Brine Fly Feeding Frenzy at the Great Salt Lake

The Great Salt Lake in northern Utah is a vital resource for many of North America’s birds.  The lake is 75 miles long, 30 miles wide and has 1680 square miles of surface area and 335 miles of shoreline.  Surrounding the lake are 400,000 acres of wetlands.  Roughly 5 million birds of 257 species rely on the lake and those wetlands for food, sanctuary, breeding and as a migratory stopover.

And one of the primary reasons the lake is such an asset for them is the endemic brine fly, Ephydra hians.  

 

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Brine flies can be intimidating because of their massive numbers but they are truly innocuous.   They are found only within a few feet of the shore, they don’t bite and won’t even land on humans.  And they’re a huge nutritional resource for birds.

In the photo above, the blurry brown bits in the background are brine flies in flight that have been stirred up by the feeding California Gull.  Different bird species have varying feeding methods with these flies – this species often stakes out a few feet of feeding territory on the shoreline and then rushes through the hordes of flies on the sand, stirring them up, and then snaps them out of the air in large numbers.  This bird is looking down the shoreline and preparing for its next run on the flies. 

 

 

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 Most of the flies are resting on the sand until the bird makes its run.  As the gull rushes along the shoreline the flies are disturbed into the air and the bird snaps up as many as it can.   

 

 

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 Sometimes the gulls will raise their wings as they run forward, in an obvious attempt to scare up more flies for the meal. 

 

 

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 This angle on the action gives one an idea of how many flies a gull might get in just one gulp. 

 

 

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 At times the flies are so incredibly thick that parts of the gull are almost completely obscured by them.

 

 

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  Various species have different feeding methods.  Franklin’s Gulls generally don’t rush forward to scare up hordes of flies.  Instead they tend to stand in place and just pluck them out of the air a few at a time.  

 

 

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Brine flies have an interesting and complex life cycle.   During summer the female lays her eggs on the surface of the water.  The eggs hatch into larvae which spend the winter on the lake bottom in an inactive state.  In the spring they feed mostly on cyanobacteria (one of the few life forms that can survive the very high salinity of the lake).  When water temperature becomes warm enough the larvae metamorphose by growing a hard brown pupal case around their bodies.  Inside that pupal case they begin to transform into adult flies.  When the adults emerge from the pupal case they rise to the water surface and fly to the shoreline.  These adults live for only a few short days, just long enough to mate and lay eggs.   

The pupal cases also come to the surface and are blown into large rafts on the surface.  In the image above you can see rafts of these pupal cases about to be blown on to shore. 

 

 

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 These are the thick mats of pupal cases that have blown onto the shore of the Antelope Island causeway.  There are also millions of adult flies on top of the mats.  The lake has about 335 miles of shoreline so you can imagine how many flies it supports.  Biologists have estimated that number to be roughly 100 billion! 

 

 

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 Here’s a closer look at a handful of pupal cases.  You can also see a few newly hatched pale adults among the cases.  The adult flies soon become darker in color. 

 

 

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 Obviously, many species besides gulls feed on the different stages of the life cycle of the brine fly.  Here, American Avocets join in the feast.  At times birds will be this thick for miles along the causeway to Antelope Island. 

Some see brine flies as a nuisance but because of their role in supporting bird life I love them.  I wish I could say the same about those damned biting gnats!

Ron

5 Comments

  1. Again thank you for sharing your knowledge. A great teacher, i’m sure your students loved you.

  2. I watched this same phenomenon along the causeway and attempted to get some photos, but they were throwaways – nice work!

  3. Ron, very interesting story about these brine flies, I had no idea about their life cycle and how many there can be. I believe I have encountered these at Mono Lake in California and thank goodness they do not bite!
    Jim

  4. Wonderful. As always. Echoing Robert’s comment. I am so glad that I discovered you. Thank you.

  5. Wonderful post Ron. Great photos and information.

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