How acid or alkaline is your lake?

A characteristic you can’t see or feel can have subtle or significant effects on life in your lake. It’s called pH, and it’s a measure of how acid or alkaline your lake’s water is.

We know that water molecules contain two atoms of hydrogen and one atom of oxygen (H₂O). However, some of those molecules actually exist as positively charged hydrogen ions (H⁺) and negatively charged hydroxide ions (OH^-).

In pure water, those ions exist in essentially equal numbers. But when chemicals are added to water, the balance can shift in one direction or the other. A solution with more hydrogen ions is acidic; a solution with more hydroxide ions is basic, or alkaline.

pH is measured on a scale from zero (extremely acidic) to 14 (extremely alkaline). Pure water, which is considered neutral, has a pH of 7. Relating this to common substances, lemon juice is a fairly strong acid (pH just over 2), while household ammonia is strongly alkaline (pH about 12).

Lake waters are not that strongly acidic or alkaline. Their pH falls generally in a range from about 6 to 8, close to neutral. Interestingly enough, natural rainwater is fairly acidic (pH about 5.6), and air pollutants like sulfur dioxide and nitrogen oxides can lower the pH significantly – causing the harmful phenomenon of acid rain.

Fortunately, most lakes contain substances that neutralize (or “buffer”) acids, thus keeping the pH stable. One of the most important of these is calcium carbonate (limestone).

How does pH affect life in your lake? That’s complicated, but it determines how well certain fish species, plants, insects and other life forms survive and reproduce. For example, at pH below 6.5, walleye spawning is inhibited, and smallmouth bass disappear below pH 5.5.

pH can also determine the extent to which certain pollutants are released into the water from sediments in the lake bottom. For example, a change in pH can cause more phosphorus to dissolve in water, making it available to feed algae. In addition, many scientists believe that higher acidity is related to the release of toxic mercury into lake water. The mercury then can accumulate in fish.

pH and its effect on lake life is a complex subject. In healthy lakes, the effects are mostly subtle – pH is just one of many qualities that make each lake unique.

Who decides where the school goes?

Hundreds of perch schooled off our pier last week. The problem? They were an inch and a half long, which means nano hooks, water flea bait, very sharp filet knife.

Looking down at that swarm of black-striped fry, moving in unison, I couldn’t help wondering: What holds that school together? Why are they schooled in the first place? And which fish decides where the school goes?

The first thing to appreciate is that these fish don’t “decide” anything. They don’t form the school out of conscious strategic thinking. The behavior is built into their genes; it conveys certain evolutionary advantages that promote survival.

First off, it’s easier for a predator to track down and capture a solitary fish than to eat fish in a school. This seems counter-intuitive, since we’d think attacking a school would amount to the proverbial “shooting fish in a barrel.” However, scientists have found that a school confuses predators. A school moving together, the sides of multiple small fish flashing in sunlight, can appear to a predator as one large fish, discouraging attack. In addition, the sheer numbers of fish in a school disorient predators, making it hard for them to zero in on one individual.

Another advantage to schooling is that more eyes watching means greater ability to find food. Schooling also helps fish conserve energy – in effect they’re able to draft on each other. The principle is the same employed by bicycle racers, one closely following another to reduce wind resistance.

But how does a school of fish move as one? According to an article on the North Carolina Aquariums website, “Each fish maintains an exact spacing from its neighbor. As they swim, they follow the movements of their neighbors and change their course in unison. Vision is the primary sense used to hold their place in a school. Visual markers play a big role – each member of a school follows some key feature of the fish around it, usually a stripe or spot on their bodies, fins or tails. The vibration-detecting lateral line, a row of sensory cells that runs along the sides of the body, also provides information about neighbors’ movements.”

A closer look at the school of perch off our pier showed the individual fish contentedly picking off white specks in the water – likely Daphnia (water fleas) or some other zooplankton. Those of us here on Birch Lake can only hope the schooling behavior helps those perch grow to catchable, edible size. Time will tell.

The functions of fins

The fins on flashy 1950s cars were, practically speaking, useless appendages, all style and no substance. The fins on fish, though, are a different matter.

Fins may add to fishes’ beauty – take for example the “sail” on a marlin – but they are also highly functional, as essential as arms and legs are to us humans.

