I am not a scientist or a quasi-scientist. I occasionally fish but not enough to consider my personal empirical information valid. OTOH, logic can be our friend. If in year "n" 5 boats collect 50 fish a day and in year "n+x" 50 boats are required to collect 50 fish a day then something has changed. Possibilities are; 1) there are fewer fish by a multiple of 10; 2) fishermen are 10 times dumber than x years ago; 3) fish are 10 times smarter than x years ago. Of these three possibilities the first is the most likely. There may be other explainations and it is possible that the fish are much smarter (given some of the individuals I am forced to work with it is possible that the fishermen are 10 times dumber).

One possible explaination is that all the well meaning bans of this and that type of fishing (deep water nets etc.) has made the fishing industry so much less efficient.

One possible explaination is that the fish have all turned gay due to over-population of liberal minorities and are just not breeding as fast as they used to.

One possible explaination is that ....

But it seems likely that as the human population rose and it's appetite for seafood remained steady and high our consumption rate eventually surpassed the fish breeding / growth rate with the natural consequences being depletion of stocks.