How DO they breathe down there?

As a kid I once asked a wiseacre friend what he’d done in swimming class that day. He said, “We learned to breathe underwater.” For a long moment I actually believed him; I imagined puckering my lips down to a tiny pinhole opening and somehow sucking the air from the water.

That’s crazy, of course. But then how do fish (and for that matter other underwater creatures) manage to breathe? The short and easy answer is: Fish have gills. A closer look reveals just how remarkable this ability to breathe underwater is.

Now, up here above the water’s surface, the air we breathe contains about 21 percent (210,000 parts per million) of oxygen, which of course is the gas on which we depend. Water contains oxygen in solution, but at nowhere near a comparable percentage. Even very high-quality water, such as in a trout stream, contains no more than about 8 parts per million. That’s 0.0008 percent.

Now, even in the oxygen-rich environment in which we live, it’s pretty remarkable that our lungs can draw in enough oxygen to keep every cell of every bone, muscle and organ in our bodies functioning. For a fish to get enough oxygen out of such a scant supply borders on the miraculous. The fact there is a scientific explanation doesn’t make it any less interesting or less wondrous.

Gills actually work on the same basic principle as our lungs: Exposing a huge number of tiny blood-carrying vessels (capillaries) to the oxygen source, so that the oxygen can migrate in (and carbon dioxide can migrate out). Our lungs contain millions of tiny sacs called alveoli, extremely rich in capillaries, where the exchange of gases takes place.

Fishes’ gills, on the other hand, have a structure of rows and columns of specialized cells, called the epithelium, that can absorb the much smaller concentrations of oxygen found in water. In general shape and form, gills look like a car radiator. Most fish have four gills on each side. There’s a main bar-like structure with multiple branches, like a tree, that gradually branch down smaller and smaller, an arrangement that exposes an enormous (relatively speaking) surface area to the water.

Fish pull in water by lowering the floor of the mouth and widening the outer skin flap  (operculum) that protects the gills. The fish then raises the floor of the mouth, and a fold of skin forms a valve blocks the water from rushing out. This increases the pressure inside the mouth, forcing water out and across the gills. The blood exposed at the gill surfaces contains less oxygen than the water, and so the oxygen migrates from the water into the blood. From there it is pumped around the fish’s body.

One advantage fish have is that, being cold-blooded, they have lower metabolism than we do and need less oxygen relative to their size. If fish were warm-blooded and needed oxygen for energy to sustain their body temperature, they would not be able to survive on the amount of oxygen the gills can extract from the water.

There’s another little issue fish have to deal with: Maintaining the right amount of sodium in their bodies. Saltwater fish live in an environment that is saltier than their bodies, so the gills tend to absorb an excess of sodium, which needs to be expelled. In fresh water, it’s just the opposite: The fish must have a mechanism to keep from losing sodium through the gills into the less-salty water around them. These mechanisms are perhaps a subject for a different time.

Anyway, it’s easy to see just how full of it my wiseacre friend was when he talked to me about breathing underwater. If we want to do that, it is much easier to by scuba gear than to grow a set of gills.

Let's be clear - A snorkel is a great learning tool

For getting to know your lake, there's nothing quite like a mask, flippers and snorkel. I love slipping them on and exploring up and down the shoreline from our cabin on Birch Lake, near Minocqua, Wis. This year our lake is much clearer than in the past, for no reason I can discern. The days have been hot and the skies clear, so one would think conditions should be right for algae (and we do now and then experience a minor bloom here). Instead, the water is crystalline compared to what we're used to. When anchored on my favorite rock bar for evening walleye fishing, I can clearly see the anchor resting beside a boulder five feet down.

Therefore, while often I have taken my snorkel gear to other nearby lakes with clearer conditions, I have kicked my way around Birch in recent days. The neat thing about snorkeling is that the action of the flippers keeps you buoyant with little exertion. That means you can snorkel fairly long distances from shore without worrying that you may get too tired to make it back. (Some people prefer to wear a life vest when snorkeling over deep water, just in case.)

