Special Shout out to Cody Craig and Texas State University Cody is a Ph.D. student at Texas State University, and researches endemic fishes of the Edwards Aquifer in central Texas. Cody was kind enough to give us an overview of his research and the importance of these unique fishes.
Importance of the Edwards Aquifer to Endemic Fishes of Central Texas
Anyone who has taken a midsummer dip in Barton Springs, floated the Comal or San Marcos rivers, kayaked the Medina River, fished the Devils River, or rope swung into the Frio realizes the springs of central Texas are special places. The cool, clear springs of the hill country are a stark contrast to the semi-arid landscape they are situated in. Although each spring is beautiful in its own right, the source of all large central Texas springs is the same. Every drop of spring flow comes from the Edwards Aquifer, which supplies springs with stable water quantities and qualities. Stable water qualities are what makes these waters so attractive for humans, with very clear water and unfluctuating temperatures that range from 68-72 degrees depending on where you are (spring temperatures are the average yearly temperature of that location).
In addition to supplying a summertime refuge for Texans that are willing to brave the first jump in (there is a reason why it’s called the Frio River!), these springs emanating from the Edwards Aquifer are also home to many endemic fishes that are found only in central Texas. As far back as the 1880s, the first ichthyologists (people who study fish) discovered these springs’ harbored unique species. Today there are 15 recognized fish species that are associated with the springs, including two sub-terrainan species that are found below ground in the aquifer itself. Reason(s) that the remaining 13 (above-ground) species are only found in the springs has been a long standing mystery that we are just now starting to solve. Springs eventually run in to larger rivers that have water quantities and qualities that vary more than the stable springs (e.g., Barton springs flows into the Colorado River). However, except for a few relatively newly installed dams, spring fishes have long had access to the non-spring rivers, but rarely leave the stable springs. Conversely, riverine fishes are less abundant and have a lower density within the springs. So there is a unique distribution of fishes in these spring/river systems, an apparent community segregation. In addition to community segregation, a study also found a positive relationship of the spring fish community based on discharge. Which means the greater the discharge of a spring, the more spring fish species and numbers of spring fish there are.
So what is it about the springs that makes them such good habitats for spring fish, and why do other fish avoid them? It was hypothesized in the 1990s that spring fishes have physiologies that perform better in stable cool water environments of springs, while spring temperatures are sub-optimal for other riverine fishes that preferred warmer temperatures. Support for this reasoning came from a study in the Devils River that showed segregation in spring and river fish communities in the summer, but mixing of fish communities in the fall, when river temperatures were similar to that of the spring flow temperatures. Further support came from a soon to be published study where one spring fish and one riverine fish species were placed in a spring (72 degree) or a typical summertime (86 degree) temperature and were fed 5 food items. The spring fish ate first more often and ate more food at spring temperatures, whereas the riverine-associated fish ate first and more at warmer riverine temperatures.
Although more work needs to be done to find the exact physiological mechanism, endemic spring fishes seem to be dependent of both the water quality and quantity of springs from the Edwards Aquifer. So the next time you decide to swim, fish, snorkel, or tube these unique systems of central Texas, be on the lookout for these endemic spring fishes that few people know to look for. Take pride in Texas being home to some truly unique and beautiful fishes that can be found nowhere else on earth!
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