Hood Canal avoids a major fish kill following unwelcome conditions

Southern Hood Canal avoided a major fish kill this year, but for a few days in September it looked like conditions were set for low-oxygen waters to rise to the surface, leaving fish in a critical state with no place to go, experts say.

Data from the Hoodsport buoy show the rise of low-oxygen waters to the surface over time (purple color in top two graphs). // Graphic: NANOOS

Seth Book, a biologist with the Skokomish Tribe, has been keeping a close watch on a monitoring buoy at Hoodsport. Dissolved oxygen in deep waters reached a very low concentration near the end of September, raising concerns that if these waters were to rise to the surface they could suddenly lead to a deadly low-oxygen condition. This typically happens when south winds blow the surface waters to the north.

“I started asking around the community to see if anyone had seen evidence of low DO (fish at surface; dead fish; deep fish being observed or found in fishing nets at surface; diver observations) and luckily I had no reports,” Seth wrote to me in an email.

The top graph shows how dense seawater from the ocean, which is somewhat higher in oxygen, arrives near Hoodsport in early September. That’s the dark blue water at the bottom. Over time, the blue color displaces the purple colors, which represent water with almost no oxygen. From Sept. 21 to Sept. 25, this very-low-oxygen water reaches the surface near Hoodsport, raising concerns. It happened again on Oct. 2, but the purple color was less intense, meaning the oxygen levels were not as low.

We don’t know how widespread this surface low-oxygen condition was in Southern Hood Canal this year, since the monitoring buoy is in a fixed location. Perhaps the fish were able to move around and escape the low-oxygen waters — which did not come as suddenly nor last as long as in some previous fish kills.

The purple color now seems to be gone from top to bottom in the top graph, and we should expect that winter storms will stir up the water and bring more oxygen from the surface to the waters below. This annual process is expected to start again in the spring, when sunlight and nitrogen from the land encourage the growth of plankton, which die and sink to the bottom, where their decomposition sucks up the available oxygen.

Seth points out in his email that the temperature of the water down at 80 meters (262 feet) is higher than we have seen in recent years — except for those years when a warm-water “blob” developed off the West Coast and worked its way into the inland waterways.

Jan Newton, an oceanographer with the University of Washington, said she and other experts are reviewing the data to see if this warm water might be a residual effect of the blob or the result of other conditions.

Seth rightfully credits the Northwest Association of Networked Ocean Observing Systems (NANOOS) with the wealth of data that helps us understand what is happening in our waterways. NANOOS is a partnership of more than 50 various entities, and Jan Newton is its executive director.

Related blog posts in Watching Our Water Ways:

  1. Sept. 7, 2016: Close call, but Hood Canal may escape a major fish kill this fall
  2. Sept. 1, 2015: Low-oxygen scenario following unusual course this year in Hood Canal

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