Summer is undoubtedly the most stressful time of the year to be a fish in Central Texas.  If you’re lucky enough to have water, then it’s probably warm (if not hot) with unfavorable oxygen levels because warm water cannot hold as much oxygen as cooler water.

In the last two weeks we have experienced fish kills at Inks Lake and Lake Travis.  Both are due to “natural” conditions and summer stratification of the reservoirs.  Let’s start by going back to this past April – when water temperatures were cool and dissolved oxygen levels high.

All that begins to change as water temperatures warmed heading into summer.  Surprisingly to some, water temperatures are a function of both the daytime highs and the nighttime lows.  When the nights don’t cool, then neither do the lakes.   Recently, surface water temperatures in Lake Travis have been hovering in the low 90s.  However, what many folks don’t know is that the water temperature at the bottom of Lake Travis is currently 54°F.  That’s a pretty steep thermal gradient (warmer water over cooler water) and one of the reasons water skiers wear wet suits in August as they ski the upper stretches of Lake Austin.  This cooler water is also devoid of dissolved oxygen.

The thermal gradient that separates warmer, productive water from the cooler bottom water also prevents oxygen from reaching the deeper depths.  It’s as if “mother nature” slid a pane of glass between the two layers – there’s no mixing between the two.   Sunlight keeps plants and algae producing oxygen in the upper layer and that’s where most of the fish hang out.  There is no way to replenish oxygen in the deeper depths (greater than 50-60 feet) and decomposition of organic material uses up what little oxygen is present.   Before too long, the bottom waters of deep storage reservoirs such as lakes Buchanan and Travis, become hypoxic (a fancy term for “devoid of oxygen”).   This year all the Highland Lakes are showing signs of thermal stratification and accompanying hypoxia (link to charts).  And it’s that bottom water that is released each day during hydro-generation. 

A fish kill was reported on Inks Lake last week (August 16th).  Most likely due to fish becoming trapped during hydro-generation and suffocating from the lack of oxygen in the water.  Figure 1 shows dissolved oxygen (DO) in the headwaters of Inks Lake (blue line), about 1-mile downstream at the Hwy 29 Bridge (green line) and the corresponding generation releases (red line).   Dissolved oxygen levels are high each afternoon as a result of algae and plants producing oxygen until hydro-generation releases begin, then DO levels plummet to near zero as the cold, hypoxic water from the bottom of Lake Buchanan is passed downstream.   What we don’t know is “why don’t fish die every day?”  or every year?  This same situation is present every year, usually from July through October but the last reported fish kill was back in 2007. 

Figure 1: Dissolved Oxygen at Inks Lake Headwater and Hwy. 29 Bridge during Hydro-generation

Now for the Lake Travis fish kill.  The same situation exists in Lake Travis…warm, productive water over cooler, hypoxic water.  The thermal gradient is in place at about 40 feet.  What complicates Lake Travis is that there appears to be a layer of oxygenated water between 80 and 140 feet.  However, there is no way to replenish the dissolved oxygen at those deep depths.  So, over the summer that layer of oxygenated water diminishes until there’s not enough oxygen to support fish and they “pop” to surface as was reported last week (August 24th).  Specifically, catfish, striped bass and drum with distended air bladders were reported near Mansfield Dam.   Our hypothesis is that these fish have been “trapped” in the layer of oxygenated water for some time but now the oxygen has run out.  This phenomenon is fairly common across the southeast U.S. and referred to by some as the “thermal squeeze.” 

In the graph below, the blue line represents the dissolved oxygen profile from the surface to the bottom.  The yellow layer, had enough oxygen to support fish, the orange layers don’t.   As the summer progressed, the yellow layer become more and more narrow, until it eventually disappeared.

Figure 2: Dissolved Oxygen Profile for Lake Travis (August 2010)

This thermal stratification and hypoxic condition will remain in place until the ”fall turnover” occurs later this year once the weather changes and cold fronts begin reaching central Texas to cool the surface waters of the Highland Lakes.