The hydrilla monster– a murky mystery

It came from beneath the deep

From a far off, foreign land comes a plant that dominates lake ecosystems, changes animal habitats, ensnares boat propellers, and corrupts anglers’ willpower…  Then again, maybe it’s not so bad.  The truth appears murky.

Prepare for the invasion!

Hydrilla Hydrilla verticillata invaded Florida from Sri Lanka and India in the 1950’s, most likely through the aquarium trade as it makes for a nice aquarium plant for fish.  It’s a submersed aquatic plant that forms a dense canopy near the surface, kind of like a mushroom or rainforest.

Acres and acres of a wild green yonder

When hydrilla forms large patches of dense canopy, it can ensnare boat propellers, fishing tackle/gear, water skis, and swimmers.  These patches can also change water quality dynamics, like dissolved oxygen and temperature, from the historical environment typical to that lake.  For example, dense hydrilla is associated with reduced dissolved oxygen due to a reduction in light peneteration (most of the light used in photosynthesis is absorbed in the top layers of the hydrilla canopy) and high rates of respiration.  Fish need dissolved oxygen for growth and survival, so aquatic plant managers assume that some fish might respond negatively to dense hydrilla.  But without plants, there is very little structure for fish to hide from predators and potentially reduced photosynthesis rates during the daytime... A conundrum!  Even still, due to a broad environmental niche it inhabits back in Sri Lanka and India, hydrilla can outshade and outcompete native vegetation near shorelines or grow in deeper and darker offshore waters that were once open water.

“Come at me bro!”– What a monster!

Particularly large infestations of hydrilla have nearly shut down some lakes’ recreational boating and fishing resources.  For example, angler use of the trophy bass fishery on Orange Lake, Florida dwindled to an all-time low in the late 1970’s due to accessibility problems caused by a whole mess of hydrilla that grew to very dense surface coverages.  Aquatic plant managers had to intervene and reduce the hydrilla coverage using a variety of mechanical removals and chemical herbicides to open the lake back up and help anglers get back to the lake.

A hardcore hydrilla harvester– “Bro, do you even lift?”

Dense areas of hydrilla also impede our ability to use typical sampling gears to study fish populations.  It’s nearly impossible to use nets or electrofishing to evaluate whether fish are impacted by hydrilla infestations.  Since hydrilla changes so much of within a lake, resource management agencies around the US Southeast are very interested in answering the question, “are fish negatively impacted by hydrilla?” to see if they need to specifically manage high quality fish habitats.  If fish do just fine in these habitats, they could focus their efforts on a myriad of alternative objectives like native plant restoration, endangered species conservation, or creating better access for recreational boaters, anglers, duck hunters and homeowners. 

But these sampling limitations left us with untested assumptions on fish use and impacts caused by this invasive plant.  Due to this intriguing question, we were recently tasked by the Florida Fish and Wildlife Conservation Commission with improvising a new sampling gear to quantify whether fish use hydrilla as habitat, the amount of fish that use it, the species that use it, and how/why they select for it.

Loft thy holy video camera at thy foe!

… and I became the fish voyeur

As I mentioned previously, hydrilla grows a surface canopy at the surface; but underwater it has a lot less structure to impede equipment.  Our research lab at the University of Florida came up with the idea of using underwater video cameras to essentially spy on fish below the hydrilla canopy.  We used a variety of tank, experimental ponds, and lake samples of fish utilization of hydrilla habitats to evaluate how effective this sampling design was and what kinds of fish we could sample in these problematic areas.

Every breath you take, I’ll be watching you.

What did we find?  Our recent research indicated that the underwater video cameras tracked population-level trends very well both seasonally (it picked up large population increases in the ponds from June to October) and between ponds (it picked up differences in abundance between ponds of different population sizes) on bass and sunfish species inhabiting dense hydrilla.  When we took it to Lake Tohopekaliga, we used the cameras to collect information on fish and wildlife species such as: chain pickerel (panel A), lake chubsuckers (B), juvenile and adult largemouth bass (C and D respectively), juvenile and adult sunfish (E and F respectively), American alligator (G), bluefin killifish (H) redbelly turtles (I), and more!

I love the taste of camera in the morning…

We also took some measures of fish habitat quality at each sample location to evaluate why these fish might utilize hydrilla as habitat.  We found that fish select for areas of hydrilla in large part dependent on the dissolved oxygen levels available and the amount of structure around.  Habitat structure plays a key role in reducing a fish’s risk to being eaten by a big bass or osprey, but also restricts that fish’s ability to see/move to eat a juicy dragonfly larvae or a smaller fish.  Areas with high dissolved oxygen and low-intermediate structure appear to be more optimal (or hospitable) than others. 

Maybe largemouth bass like the taste of cameras too...

Interestingly, fish changed their habitat selection patterns dependent on the number of fish around them where with higher numbers of fish and they more likely they are to move into previously ‘inhospitable’ areas.  This is like how some people move away from city-areas with high competition for limited jobs and into smaller towns with reduced competition.  This indicated to us that potential negative impacts caused by hydrilla might ultimately depend on the density of the fish population, but often times fish seemed to do just fine in hydrilla.

An interesting find indeed! – Fish habitat selection in dense hydrilla habitats changed based on the densities of fish in the population

We found that many fish species, even adult largemouth bass (Florida’s #1 freshwater recreational fish species) will occupy areas of dense hydrilla and that some fish populations can grow at high rates in these environments.  Fish even used areas that have very little dissolved oxygen!  This counters some of the previously untested assumptions that resource managers were forced to rely on for their management strategies.  

These bluegill seem to do quite fine in dense hydrilla at Lake Tohopekaliga

This is only the beginning stages of this type of work.  We now have a tool to evaluate fish habitat use in dense hydrilla and other invasive aquatic plants.  Furthermore, we have some predictions to test based on some of our early ecological findings on density-dependent habitat selection patterns.  These new methodologies sampled a fairly typical fish community in a large Florida lake and we can now begin to uncover some of the ecological mysteries in the habitats of this formidable plant.  This should help resource managers.  And maybe we’ll find that, at least for the fish, it’s not such a monster after all.

In low dissolved oxygen even these sunfish seemed to enjoy their afternoon swim