Deciphering Plants Under Ice
November 29, 2017
Well before digital lake mapping and Google Earth unveiled the structural secrets of so many water bodies, the winter mission of several friends and I was the exploration of lakes small enough where we could cover the entire basin and sniff out the most expansive underwater forests within a half day's fishing.
Alternately drilling and looking beneath the ice with an underwater camera is the most logical way to learn a new lake's secrets. But GPS, modern mapping, and satellite imagery now reveal so many high-end possibilities for fish contact that it's now possible to cut a single hole on a waypoint, and find yourself face-to-face with an impressive assembly of fish.
One of the coolest discoveries I made a few winters back initially revealed itself on a satellite image — a visible trench or perhaps an old creek channel snaking its way through a massive field of submersed vegetation. The image shows the trench clear as day, and with Google Earth, you simply place the cursor over any segment of the map image to reveal its waypoint coordinates. We fished a particular section of this long trench starting two winters ago and have caught crappies and big perch here on every wintertime visit.
It reminds me of the small clear spots amid fields of pondweed (cabbage) fished during several North American Ice Fishing Championships, where 1- and 2-pound crappies anchored tournament winning limits. In the case of the NAIFC events, the winning anglers ran side-imaging sonar before ice-up to reveal small openings within these expansive vegetated flats.
This reminds me of something Dave Genz has said about the winter tendencies of sunfish and crappies. "The main wintertime mission of panfish is to hide from pike and other predators." He's right, and that brings up two critical factors in terms of aquatic vegetation: edges and patchiness.
A Patchwork of Plants
Something longtime Minnesota Department of Natural Resources Research Scientist Paul Radomski told me will, I suspect, always be relevant and perhaps overlooked relative to vegetation: "A patchy field of vegetation composed of many different plant species creates a lot of edges, nooks, and tunnels panfish need to both hide from predators and pursue prey. Multiple fish age-classes and species depend on a diversity of plant habitats for feeding, spawning, and refuge."
The concept of patchiness remains significant today, and increasingly so, as invasive plant species overgrow natives and decrease species diversity. AnglersÂ should care because a diversity of plants brings a balanced environment in which each species offers something unique from which other species of fish or invertebrates can benefit. Native plant species coexist best with other natives, while invasives such as Eurasian watermilfoil overgrow native plants and reduce the patchiness of littoral zones. Ultimately, fish and invertebrate diversity suffers, too, making for a dull fishery.
"Plant diversity is often highest in heterogeneous lakes," Radomski says. "These are generally larger or deeper waters with various types of bottom substrate, and more convoluted depth variations. Oppositely, homogenous lakes, which are often more turbid, host greater phytoplankton (algae) densities relative to rooted macrophytes. Ideally, you like to see an abundance of submersed plant species, too, because they increase the probability that at least one will be winter hardy."
Radomski's point about winter-hardy plants points to another key element affecting under-ice fish location. Fish seek vegetation for at least three main reasons: pursuing prey and evading predators, oxygen production, and perhaps to absorb the sun-warmed waters surrounding vegetation.
Light, Oxygen, and Other Monkey Wrenches
In terms of under-ice oxygen, sun penetration is everything. Sunlight feeds rooted plants where ice is clear and thin and snow-cover is minimal. Even nominal light below the ice can provide enough energy to fuel plant photosynthesis. One winter I viewed an expanse of shallow water lying beneath an area that had been plowed for a vehicle turn-around. Nearby, ice shelters sat just off the deep edge. Surprisingly, vegetation beneath the cleared ice looked upright and healthy, with scads of sunfish, pike, and bass on patrol. Meanwhile, in every other shallow area of this small lake, beneath over a foot of snow, an Aqua-Vu camera revealed nothing but decaying vegetation, and very few fish.
Typically, although sunlight can induce photosynthesis and oxygen production in winter-hardy plants, at least intermittently, dissolved oxygen (D.O.) levels in most shallow zones continue to decline throughout winter due to decomposition, which uses up oxygen. In mild winters — or in the extreme example above — oxygen replacement via plant photosynthesis can be enough to sustain life in littoral zones all winter. (We don't recommend plowing snow to create fishing hot spots, however.)
Opposite conditions, such as turbid water, and thick, cloudy ice and snow deeper than a foot greatly limit light and thereby plant-produced oxygen. This means increased plant decomposition and plummeting D.O. concentrations. In these lakes, entire fish populations may eventually congregate in basin areas 20 to 50 feet deep. As winter progresses, the fish may rise higher in the water column until at late ice they literally linger just below the frozen surface.
