We have stressed in past blog posts the importance of details when it comes to lake and pond management. One would think that common sense would tell one that details are important no matter what field one is talking about, whether it be fisheries science or rocket science, landscaping or construction of a skyscraper; but, judging from our experience, it is common in the world of lake and pond management for key, essential details to get overlooked.
There’s a prominent fish hatchery in a state directly south of us that recently updated their website. As part of the new look, they added a Q & A section, with a handful of questions that ostensibly they get regularly from pond owners. One of those questions is, “Is pH important?”
Bear in mind: this section of the website was not presented as speculative information, or fanciful opinions of a marginal employee of the hatchery. It’s presented as being authoritative, expert advice from the owner of the hatchery, who is announced as a degreed biologist.
His answer regarding whether pH is important? “Not really. Alkalinity is more important.”
It’s really not possible to issue a more incompetent, irresponsible piece of advice to a lake or pond owner. If my statement seems harsh, consider the following.
pH is often referred to as the, “master regulator,” of fish biology. It has been referred to as such because it controls not one but a whole range of key physiological processes in fish that in turn determine the health of the fish and even whether it lives or dies. pH below 5.5 limits survival of fish in the egg and larval stages; largemouth bass, for example, cannot spawn successfully at pH below 5 (Swingle 1956; Buck and Thoits 1970). Hemoglobin in fish blood, the substance that transports oxygen throughout the fish’s body, has half as much affinity for oxygen at pH levels below 6 as it does at higher pH levels: it’s actually more difficult, much more so, for fish to get oxygen from the water at low pH levels. Low pH causes carbon dioxide to bind to the receptors where oxygen molecules would otherwise bind, further hindering the fish’s ability to get oxygen from the water. Osmoregulation, the process whereby fish constantly regulate the salts in their tissues via osmosis at the gills and skin, is more difficult at low pH. A wide range of metals and compounds that are common in freshwater ponds and lakes are more toxic at lower pH levels. pH levels below 4 are considered lethal for most freshwater fish. And pH not only affects the fish directly: it also affects their entire environment. Many aquatic invertebrate species are eliminated at pH levels below 4.5, as are many species of aquatic plants, key habitat for invertebrates and small fish alike.
We have evaluated several ponds and lakes that had pH below 5. Though these waters varied widely in other parameters such as dissolved oxygen, phytoplankton abundance (the foundation of the food chain), habitat, and alkalinity, they all had two things in common: very few fish, of any species, and few large fish (usually none).
A couple years ago we electrofished a nine-acre pond in South Carolina that had a pH of 4.7. Despite spending more than two hours electrofishing, more than twice the amount of time we typically need to capture over a hundred fish even from a one-acre pond with good water quality, no largemouth or other predator species were captured in the survey. Normally when a pond has bluegill in the absence of predators, the bluegill will be badly overpopulated, with thousands of fish per acre; in this pond, in two-plus hours of electrofishing, we captured fifty-one bluegill and six redear. Forty-six of the bluegill were under five inches in length, with most of them being under two inches. The low pH was suppressing bluegill reproduction; and if bass had been stocked at any point, they had failed entirely due to the pH. Now that the pond has been limed and more bluegill stocked to replenish their numbers, the bluegill are thriving, and the newly-stocked largemouth are growing rapidly.
In May of 2016 we electrofished a 50-acre lake in middle Tennessee that had previously been managed by the largest pond and lake management company in the Southeast. Said company has more of the best accounts in this state, at present, than we do, much to our amazement and chagrin; however, we regularly get called by landowners who have just cut ties with said company because they had done a mediocre job. In this particular instance, that big company had told the lake owner that his bass were not growing because they were overpopulated, and that he needed to remove twenty pounds per acre, per year, of small bass. Bass harvest is indeed a valuable tool for managing smaller lakes and ponds of a few acres; if there are too many small bass, that means less food for the big bass, and fewer bass that make it to a desirable size in the first place. But too many consultants have a knee-jerk response if they see three one-pound bass in a survey and rush to proclaim the lake is overpopulated with largemouth, when oftentimes it’s simply not the case, and some other factor is to blame for poor bass size. In this particular instance, we electrofished this beautiful, scenic fifty-acre lake for nearly three hours…
And captured a grand total of twenty-six fish.
We did indeed capture more predators than bluegill, nineteen bass and two channel catfish to only five bluegill. But more important than the numbers of either species is the total number of fish, which is exceedingly low for a lake that size, the lowest we have ever found. As a reference point, a little over a year after that survey, we electrofished a 40-acre lake, also in middle Tennessee, for two hours, and captured 79 bluegill, 74 largemouth, and ten redear. So even though we found almost four times as many largemouth, we also found almost sixteen times as many bluegill, and the bluegill were larger on average as well. Because bluegill are one of the most prolific spawners of any freshwater fish, they will typically outnumber the largemouth even when the largemouth are overpopulated, unless there is a water quality issue.
The 40-acre lake did have a much better phytoplankton abundance: visibility was only thirty-three inches, compared to the ninety-six we found at that 50-acre lake. But high-clarity water can still have thriving fish populations; we electrofished a 150-acre lake a few weeks ago that had 96” visibility, and we still captured 268 fish, with 205 of those being bluegill and redear. We had 62 largemouth in the survey, including five that were over eighteen inches and five more between fourteen and sixteen, and yet those bass had not eaten all of the bluegill, or even close. The 150-acre lake had good amounts of cover in the form of weedbeds, but the 40-acre lake had minimal cover. What both of those lakes had in common was a pH level that allowed normal bluegill reproductive success: the 150-acre lake had a pH of 6.8, and the 40-acre lake had 8.7.
The pH of the 50-acre lake? 4.5.
Its alkalinity was 120, well within the ideal range of 50 to 200. So if the lake owner had had the hatchery from south of us evaluate his lake, he would still have an under-performing lake.