So, what exactly do these fins do? Let’s start with the tail fin, or what the scientists call the caudal fin. It’s mainly for propulsion. If the fish’s muscles are the engine, then the tail fin is the propeller. A few whips of the tail and the fish can be off in a flash, chasing prey or fleeing a predator. The tail fin also contributes to steering.

The dorsal fin, the one that runs along the top of the spine, adds stability during travel, a little bit like the centerboard on a sailboat or the fletching on an arrow. In many fish the dorsal fin also contains spines. It can lie flat or be unfurled, spines vertical.

The expanded dorsal fin can help protect a fish by making it appear, to a predator, larger than it actually is. The spines themselves can also deter attacks. If you’ve ever been “speared” while unhooking a perch, imagine how that would feel to the inside of a larger fish’s mouth. The anal fin, on the fish’s underside forward of the tail, also lends stability.

Deep-bodied fish, like bluegills or crappies, generally have larger dorsal and anal fins because they need more support to hold themselves upright.

Pelvic fins, on the underside coming off the belly, also help the fish stay level, providing stability against rolling from side to side. The pectoral fins, generally on the side of the fish just behind the gills, add stability and help the fish maneuver and control depth. Pelvic and pectoral fins also act as brakes – when flared out they help the fish slow down and stop.

Fin sizes, shapes and configurations vary with fish species, where they live, and how they feed. If you’d like to know more, there's a great online presentation from the the Minnesota Department of Natural Resources.

Tales in the scales

Alive without breath, as cold as death

Never thirsty, ever drinking

All in mail, never clinking

The answer to this riddle from J.R.R. Tolkien’s The Hobbit is: Fish. And the “mail,” of course, refers to fish scales.

Scales are fascinating structures that can tell a great deal about the fish in your lake. By looking at scales from fish taken during test nettings, scientists can tell how old the fish are, how fast they have grown, whether they has been seriously ill or stressed, and more.

The types of fish in our area lakes are hatched with all the scales they will ever have. Scales originate from points in the fishes’ skin and overlap like shingles in a roof. They grow larger as the fish ages by adding to the outside edge. The scales show growth rings somewhat like those seen in the cross-section of a tree trunk. A difference is that while trees add just one ring per year, a fish scale may gain multiple rings in a year.

Still, each year does leave a distinct mark, especially in our climate. Because fish are cold-blooded, their growth slows significantly as they spend winter under the ice. A thicker ring, called the annulus, forms during these times.  

Scientists can learn a lot by studying these rings. For example, the distances between the annular rings reveal the approximate length of a fish at each age up to the current one. That’s because the rings are placed in proportion to the total length of the fish. So, suppose a scale from 12-inch walleye, three years old, has its first ring one-third of the distance from its focal point to the outer edge. That fish would have been four inches long at the end of its first year.

Using scales to tell the age of individual fishes, biologists can learn about the growth rate of a lake’s fish population. Because fish grow more slowly when they reach sexual maturity, scientists can use rings on scales to estimate the age at which fish began to spawn. This helps in setting fishing regulations to make sure most fish can spawn at least once before they are caught and removed.

A few other facts about scales: Fish that swim fast and live in fast-flowing water often have small scales, while fish like carp that live in slow or still water tend to have larger scales. Some fish have smaller scales toward the tail, providing more flexibility there. If a fish loses a scale, such as to injury, it grows back, all at once – so that scale will not show growth rings.

Freshwater jewels

You’ve probably seen neon tetras, bleeding hearts, clownfish and yellow tangs in aquariums. In all likelihood a fish just as beautiful, or more so, swims in your lake.

For my money, no fish is prettier than the sunfish – officially the pumpkinseed. I sometimes question why they are named for something as nondescript as those off-white ovals pulled from Halloween jack-o-lanterns. But of course the name “sunfish” belongs to a family of fishes that include the bluegill. Still, what the experts call pumpkinseeds are always sunfish to me.

Bluegills are pretty fish in their own right, but they look pale next to sunfish: Those eyes with bright-red iris around the dark pupil. The wavy lines that radiate from the mouth across the gills, in a color like aquamarine charged by blacklight. The deep black gill spot with the accent of brilliant red. The subtle pattern of blue-and-emerald vertical bars across the golden body. And then that bright yellow-gold belly.

