Density Dependent - True or False??

Hypothesis such as – Every fish returned equates to 5000 more eggs being deposited, so 10,000 fish returned to the river equate to blah de blah de blah!! Then, in the next paper, explaining that salmon populations within river systems are in fact “Density Dependent”, which in layman’s terms means, Due to environmental factors /limitations e.g. –10,000 fish will produce the same number [maximum output for that given river] of smolts to that 50,000. So, no matter how many juvenile fish are produced, the river can only feed/sustain “X” number in it at any one time. The remainder are always excess/surplus to requirement and will die or be eaten! Any population is much the same. Again, in laymen’s terms, Ethiopia and Somalia, although extreme, are good examples of this. So, what does this mean? Well, if the above is in fact true; with regard managing our “recreational fisheries”, finding and understanding the minimum/maximum number of spawning adults in “your particular” river will be extremely important, because, if the population falls below the figure of e.g. 10,000, which, somewhat contradictory to the above, we are told is not the case here in Scotland, but would appear to be the case elsewhere, then the population could very quickly move into terminal decline.
Concluding and thinking rationally about the above, it would appear to me that – If, as we have always been told is the case, our rivers are producing their maximum number of Smolts, and they are in fact “Density Dependent”, then dramatic reductions in the population of Atlantic Salmon cannot be related or attributed to anything going on in fresh water, or, for that matter, the number of adult fish returning to spawn, but must be related to changes going on in the ocean, I.e. the increase from 5 to 200 thousand Grey seals around the UK coast over the past 100 years? Naturally changing weather patterns creating more intense low pressure systems moving across the Atlantic, which in turn cause changes in sea surface temperature in different parts of the ocean affecting timing of Plankton bloom in that area may also be a cause and due to the MSW salmons long migration will undoubtedly influence natural peaks and troughs seen in populations of returning adult salmon, but the catalyst for long term decline ultimately lies with Drift-netting with monofilament and increase in numbers of natural predators. Stop those two things; keep the water they swim in clean and bingo, no need for any more stupid and expensive projects to keep us all bamboozled and going round in circles achieving nothing fast.