From the 3,000 or so eggs that a female salmon spawns, on average only two will grow into fish that survive until adulthood.
Misfortune, predators and man all combine to make the future bleak for the tiny, newly hatched salmon, wriggling like worms in the water tanks at the Fisheries Research Services unit at Almondbank, near Perth.
It is sad, but knowing this does make the P7 class from North Muirton Primary, in Perth, feel a little less guilty that many of their charges had died. They had been given salmon eggs to hatch in a tank as part of the Salmon in the Classroom project organised by Scottish Natural Heritage.
"It was when we brought them indoors," explains Rhys Clark, aged 11. "I think it might have been because of the dramatic temperature change in the water."
"Lots died all the time," adds Katharine Balbrinie, also 11. "We had to fish them out every day."
Fortunately, the class still had a generous nursery of salmon to release into the River Tay a few weeks later. However, it wasn't until the field trip to the research unit last month that they discovered the high death rate.
"I'm not so upset now I know that so many die in the wild," says Louise Cowan, 11. "And anyway, there were still lots spare to put into the river."
The hour-long visit to the Almondbank unit is part of the four-staged project which runs from January to June. It began with eggs and equipment being delivered to the classes involved in the project, and Elspeth English, the project leader, introduced the children to the life cycle of salmon and their environment. Soon the pale, round eggs hatched out to become alevin, which look like tiny, red tadpoles. After a few weeks, the pupils visited Quarrymill to release them into the Tay.
Twelve schools in Perth and Kinross, Angus and Stirling have been involved in this project. Across Scotland, around 100 schools have linked up with their local fisheries trust to receive eggs and study the life cycle of salmon. Scottish Natural Heritage is soon launching an online version of Salmon in the Classroom that will allow even more schools to take part next year.
The project began in south-west Scotland, when the Galloway Fisheries Trust began providing eggs to schools in 1991, but only the Perth project lets pupils see the life cycle of salmon at Almondbank. Huge tanks in a large hangar at the end of a single-track road, house fish from egg to adult.
"It helps to put the growth cycle of the salmon into an ecological and environmental context," says Mike Miles, a biologist at the unit. "They can also see the inter-relationship between other organisms in the food chain."
The pupils were first split into groups of up to 10 and taken into the hatchling area by Mr Miles. Lining the wall are tray-like tanks containing up to 750,000 alevin, the number needed to sustain levels of salmon in the wild.
After that they moved on to see the fry, just a few weeks old but already developing the strips that help to camouflage them in the weeds of rivers.
Next were the tanks of smolts, salmon ready to start the journey downstream towards the sea, and then a tank of fully grown silver salmon. Feeding time had every pupil riveted as the large fish lashed around to reach the food.
The children where then introduced to predators, such as otters and mink - stuffed specimens borrowed from Perth Museum - and live trout and pike swimming in the unit's artificial river, to illustrate some of the reasons while only two eggs ever make it to adult fish.
The visit closed with an insight into what salmon eat.
"The whole project is fantastic," says North Muirton Primary teacher Karen Crochart.
"We have been doing classification of animals this term and I thought it would fit perfectly. I've had to do hardly anything, except look after the eggs. All the risk assessment is done for you, and Elspeth has organised everything, including the bus.
"I think my pupils found it really interesting.
"What they eat fitted in well with our vertebrate and invertebrate work,"
Rhys agrees. "Seeing the eggs hatch, putting the alevin back in the river, then coming here has made it seem more real."
The final stage of the project will take the children back to Quarrymill in June to go electrode fishing, when the fish will be stunned, without harm, and removed from the water for closer inspection. All are hoping that it will be their salmon that are picked out of the water.