The Atlantic Salmon - Is a highly adaptive, but also, a less complex species than some would like to believe. As with most other wild fish, Salmo Salar is driven by two strong primeval instincts - To eat and to reproduce! Nothing else! The same could be said some humans I know, but that’s another story!
If we remember these two simple points, it becomes easier to understand
What remains constant in their world? Light – Moon – Stars – Tide and Threats!
Threats to salmon come in many forms, the biggest by far being that of human activity. But if we take that out of the equation for now, it will enable us to focus on the salmon in a natural sense.
We shall start at the beginning, the moment of fertilization.
Our egg will be one of many, and I use the word many because although scientific data can provide us with rough estimations, it cannot provide us with exact data regarding how many of the females eggs are actually fertilised. As would be obvious to anyone, this figure will change dramatically if spawning were taking place in ideal or adverse conditions. The effects of a large spate at this time would dramatically decrease the number of eggs fertilised. From this we can deduce – Weather, a natural element, is playing a part right at the beginning. But, as with all fish, the salmon has an insurance policy – Large numbers of eggs.
A large number of fertile eggs are now buried in the gravel. Slowly Developing! The important word here is SLOWLY! Salmon eggs take longer to develop and hatch during colder than average winters. During extremely mild winters and higher water temperatures, eggs may begin to hatch in the month of February, with alevins emerging from the gravel during March, a time at which the river does not produce an abundance of natural food. If, on the other hand, the winter is extremely cold and we have a “proper winter”, this will not take place until April, a time the river will obviously produce more natural food, leading to the survival of more fish to the fry stage. As spring fish tend to spawn slightly earlier than both their summer and autumn cousins, the effects of a colder winter would have obvious benefits to that particular cohort. On the other hand, couple earlier spawning with a mild winter and you have a high number of offspring from this cohort hatching far too early in the year. Prolonged frost can also have an adverse effect on our developing eggs. Streams used by spring fish tend to be small and higher up the river system. In extreme cases these will dry up, leaving eggs starved of oxygen. Thankfully this does not often happen, and when it does, salmon always have their insurance policy of safety in numbers. However, add water abstraction schemes and land drainage and over a period of time you create potential for disaster to that particular cohort. Mind you, when we are talking about huge numbers of eggs, their loss will be someone else’s gain. Offspring from both summer and autumn salmon, because they tend to spawn in the main river, will have less than normal competition for food! Not really complicated at all.