Years ago (and I’ll admit I did this illegally, without a permit) I kept a couple of sunfish in an aquarium – they looked great amid the green artificial weeds, lit by the fluorescent lamp, and our toddler daughter loved watching them.

Fortunately for us all, sunfish are extremely common. They’re generally not as abundant as their bluegill cousins, but you can find them in almost any lake. They live in the shallower water and in the weed beds, eating mostly insects and their larval forms. They prefer water temperatures in the low to mid-70s. They live up to 10 years and can grow to 8 or 9 inches, though you won’t often see specimens that size around here.

Sunfish are easy to catch. They’ll take almost any live bait – worms, grubs, crickets, small leeches – but also artificials like dry flies, poppers and small spinners. When fishing with kids, nothing will bring more cries of delight than one of these jewels, popped from the lake, multiple colors glistening in an evening sun.

Why can you see better into water with polarized glasses?

Many of us know we can discern more features in shallow lake water when wearing polarized sunglasses. It’s easier to see fish we want to catch, observe bottom features, or discover treasures like lost fishing lures with those glasses on. But how exactly do polarized lenses work?

They do it by filtering out reflected light. Light consists of waves that are oriented in all directions. Light waves that reflect off water (or any surface) are oriented horizontally – think of a flat sheet of paper, parallel to the water, coming toward you edgewise. That light is said to be polarized.

Now, the lenses of polarized sunglasses are specially treated so as to form filter that acts like a vertical picket fence (except that the spaces between the fence “slats” are extremely small). Imagine you’re holding a long rope that’s tied off against a tree. Between you and the tree is a picket fence, and the rope passes between two of the fence slats.

If you were to move your end of the rope rapidly up and down, you would create a wave in the rope, and that wave would pass right between the vertical fence slats and reach the tree. Now imagine moving the rope rapidly side-to-side. The narrow space between the slats would block the formation of a horizontal wave, which would never reach the tree.

That’s what polarized sunglass lenses do to reflected light. Light that would otherwise impede your ability to see into the water is filtered out before it hits your eyes. Light waves with a vertical orientation are allowed to pass through, revealing all those secrets from below the surface.

So, when on your lake exploring, wear your polarized glasses. You’ll get to know a little more about your lake. Not as much as if you looked below the surface through a snorkeling mask – but that’s a subject for another time.

Can walleyes make a lake clearer?

We all like our lakes to be clear, and most of us prize walleyes as a sport fish and table fare. But is there a connection between the two? Between walleyes and water clarity?

I learned at this year’s Wisconsin Lakes Partnership Convention that there can be. Scott Van Egeren, a water resources management specialist with the state Department of Natural Resources, gave a talk on the food chain in typical lakes such as we have in Wisconsin.

To oversimplify matters a bit for brevity, the food chain starts with one-celled algae (plant plankton, or phytoplankton), which are eaten by animal plankton (zooplankton), which include small crustaceans like Daphnia (water fleas). Fish such as cisco (planktivores) eat the algae eaters, and predatory fish, including walleyes, eat the cisco.

Now, what has that to do with water clarity? Well, in general, the lower the level of algae in a lake, the clearer the water. And in general, the more algae-eating water fleas are present, the less algae there will be. But what happens if cisco (and other smaller fish) are abundant and are gobbling up the water fleas? That means fewer algae-eaters, more algae, and cloudier water.

And here is where walleyes come in. Walleyes graze on cisco. If the walleyes are abundant, they can thin out the cisco and other smaller fish considerably. That means the water fleas and other algae-eaters have a chance to thrive, and the algae population goes down. And the water is clearer.

Now, of course, water clarity has to do with much more than just the walleye population. An important factor is the level of nutrients – which cause algae to thrive and can lead to nuisance blooms. Other factors include lake depth (deeper water has more capacity to absorb nutrients), surrounding land uses (which can contribute sediment in runoff), and wind and wave action, (which especially in shallow lakes can stir up sediments from the bottom).

But it’s interesting to think that a healthy population of walleyes can have a benefit beyond providing mornings and evenings of excellent fishing.