Many of us have experienced winterkill situations, and at first we think we've found a fishing mecca, with giant schools of all species hovering just beneath our boots. Problem is, the fish are typically highly stressed, uninterested in biting, and taking up the last available oxygen.
The big monkey wrench in the oxygen equation is that anglers have no easy way to measure D.O. If, for instance, we were able to identify a beautiful vegetation bed bubbling with 6 parts per million of oxygen in late January — surrounded by anoxic conditions elsewhere — the spot would house fish.
The Need to Feed
Many of us have long suspected fish such as perch, crappies, and sunfish make short sojourns into low-oxygen water to take advantage of specific high-energy food sources. What it tells us is that ample oxygen on a certain structure is no guarantee of fish presence. We know, for example, that many fish species glean plentiful protein by binging on chironomid larvae living in deep anoxic (less than 0.5 ppm D.O.) soft-bottom regions. Bloodworms actually derive their name from the red hemoglobin in their bodies, the mechanism allowing them to live in anoxic lake zones. Ice anglers frequently observe the reddish-hued chironomid larvae burped up by crappies, perch, and sunfish, while fishing in deep basins.
In shallower waters, we've also seen panfish make periodic use of decaying patches of native northern watermilfoil and coontail, plant types that often attract swarms of decomposer invertebrates. Decaying vegetation is an important food source for backswimmers, water boatmen, and crayfish. As often as we catch big 'gills packed with crunchy, beetle-like critters feeding on dead plants, it's surprising that many anglers still exclusively adhere to the ostensible "green weed" connection.
The reality is not all living plants appear green under water. Several appealing species of pondweed, for example, display brown or red live foliage. Too, plant stalks often stay standing, alive or dead, and continue providing some measure of fish cover. Finally, in deeper or spring-fed waters, ample oxygen can remain even within flats of vegetation all winter, throwing anglers yet another curveball.
The point of these non-conformist panfish/forage scenarios is that hunger is perhaps the most powerful motivator of all. Even if it means holding your breath for a while — or chancing an encounter with a predator — the need to feed eventually wins out.
Beyond that low-oxygen regions can still host short flurries of feeding activity, pods of panfish must also continuously contend with predators. Viewing a solitary pike with a camera might not mean a thing. On the other hand, finding or catching multiple Esox in an area is a solid clue, greatly elevating your odds of contacting 'gills or crappies there.
This brings us back to the patchy concept. The more different plant species fish encounter on a spot, the more edges, nooks, and crannies they can utilize for foraging and concealment from predators. Plant species diversity also indicates invertebrate diversity, potential baitfish use, and ultimately, a wider selection of available forage. Oppositely, spots that cultivate but a single plant type greatly reduce patchiness and edges and make it unlikely panfish will find something good to eat there.
The one qualifier accompanying a diverse mix of vegetation is that such patchy, edge-rich landscapes can make panfish trickier to find. When pike or big largemouths share the bed, panfish, bluegills, and crappies often hunker down into the densest, bushiest plants, including nearly impenetrable species like coontail, northern watermilfoil, or wild celery.
Alternatively, panfish often band together in small pods above vegetation, where predators constantly hound them on and off the shallow edge. For us, this means short windows of angling opportunity. Catching a single sunfish immediately draws a bully bass or pike, sending panfish off into the abyss. Sometimes, the best course of action is to attempt to catch and then relocate a menacing pike back into the water a good distance from the original spot.
Some of the biggest bluegills and crappies I've seen, however, opt to hunker down into nearly impenetrable shallow cover — sometimes no more than 3 feet deep, even in midwinter. Often, the only way anglers can find these concealed panfish is to penetrate the cover with a compact, underwater camera, weighted with a clip-on lead depthfinder. Orienting the optics in an upward angle — up-viewing — is an approach devised by several top ranking NAIFC anglers I've mentioned in previous articles on the subject. In dark, dense vegetation, up-viewing reveals fish by silhouetting them against the light underside of the ice. Similarly, catching these hidden fish necessitates compact, heavy tungsten jig-and-plastic combos, which penetrate cover without hanging up on leaves.
On the ice, deciphering live vegetation from decaying plants often helps you identify individual species. What you eventually discover is that some plants wither in summer or fall while others grow, thrive, and produce oxygen under ice. Underwater study also reveals clues such as which plants most appeal to sunfish versus crappies or other fish; which plants tend to coexist on the same structure; and the highest percentage combination of elements that attract fish, time after time. Here's a glance at some of the more common plants under ice.
