The most important fishing grounds of the world are located where warm and cold oceanic currents meet. The convergence of these currents leads to the mixing of waters, which creates an ideal temperature and environment for the rapid growth of phytoplankton (microscopic plants). Since phytoplankton is the primary food source for fish, these regions support a massive and diverse marine population.

Key Examples:

1. The Grand Banks (Newfoundland): This is one of the world's richest fishing grounds, formed where the cold Labrador Current meets the warm Gulf Stream.
2. North West Pacific (Near Japan): A highly productive zone where the cold Oyashio Current meets the warm Kuroshio Current.

While regions with coastal upwelling (like the Humboldt Current) are also significant fishing grounds, the specific phenomenon of warm and cold current convergence is the primary reason for the productivity in the Grand Banks and the seas around Japan.