There are, with few exceptions (salmon hatcheries), no measures applied to enhance the productivity of the oceans, analogous to seeding, fertilization, or plowing and harrowing, the fisheries take what nature gives them, and nature responds in a niggardly way if the fisheries take too much. The productivity of the oceans depends on ocean climate, the upwelling of nutritional materials from the deep sea that occurs in certain areas depends on currents, which in turn depend on winds, and currents carry plankton to certain areas so the fish can thrive. The strength and even location of ocean currents can vary substantially over time, which in turn gives rise to fluctuations in the productivity of fish stocks, as well as in their migrations and location.