The fish kill may also be part of a natural 50-year cycle of oxygen levels in the canal, which has merely been influenced (but not controlled) by anthropogenic activity. Natural causes of hypoxia in Hood Canal include the timing of freshwater inflows, water layer stratification resulting from seasonal changes in surface temperature, and climate change. The oxygen level may also be partially due to a change in wind direction. The prevailing north wind generally pushes oxygenated water into the oxygen-depleted area. A sustained south wind will cut off this source of oxygen. Coastal upwelling from the Strait of Juan de Fuca bring in a surplus of nutrients into the Puget Sound, but fail to circulate oxygen through Hood Canal. Chronic hypoxia is observed year-round. This area of low-oxygen is often seen in Lynch Cove, but has been spreading towards the mouth of Hood Canal.