Despite everything man and nature throws at them, salmon are actually rather good at reproducing themselves and normal growth is exponential. The salmon play a numbers game where the salmon’s main strategy is to produce lots and lots of eggs (4,000 in a typical female adult of 8 to 10lb) and to accept massive losses. (This is the typical strategy for animals that don’t look after their offspring after birth). The egg numbers are mind boggling; for example, in a river with a decent run of salmon, of say 20,000 fish averaging 9lbs, the females (50% of the 20,000) will lay around 40 million eggs.
With a life cycle of generally four years (in the UK) we could surely expect to see more than the original 20,000 fish return from those 40 million eggs in four years time?! Well yes we would. If we say the salmon reproduce with a 20% increase in numbers each cycle (A reproductive success rate* of 20%), after the first cycle 24,000 adult fish would return from their offspring. This is population growth. Let’s see what happens to the population in 10 life cycles:
Over 120,000 salmon are now returning to the river from the original 20,000 showing excellent and consistent population growth. Of course, this could only happen if the river system can cope with the number of juveniles produced – what we probably see is lower reproductive success as capacity is approached/reached. But for now, let’s assume the river system has capacity enough for the young ones. (which is almost certainly the case in most rivers these days).
But what happens if the salmon face problems during their life cycle, e.g. poor spawning habitat, siltation, poor gravels, predation, pollution, lack of food, blockages, sea netting, global warming etc, etc. Clearly the reproductive success may go down and for most rivers 10% is more realistic these days. What happens then?
We still see decent growth over the life cycles. Slower, but 20,000 salmon have still grown in number to over 50,000.
But we’ve forgotten something! — Anglers!
We also need to factor into the equation anglers keeping “one for the pot” (or worse) and, of course, in-river netters and poachers. So let’s add a Catch & Keep* rate. Let’s say this is typically 10%. Surely no harm done there – just a small crop from an otherwise healthy run of fish? Well let’s see what happens to the population in the chart below:
Oh dear! The chart above shows that in a river with 10% reproductive success rate and 10% catch and keep rate the population will slowly but steadily decline. But perhaps this catch rate is conservative on rivers where all angling methods are allowed. Here a 20% catch & keep rate may be more realistic. Let’s see what happens then:
You now have consistent, serious decline. Recognise the above trend?
Below is a little program for you to play with numbers. But be warned, you may get some nasty shocks.
NB: Please remember that in a four year period – four of these life cycles are under way in parallel. The charts above do not represent the total population of salmon involved with the river. They do show what you might expect to return each year though in that particular year group (assuming no mixing of year groups).