Chara (muskgrass): Actually a sediment-stabilizing algae, chara often grows in deeper waters up to 20 to 25 feet, with the plants ranging from ankle- to knee-height. Easily identified by its calcium-encrusted branches, chara exudes a skunk-like odor when brought to the surface. In lakes with excellent water clarity, chara hosts ample invertebrates and can provide excellent winter habitat for grazing perch, walleyes, and smallmouth bass.
Clasping-leaf pondweed: Alongside large-leaf pondweed, clasping-leaf pondweed is one of the more ubiquitous "cabbage" species in the ice belt, rooting and growing toward the surface in up to about 13 feet of water. Commonly confused with invasive curly-leaf pondweed, native clasping-leaf lacks the serrations or teeth seen on the edges of curly-leaf plant leaves. Leaves are wide and wavy with a broad base that "clasps" the stem, and are often colonized by macroinvertebrates. Like other common pondweeds, clasping-leaf can overwinter in clear-water environs with minimal ice cover, providing cover and foraging opportunities for many fish species.
Coontail: This common native macrophyte tolerates cold water and low light levels, allowing it to frequently overwinter as an evergreen. Live winter coontail has a bushy appearance, and unlike most plants isn't always rooted to the bottom, allowing plant masses to drift between different depths, typically 10 to 20 feet of water.
Providing nearly optimal winter habitat for bluegills and occasionally crappies, coontail hosts abundant invertebrate life, including mayfly, caddisfly, and midge larvae. When you find scuds (a small shrimp-like crustacean) swarming coontail plants, there's a great chance you have trophy specimens of available fish species — rainbow or brown trout, impressive sunfish or perch — which all grow large fast on a scud diet. Commonly associated with pondweed, finding the two species in proximity provides one of the most fish-attractive plant combinations in any lake.
Curly-leaf pondweed (crispus): This problematic Eurasian exotic outcompetes native plants by sprouting leaves in late fall, growing slowly under the ice and exploding to the surface shortly after ice-out, crowding out native plants. Found primarily in softer sediments and in lakes with high fertility, curly-leaf pondweed sometimes attracts groups of large sunfish and crappies. When curly-leaf dies back in summer, previously hidden juvenile bluegills expose themselves to predation, reducing their numbers. The plant's early-spring propensity to overgrow shallow areas can limit bluegill reproduction. In late winter, just before ice-out, new growths of curly-leaf can attract shallow panfish.
Elodea (common waterweed): Elodea's closed compact structure isn't ideal as fish habitat, but may attract fish as part of a patchy landscape and can be beneficial as a winter oxygen producer. Plant stems sprout waxy green leaves that crowd together. Typically, it grows in softer sediments enriched with organic matter. Some of these areas lie in shallow water, though the plant can grow to depths beyond 20 feet. Like coontail, elodea tolerates low-light conditions, often allowing it to photosynthesize through winter. Fish often favor because it produces considerable oxygen. Elodea is also associated with midge larvae colonies, a key winter food for many fish.
Eurasian watermilfoil: Notorious for its ability to spread rapidly, exotic watermilfoil can spread when a single stem fragment breaks loose and re-establishes roots. Eurasian watermilfoil has difficulty becoming established in lakes with healthy populations of native plants — but moves in rapidly when mechanical or chemical clearing of vegetation occurs.
Eurasian milfoil rarely overwinters, but quickly outgrows native plants beginning in early spring. The best way to differentiate it from native watermilfoil species, including northern watermilfoil, is that the Eurasian variety has at least 12 to 14 pairs of leaf divisions (leaflets). Northern watermilfoil typically has just 5 to 9 pairs.
Northern watermilfoil: One of seven native watermilfoil species in the ice belt, this plant creates excellent fish cover, but rarely overgrows at the surface like exotic Eurasian watermilfoil. Northern milfoil commonly roots in soft sediment in relatively clear-water lakes, growing from the shallows to as deep as 20 feet. This plant's presence usually indicates excellent water quality.
Resembling coontail in its bushy appearance, northern watermilfoil provides terrific fish and invertebrate habitat. Although it generally decays in winter, it can continue attracting fish, as its feathery foliage traps silt and detritus that attracts small aquatic insects. It's also a favorite food of crayfish.
Large-leaf & fern pondweed: Plants anglers call "cabbage" are often one of these two common macrophytes. Their broad leaves furl away from lanky stems, providing cover for many fish and habitat and food for invertebrates. Under ice, they remain alive only where light penetrates clear water. Live pondweed remains upright and intact, brownish leaves absorbing sunrays and generating oxygen. These plants commonly intermix with coontail. Both living and decaying pondweed can attract ample invertebrate life.
*In-Fisherman Field Editor Cory Schmidt, Brainerd, Minnesota, is an avid multispecies angler, with a keen interest in interactions of fish and their environments.