Don’t let pond vegetation get ahead of you
The first sign that my pond was in trouble this summer was a dead, juvenile grass carp.
I found the kill unusual, as grass carp – this was one of a pair of sterile juveniles I stocked to control algae – are very hardy. At first I wrote it off to possible mishandling; I accidentally had dropped it en route from pond specialist Schooner Farms at Weston to slipping it into my pond at Froggy Bottom. Maybe internal injuries had caught up with it in several weeks.
But then I found a dead largemouth bass, a nice one, maybe a pound and a half. I knew still, dead, extended, early heat this summer was not good for ponds, but rather it is ideal for growth of pond vegetation such as the rooted green algae, chara – my pond’s particular nemesis.
This summer, thick mats of chara practically exploded, such that bullfrogs could hop across the surface hardly getting their feet wet. The mats were as thick in mid-July as I might expect in mid-August most summers.
I had hoped that the carp – which also are called triploid or sterile white amur – would get a start on munching down the chara and be sufficient to maintain a balance. But no, not this summer. When the plant world goes wild, there is no keeping up or catching up, especially when you get behind. I was laid up since late May with a partially torn hamstring and couldn’t get after the chara mats that I saw forming.
Just lately I have rehabbed enough to start mucking out the thick, heavy mats using a big heavy-tined Minnesota lake rake. The chara feel raspy to the touch from calcium carbonate deposits, or stoneworts, that the long strands accumulate. Hauling it is tough work.
As I pulled and stacked mounds of the stuff on the bank, I found more and more dead fish, some of them seemingly trapped, dead, deep within the thick algae mats. It was like the chara had acted like fatal gillnets, though that is just my idle speculation. Most of the dead fish were large (for my little pond) largemouth bass, including at least half a dozen 2- and 3-pounders. Turtles had eaten half of two of them by the time I dredged them up. Some dead bluegills of 2- and 3-inch length and tiny young-of-year fish also were dead. I was not happy.
It took two afternoons of grunting and groaning, but I got most of the chara hauled out. I will let it dry a few days and haul it by wheelbarrow to my compost piles.
I hope that the remaining grass carp alone can keep the vegetation under control now that I have tried to even the score. (Its predecessor did so well over several years that it eventually all but ate the pond clean – so clean that I had to remove it as it was rooting up the bottom searching for roots and such to eat. That totally muddied up the water clarity, which in turn suppressed any vegetation, which is not ideal either in a “natural” pond.)
Mike Wilkerson, fish management supervisor for District 2 of the Ohio Division of Wildlife at Findlay, said this summer’s weather pattern has created the perfect storm for fish kills in ponds such as mine. The heat, lack of rain, still air, and lots of sunshine have been just what vegetation needs to grow explosively, he agreed. He said that even larger reservoirs are showing low dissolved oxygen levels. Pleasant Hill in north-central Ohio, for instance, showed very little dissolved oxygen below three feet down.
Wilkerson said that chara gives off oxygen in the daytime, but at night it uses oxygen from the water. It is all part of the photosynthesis cycle of plants. But as the mats in my pond grew and grew, they used up so much oxygen that some of the fish, too many, virtually suffocated. Their great mass outstripped the oxygen supply for the fish.
One option to avoid lake-raking and deploying grass carp would be to install an aerator pump. Aerators tend to boost oxygen levels and suppress algae growth. For now, I put a submersible pump in the pond, connected to a short length of hose to splash water to the surface and oxygenate it. It is a tactic I have used previously. It is not a perfect solution, for the anoxic of oxygen-depleted zone lies in the deeper, cooler water of the pond, where the bigger fish also tend to hang out. But my pond is small enough that I am hoping the eventual mixing of oxygen-rich water will spread throughout.
In any case, it was another lesson learned: Don’t let the vegetation get ahead of you, or you’ll be scooping dead fish from your pond. Especially this summer.