I always enjoy the new scenery I encounter when snorkeling, and I almost always learn something new about my lake. This year I've learned that our rusty crayfish population is bigger than I would have thought just from what I see around my pier. They are everywhere, and some of them are huge. It tells me I need to augment the the efforts of the Friends of Birch Lake and get a couple of my own traps to place along the edge of our reed bed. I also noted some patches of cabbage weeds down the shoreline from our place -- a good sign, after the cabbage beds were all but wiped out years ago when the rusty crayfish population exploded.

The best thing I learned, though, was the exact nature of the spot a short distance from my pier where a brother and I have caught numerous walleyes at times. About 50 yards down the shore and about 50 feet out from the reed bed lies a tangle of brush, probably placed there deliberately as a crib years ago. Hovering over it, I looked down on a couple of smallmouth bass in the upper branches and, down deeper, a dozen or more walleyes, finning in place. They seemed not to notice as I quietly passed over. I fished that brush pile the same evening and caught -- nothing. I suppose that says something about the fickle nature of walleyes, or about my skill as an angler.

After recent heavy rains, the water of Birch Lake remains clear. There are more areas of the lake to explore. You could certainly do worse than to spend some time getting to know your lake from a whole different perspective -- with flippers on your feet and your face behind a mask of glass.

How do you like these odds?

I wrote recently about seeing near my Birch Lake pier a school of hundreds of what I believed to be smallmouth bass fry. Looking at all these tiny fish and envisioning many more such schools around the lake’s perimeter, one could assume the lake will soon be chock full of smallmouths.

The reality is far different. The odds of survival for these fry are exceedingly long. One scientific study used DNA tracking to estimate the success of spawning smallmouth bass on a lake in Ontario. To make a long story short, the study found that only 27.7 percent of male bass that acquired eggs (it’s the male who guards the young after the eggs hatch) had at least one offspring survive to the fall young-of-the-year stage. Just 5.4 percent of all the spawning males produced 54.7 percent of the total number of the fall young-of-the-year, which range in size from 1 1/4 to 3 inches.

To look at it another way, consider that female smallmouth bass deposit anywhere from 2,000 to 10,000 eggs on a spring spawning bed. Even under the best conditions, most eggs don’t survive. They’re vulnerable to changes water temperature and oxygen levels, flooding or sedimentation, disease and predation (as from panfish and crayfish).

When the eggs hatch, the larval fish live off a yolk sac attached to their bodies. Once the yolk sac is fully absorbed, the young fish, called fry and about an inch long, rise from the bed and start eating on their own. For a time the male bass protects the school, but eventually he leaves and the fry scatter. They survive on tiny crustaceans until they are big enough to eat aquatic insects, then larger crustaceans and fry of other fish species that spawned later. As the fish grow, they face the same threats as the eggs – in addition to which all manner of predators feast on them.

When they’re small, they get attacked by bluegills, perch, pumpkinseeds and sunfish. As they grow, they become prey for walleyes, northern pike and muskies. Other enemies, again depending on the fishes’ size, include kingfishers, loons and herons, mink, frogs, and some snakes. The end result is that only a tiny fraction of the eggs laid in a spawning bed, and only a tiny fraction of the fry I see near my pier, ever become adult bass that I try (with limited success) to catch. Yes, nature can be a cruel mother. I am certainly glad the odds of survival for my new grandson, Tucker, are considerably better than for a newly hatched smallmouth bass.

They're not just minnows

Judging from what I saw while uncovering the boat on return to Birch Lake yesterday, some fish have pulled off a highly successful spawn. A large school of fry skittered off as I waded into the water to untie the boat canvas. They were about an inch long and, individually, looked like little more than slender shadows cast against the sandy bottom.

