Closing Time

I hope you were among the fortunate souls who spent last weekend at their lake homes or cabins. I met several such folks as I took a solo paddle, my last of the season, around the shoreline of Birch Lake, at Harshaw.

This was a prototype October Saturday afternoon, clear sky, temperature mid-50s, the softest of breezes, the lake’s surface smooth, oaks and birches still holding their colored leaves, the air scented like (to borrow a phrase from Garrison Keillor) fine brandy.

When traveling alone in our red Kevlar Old Town, I always assume the bow seat and paddle stern first; sitting farther amidships keeps the canoe flat instead of nose-up in the water. At this season there’s something appropriate about paddling “backwards”: The trip is more about looking back than forward.

You tend to think, as autumn closes down, on what was instead of what will be. My annual spring canoe reconnaissances are about watching for life in the shallows, spotting painted turtles released from hibernation, following smallmouth bass across the reef on the lake’s east end, spying on walleyes hunkered deep in sunken tangles of brush.

On this mid-October ride, there was of course little life to observe other than a somewhat heavier-than-usual clouding of green algae. The fish had gone deep. Several small ducks in a cluster skittered away and up well before I could get close enough for an identification.

I did encounter several lake neighbors enjoying the day in various ways: one man disassembling a pier, ratchet wrench periodically rasping; another enjoying a drink while seated atop a short stairway of timbers; a woman at the end of a pier with a small black dog that barked at me sharply; a man and wife prepping a pontoon boat for storage, two fishermen in boats working rocky points, presumably for muskies.

From here on there would be few days like this. It’s hard at such times not to regret the decline of the seasons and to long, far prematurely, for spring. It’s too soon to embrace the idea of November’s bleakness and then the long winter. So, while taking in the glory of the day, we tend to scan back over the good times of spring and summer past.

As I pulled the Old Town from the lake and tipped it over on shore, for the last time until next year, the couple from three lots down paddled by in their canoe, just two more lake country folks lucky enough to enjoy this day, around or on the water.

This turnover isn’t for dessert

Right now many Northwoods lakes are going through (or soon will) something called the fall turnover. It’s a phenomenon as beneficial as it is interesting. 

Fall turnover is a restorative process, a bit like opening doors and window in a long-sealed, musty basement and letting lots of clean, fresh air course through.

A previous column in this space told how lakes stratify (form layers) in summer – warmer, lighter water above and colder, denser water below. At the height of the warm season, these layers don’t mix very much because the difference in density between surface water (at, say, 80 degrees F) and deep water (at, say, 40 or 45 degrees) is considerable.

So as the summer wears on, all kinds of materials sink from the surface water into that cold bottom layer. Plant parts, algae, fish carcasses, dead insects and more drift down and decompose, consuming oxygen. As a result, the oxygen down there can become quite depleted.

What would happen if your lake remained stratified all the time? Those deep waters would become largely lifeless, hospitable mainly to organisms that thrive in anaerobic (without oxygen) conditions.

But fortunately, along comes the fall turnover, generally sometime in late September or early October (likely on the early side this year because of all the chilly weather). In simple terms, what happens is that the surface water gradually cools, and the difference in density between the surface and deeper water decreases, so that eventually wind and wave action can mix the layers together. And that means the lake, from surface to bottom, becomes infused with oxygen.

This is great for all manner of lake creatures – especially fish that dwell in the depths – that need oxygen to make it through the winter.

How can you tell if your lake has turned over? Well, for one thing, the water suddenly becomes cloudier than usual because the mixing action brings up nutrients and debris from the bottom. You might even notice a hint of sulfur scent (like rotten eggs) as decomposing material comes to the surface. When the turnover is complete, the water becomes clear again, likely more so than in high summer.

Some anglers say fishing is tougher during the turnover because with oxygen available everywhere, the fish are more scattered.

Different lakes experience fall turnover in different ways. Deeper lakes take longer to turn over. Shallow lakes may not turn over at all because they never actually stratify in the first place – wave action keeps them well mixed all through summer. The turnover itself can play out in a few days in some lakes, or during a week or more in others.

So watch for signs of turnover in your lake. It’s another seasonal milestone, like ice-in and ice-out, that can be fun to track over the years.

The bounty of the benthos

Leaving an airport, you see signs that say Ground Transportation. After flying at 36,000 feet and a few hundred miles an hour, that travel mode seems quite unglamorous.

So it is with life on at the bottom of a lake, which the limnologists (freshwater biologists) call the benthos. Up above in the water column the fish are like the aircraft and birds of our dry-land world. Creatures less appreciated live on (an in) the “ground” below.

It’s appropriate at this season to think about the benthos, because that’s where a lot of lake life is heading as the water gets cold and winter comes on. The term “benthos” comes from a Greek word, “bathys,” which means “deep.” It’s a zone much richer in life than most of us appreciate.

Of course, crayfish live on the bottom, as do clams, mussels and snails. Aquatic insects like mayflies and damselflies also live on the bottom, or buried in sediment, at stages of their metamorphosis from egg, to nymph, to winged adult.

These creatures are important links in the lake food chain. They eat algae or sunken plant matter and in turn provide food for fish (as anyone who has ever caught bluegills with nymphs or perch with wigglers can attest). An assortment of worms can also be found in upper layers of bottom sand and muck.

Leopard frogs and bullfrogs become benthos dwellers in winter. They do not (as many believe) dig into the bottom – the sediment contains too little oxygen to get them through until spring. Instead, they lie on the bottom, or only partly bury themselves. Some may even swim around slowly from time to time.

Painted and snapping turtles, on the other hand, do burrow into soft lake bottom mud and hibernate. In that state, they need very little oxygen and can absorb it through exposed mucous membranes in the mouth and throat.

An important function of the small benthic creatures (the worms and inserts) is that they allow scientists to assess water quality in a lake (or stream). A researcher can take “grab samples” of the bottom sediment, sort out and identify the organisms it contains, and get a good idea how healthy the lake is.

One measure they use is species diversity. In general, the more different creatures they find, the better the water quality. Another criterion is pollution tolerance. If a bottom sample is rich in immature forms of mayflies and stoneflies, which are sensitive to pollutants, that indicates good water quality. But if only midges and worms are present, that signals polluted water.

So while we get ready to “hibernate” for the winter, it’s good to think about the importance of all those creatures spending the cold season on and under the benthic blanket.

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.