. A strategy that might lead to most success would be to keep green eggs at 6°C and then reduce the temperature to 4°C during subsequent development. Genetic variations between races of salmon in different river systems may partly account for differences in reported incubation times. Several northern stocks are known to be cold-adapted and it seems clear that standardised incubation regimes are inappropriate in the absence of standardised fish and water quality

The cumulative ambient water temperatures at four Scottish hatcheries were shown to approximate those required by the industry, although some very low temperatures were regularly experienced. The technology for temperature control is generally available, but has not been widely used. To be most succussful it should be geared towards producing fish ready for first feeding by the end of April, as natural temperatures are increasing rapidly (10°–14°C), so that an open flow system can be used for feeding

A review of the literature showed that temperature effects should not be considered in isolation. Low pH, resulting from acid rain and snow-melts, was also recognised as a potential problem as was subsequent metal pollution. To be most effective, recycle systems should be designed to control both temperature and pH