The Wild Burlington Newsletter .2.03.b | Cave fish

Issue #2.03.a | Cave Fish

Well, folks, this week marks the 2nd earliest amphibian migration in the 12 years I’ve been monitoring at Shelburne Pond. Back in 2012 we had an 80 degree day followed by a downpour that brought out a slew of amphibians on March 8th. I went out last night with a few other intrepid naturalists and we spotted 2 green frogs, 2 wood frogs, and 1 leopard frog. Looking ahead at the 10-day forecast, next Tuesday night’s weather might be favorable for more amphibian movement. If the forecast holds, I’ll be heading out to Shelburne Pond around 8pm to check for amphibians (send me an email if you want to join).

   Also, I’m looking to put together a guide to amphibian migration and habitats over the next month. If anyone’s interested in collaborating on a video project on amphibians, let me know!


Herpers harassing a poor frog trying to make its way to its breeding grounds (Shelburne Pond)

Did you miss the Wild Burlington Lecture on amphibian migration? 

I’m still trying to figure out the best way to record lectures and upload the video afterwards. Until that day I’ll continue to post slides from each lecture on the event page (you can find the amphibian migration lecture here).

Slides From Lecture

The Donahue Sea Caves

Every winter I take at least one trip down to the Donahue Sea Caves in Burlington. In the early part of the 20th century, the wide trail, which now leads down to the wetland, served as a dirt road connecting farmers’ homes on North Ave to their farms in the Intervale (thanks to Sean Beckett for this great write-up: link). Eventually the road was abandoned and Rte 127 was constructed (the stretch north of the Old North End was built in 1971). Rte 127, or the Burlington Beltline, blocked water from draining from North Ave down through the Intervale and ultimately into the Winooski River. Over the past 50 years, this water has slowly accumulated in that narrow bottomland (see image below), creating an open pond flanked by an extensive cattail marsh. The (unnatural?) wetland serves as prime habitat for amphibians, fish, and lots of waterfowl (this list of birds is definitely incomplete because much of the wetland is inaccessible to birders in the summer). 

Great blue heron trails leading to the open water adjacent to the rocky cliff (Arthur Park, Burlington)

My field assistant, Cheddar Bill, standing in awe of the sea cave entrance. 

Cave Temperatures

In the winter, the shallow pond freezes over completely, safely locking all that aquatic biomass away from the terrestrial predators (such as the great blue heron I wrote about last week). But all is not lost for our hungry heron, as the warm cave keeps a tiny door open, granting predators access to the helpless pond-dwellers (in addition to the heron, this open water draws in otters and raccoons and, once they arrive, osprey and bald eagles).

Icicles hang from the roof of the cave, while a small pond forms at the base

Temperatures in the cave
Generally, the interior spaces of a cave are mostly sheltered from the outside environment, with little air flow moving from outside to the inside. Because air moves so slowly in and out of the cave, the temperature within a cave changes very slowly and is relatively stable throughout the year. The chart below (from a cave in Iceland: link) shows the daily temperature swings – both inside and outside a cave – over a 3 month period. (Note: that the scales are different for the external temperature (blue) and interior temperature (purple)). Temperatures outside of the cave can fluctuate almost 15-20 degrees F per day, while inside the cave temperatures rarely fluctuate more than 2 degrees and are consistently ~15 degrees cooler than ambient temperatures.

This same pattern holds throughout the year. And this is true of caves throughout world. The temperature that inside of a cave hovers around is roughly the average external temperature of the cave’s location over the course of a year. So if you took the temperature outside the cave every minute for a full year and then found the average temperature from your data set, you’d have a rough approximation of your cave’s temperature! Pretty neat. This also applies to ground water. 

Looking up at icicles forming at cracks in the limestone.

The sea cave, as you’ll note in the photo above, has plenty of icicles hanging from the ceiling. The entrance to the cave is quite large and so the cave does exchange quite a bit of cold winter air with the surrounding environment making internal temps somewhat more variable than for the cave in Iceland (and other deeper caves). The icicles form as ground water percolates through fractures in the rocks and freeze once exposed to the colder air. Some ground water flows directly into the sheltered pool at the bottom of the cave and keeps warm enough in the pool that it avoids freezing (in particularly cold winters I’ve seen it freeze solid).

Small fish congregating under the icicles. Dripping water stirs up the water and facilitates transfer of oxygen from the air into the water

Open Water + Oxygen

Access to the cave
I should note that water levels inside the Donahue Sea caves vary from year to year. There’s a little lip at the mouth of the cave and if the water in the pond is high enough then the open water in the cave will extend a channel that connects the cave water with the rest of the pond. This channel allows fish to freely move in and out of the cave during the winter.

Oxygen depletion
Once a pond freezes over, the oxygen dissolved in the water slowly gets used up by all the fish, amphibians, reptiles, bacteria, and invertebrates trapped under the ice. The sheet of ice over the pond prevents oxygen exchange between the air and the water (plus, as plants and bacteria are dormant, they’re not photosynthesizing and therefore aren’t producing oxygen). As oxygen levels drop, the larger, more active fish struggle to take in enough oxygen and can ultimately asphyxiate unless they’re able to move towards areas with higher concentrations of oxygen. The cave, with open water, provides an interface for the exchange of oxygen between the water and the air (particularly in places where water dripping from stalagtites stirs up the surface). This year, the cave is filled with lots of smaller fish and even green frog tadpoles that are congregating in the cave for access to warmer water (45 degrees does feel quite nice) and higher dissolved oxygen concentrations.

Not all fish make it through the winter in the cave.

One challenge is that higher oxygen levels can attract larger fish to the cave. In previous years, when the lake levels have been higher, I’ve found 12″ bass and catfish in the cave. As water levels in the adjacent pond drop throughout the winter, the fish can get trapped in the cave where they’ll inevitably use up all of the oxygen and die. Come spring time, when the ice melts, these dieoffs serve as a much needed food source for returning bald eagles and osprey. This year, the water was high enough to connect the cave with the pond, but not high enough to allow for many large fish to access the cave. Right now, for the heron, it’s a bit like shooting fish in a barrel, but as the temps continue to stay above freezing the pond will melt and the fish will disperse back into the safety of the pond. 


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