We're tempted to label any small fish we see, especially in schools, as minnows. In reality, minnows are a family of fishes defined not by size but by body characteristics. For example, carp that can grow to 50 pounds belong to the minnow family, as do the shiners, only a few inches long, that we use for bait. Members of the minnow family have one brief dorsal fin with nine or fewer soft rays. They have smooth-feeling, scales that may come off when the fish is handled. They do not have true spines in their fins. They have no teeth in the jaw but have rows of toothlike structures on the bony frame that supports the gill tissues: The teeth are actually in the throat and help grind the fishes' food. Most minnows are in fact small -- they reach a few inches to perhaps a foot long. 

So, what did I see in the shallows near my pier yesterday? My guess is that they were smallmouth bass, since those fish were on the spawning beds just two or three weeks ago. I am not aware that any other fish species have spawned since then. I wished I'd had a little dip net with which to scoop a few up and examine them. When I have done this, it amazes me how much even tiny fish fry resemble the adults they will become. There is no mistaking them. Smallmouth fry, for example, have the signature black-edged tails and red eyes. Largemouth bass have the black stripe down the side, perch the vertical black bars, northern pike the oval spots. And so it goes.

Have you seen fish fry (not minnows) in your lake? Try netting a few and taking a close (brief) look. It will allow you to see what's breeding successfully. Of course, success is a relative term -- hatched fry do not a large or stable population make. The odds of fry survival are long indeed -- a topic for another time.

About those midges

My last post mentioned that midges, besides mayflies, were hatching on Birch Lake recently. Midges of course are tiny white flies that when on the wing look like mobile bits of cottonwood fluff. Believe it or not, trout fishing enthusiasts (or should we say fanatics?) actually tie flies small enough to mimic these things.

While the mayflies at Birch were hatching a week or so ago, the midges were, too. One day hundreds of them clung to the screens of our lakeside porch; a tap on the screen sent them flying; in a few moments they were back. The next day only a few remained. I’ve been down on the lake when midges were thick, a swarm hovering around my head, and if I listened carefully I could hear a faint, collective buzzing.

These were non-biting midges, from the insect family Chironomidae, and often called chironomids. Some call them “blind mosquitoes”; others call them “fuzzy bills” because of the males’ bushy antennae. As with mayflies, if your lake has midges, that’s a sign the water quality is pretty good. Midges are an important link in the food chain in and around a lake. Fish and predatory water insects eat them, and the midge larvae help keep the water environment clean by eating organic debris.

Like mayflies, midges have interesting lifecycles. The adult flies lay gelatinous masses on eggs on the water surface, each holding as many as 3,000 eggs, which sink to the bottom and hatch in about a week. The larvae dig into the mud or, in some species (and there are many) build small tubes to live in. They feed on organic matter suspended in the water and mixed with the bottom mud. As they grow, they turn pink and eventually dark red, at which point they are known as bloodworms. The color comes from hemoglobin, the same compound that makes our blood red; it allows the larvae to “breathe” in the mud, which of course is low in oxygen.

After two to seven weeks (largely depending in water temperature) the larvae become pupae. About three days later, they swim to the surface, and adults emerge within several hours. The adults then mate; they live only three to five day and do not feed. In the heat of summer, midges may complete their lifecycle in as little as two or three weeks. Fall larvae do not pupate but instead remain in the larval stage until spring.

Have you seen midge hatches on your lake? Watch for them throughout the summer – several generations may hatch before the season turns to autumn.

Up from the Bottom

There are visitors to the screens of our lakefront porch this week. A number of mayflies and countless tiny midges cling to the screens and, for that matter, to the siding of the cabin. They’ve come up from the bottom of Birch Lake and up to our place, more than 75 feet back from the water and up a considerable hill.

One thing I know about mayfly hatches is that they don’t help the walleye fishing. But that’s not the main point of interest. Are mayflies hatching on your lake? Maybe you already know their story and where they come from, but in case you don’t, here it is.

