I really like your idea of having certain areas open for a season, and then closing that area, and opening other areas. It reminds me a lot of rotational grazing, done on farms with ruminants (goats, sheep, cattle, etc.). By rotating the pasture your animals are on, you improve the health of your pastures (by not eating them down completely), and the health of your animals (reduced parasites, and better food). I think the same principals could apply to fishing fish stocks like that, too. By not fishing too heavily in any particular area, and regularly giving certain stocks a break, we can maximize the size of individuals we catch, without negatively impacting the genes of the stocks (by killing off the large fish constantly, and only allowing smaller fish to reproduce, resulting in the overall average size to go down).