Mayflies are insects that belong to the scientific order Ephemeroptera, a name no doubt chosen because these flies are ephemeral. The adults, with their triangular wings that propel them in silent flight in a posture that resembles a man with a jet backpack, live only two nights. They have no functioning mouthparts because they don’t eat – they have no need to. During their brief time in the air above a lake, male and female mayflies mate in swarms. The female deposits eggs as she flies low over the water or dips her abdomen. Some species (there are many) even submerge themselves and lay eggs underwater.

From eggs, mayflies develop into adults through several stages of molting. Different species have different molting stages, which also can vary with temperature and water conditions. The insects in immature stages swim to the surface or grab onto rocks or plants. There, according to the Texas A&M University Agri-LIFE extension website, they molt in minutes or second into winged subimagoes, which fly quickly from the water to nearby plants. There they molt again into adults (imagoes) that fly out over the water to begin the mating cycle anew.

Mayflies are the only group of insects that molt after they have wings, says the Texas A&M website. In all other insect orders, winged forms are found only as adults. A typical mayfly lifecycle lasts one year.

Immature mayfly stages have chewing mouthparts and feed by scavenging small pieces of organic matter, such as plant material or algae that accumulate on rocks or other surfaces. Mayflies require water relatively high in dissolved oxygen, which is why they thrive in fast-flowing trout streams. If your lake has mayflies, that’s one sign (certainly not the only indicator) that it’s in decent condition.  

So how did the mayflies affect the walleye fishing here on Birch Lake? Well, yesterday afternoon I caught two, one of them a 17-inch keeper that I brought back to join another already in the freezer as a fish fry for my wife and me. I put the fish in a landing net and set it on the pier while I moved my boat cover from the pier onto shore. Before I made it back to the net, the walleye flopped free and into the water. There went dinner.

Fishing was deadly slow last evening. Could be the mayflies, could be the east wind that brewed up, could be the Birch Lake gods working their will. Could be plain old angler ineptitude. There are too many variables on which to base a conclusion.

Bass on the Beds

Birds nesting, newborn fawns trying their legs, toads and frogs singing in the swamps.  These are all spring magic, but none greater than fish spawning in the shallows of the lakes. Older brother Steve visited our Birch Lake cabin two weekends ago, and while fishing we spotted smallmouth bass tending round gravelly beds they had cleared of sand and silt so as to lay and hatch their eggs.

Now, anglers sometimes debate whether it’s OK to fish for bass while they’re bedding. I can buy the argument that it’s not altogether sporting. Drop almost anything onto a bed and the bass guarding it is compelled by instinct to pick it up and move it away. So an accurate cast to a bed is almost certain to trigger a strike. And it is in fact a bit too easy.

As to whether fishing for bedded bass harms the population, most of what I read says it doesn’t. If you release the bass where you catch them, they’ll go right back to the nests. And in my lake there are essentially no bluegills or other interlopers to eat the eggs for the few minutes when the resident bass are missing.

So Steve and I prowled along the shoreline looking for light-colored circles in the rocky, gravelly areas, finding them often next to sunken logs. Since I am at the lake much of the time and Steve is a once-or-twice-a-year visitor, I mostly just steered the boat, not fishing the beds myself, satsified just to aid and abet and show my big brother a good time.

Birch Lake holds trophy smallmouths, and Steve caught at least one measuring more than 20 inches – 20 and three-quarters to be precise, just as long as the biggest one I’ve ever caught there. He got one or two that might have crossed the 20-inch threshold but that we didn’t bother to measure. If we had been into the taxidermy thing, Steve would have had himself a trophy for the wall, but we both prefer just to let smallmouths go, in some cases after snapping a picture.

We didn’t overdo it – we stopped after had Steve caught half a dozen bass – all lip-hooked and released unharmed – and had enjoyed one of the better fishing times of his life. By the time I get back to Birch again this weekend, I suspect the spawn will be over, the beds empty, the smallmouths in a resting phase, so that I must focus on walleyes. And that’s fine. If some bass are still on their beds, I may just let them, content to observe. I'm just glad I got to witness the spring ritual of smallmouth bass creating the next generation of their kind.