Lake trout

Lake trout are pursued in deep water for most of the open-water season, and that produces a handling problem with two independent halves.

THE PRESSURE HALF. A char taken from thirty metres or more can suffer the same gas expansion that afflicts marine deep-water species: distended bladder, buoyancy the fish cannot overcome, and an injury that is invisible until the fish will not go down. Deep-caught lake trout have measurably poorer release survival than shallow-caught ones, and the difference is depth rather than fight length. Where a fishery is release-oriented and the fish are deep, a descending device is as relevant on a lake as it is on a reef.

THE TEMPERATURE HALF. These are cold-water fish, and in summer the water they are taken from may be fifteen degrees colder than the water at the surface and colder still than the air. A fish brought up quickly through a thermocline and then held in warm air is subjected to a thermal shock on top of the pressure one. Air exposure matters more here than for most freshwater species.

WHAT FOLLOWS PRACTICALLY. Decide before the fish is hooked. If you are fishing deep and intend to release, use a descender and keep the fish in the water. If you are keeping fish, keep the deep ones and release the shallow ones rather than the other way round, which is the opposite of what size-selection instinct suggests.

AND HANDLE THEM AS THE SLOW-GROWING FISH THEY ARE. Lake trout are long-lived and late-maturing, and large individuals in northern lakes may be several decades old. The species does not restock itself quickly from a good year, and a population fished as though it does declines quietly.

Lake trout occupy a habitat defined by two requirements that are in direct conflict in most lakes, and that conflict explains where they are and where they are disappearing from.

THEY NEED COLD WATER AND THEY NEED OXYGEN IN IT. Lake trout are the coldest-water char of the continent, requiring temperatures well below what most freshwater fish prefer, together with high dissolved oxygen. In a stratified lake in summer, cold water is in the hypolimnion below the thermocline — but the hypolimnion is also where oxygen is consumed and not replaced. So lake trout summer habitat is the band that is both cold enough and oxygenated enough, and in many lakes that band is narrow.

WHICH RESTRICTS THEM TO LARGE, DEEP, CLEAN, NUTRIENT-POOR LAKES. Big, deep, oligotrophic waters have enough volume of cold oxygenated water to sustain them: the Great Lakes, the great Canadian shield lakes, deep glacial lakes across the northern range. Shallow, warm or productive lakes do not, however far north they are, because a productive lake consumes its deep oxygen.

AND IT MAKES NUTRIENT ENRICHMENT LETHAL WITHOUT ANY DIRECT POLLUTION. Adding nutrients to a lake increases production, which increases the organic matter settling into the hypolimnion, which consumes the oxygen down there. The surface looks fine; the lake trout habitat has been removed from below. This is the classic mechanism by which lake trout are lost from lakes that still look pristine.

SEASON MOVES THEM DRAMATICALLY. In cold water — early spring, late autumn, under ice — lake trout use the whole lake including the shallows, and are catchable near the surface. As the lake stratifies they retreat to depth and stay there. The same fish is a shallow-water fish for two months and a deep-water fish for four, which is why lake trout fishing is so strongly seasonal in method as well as in timing.

SPAWNING IS ON CLEAN ROCKY SHOALS IN AUTUMN, in relatively shallow water, with eggs dropped into the spaces between cobble and boulder. Those shoals must be clean — sedimented or algae-covered rubble does not work — and shoal degradation is a recognised limit on natural reproduction in several major lakes.

IN THE GREAT LAKES SPECIFICALLY the habitat story is inseparable from a century of sea lamprey, overfishing and invasive-species change, and natural reproduction there has had to be rebuilt on the same physical shoals that were always there.

Lake trout reproduction is unusual among salmonids in almost every respect, and every one of those differences makes the species less able to recover from anything.

THEY DO NOT BUILD REDDS. Unlike stream salmonids, lake trout do not excavate a nest. Eggs are broadcast over clean cobble and boulder shoals in autumn, at night, and fall into the interstices between the rocks where they overwinter. The rock does the protecting; the fish does nothing after spawning.

WHICH MAKES SHOAL CONDITION THE ENTIRE REQUIREMENT. The substrate must be clean, with open spaces between stones, and free of accumulated silt, algal growth and organic sludge. Sedimented shoals lose their interstitial space, the eggs sit exposed or suffocate, and reproduction fails on ground that still looks like a rocky shoal.

DEVELOPMENT TAKES THE WHOLE WINTER, in cold water, which is the longest incubation of the freshwater fish in this catalogue and a very long exposure to anything going wrong on the shoal.

MATURITY IS LATE AND LONGEVITY IS EXTREME. Lake trout take many years to mature, spawn in some populations only every second or third year rather than annually, and live a very long time. Put those together and the reproductive output per fish per year is tiny by comparison with almost anything else in fresh water.

WHICH IS WHY THE SPECIES IS SO EASILY LOST AND SO SLOWLY REGAINED. A population fished down, or hit by an introduced predator or parasite, rebuilds over decades rather than years, and several major lake systems have required sustained stocking, sea lamprey control and long closures before natural reproduction resumed on the same physical shoals that were always there.

SITE FIDELITY IS STRONG. Lake trout return to particular shoals, and knowledge of those sites is inherited within a population rather than being general to the species — so extirpating a local population can effectively remove use of a shoal even when suitable habitat and other lake trout remain.

AND THE MANAGEMENT IS CORRESPONDINGLY CAUTIOUS AND VARIABLE, with closed seasons, slot limits, quotas and lake-specific arrangements common across the range. Nothing above states what any of them currently are.

Lake trout methods divide absolutely on whether the lake is stratified, and the two halves have almost nothing in common.

WHEN THE WHOLE LAKE IS COLD, THE FISH ARE ACCESSIBLE. In the weeks after ice-out and again in late autumn, lake trout use the entire water column including the shallows, and they can be taken by casting from shore, by shallow trolling and by fly fishing. This is a short window and it is when the species is available to anglers without specialised equipment.

WHEN THE LAKE STRATIFIES, EVERYTHING MOVES DEEP. The record's habitat section explains the squeeze: lake trout need water that is both cold enough and oxygenated enough, and in summer that band can be narrow and a long way down. Reaching it requires downriggers, leadcore, wire line or heavy jigging, and the fishery becomes a matter of finding a depth before finding a place.

VERTICAL JIGGING OVER STRUCTURE IS THE PRECISION APPROACH — locating fish on structure at depth and working a heavy jig or spoon to them. It is efficient and it is entirely dependent on finding fish first.

TROLLING SOLVES THE SEARCH AT SCALE on big water, covering ground at a controlled depth along contours, and it is why the discipline is so equipment-intensive on the great lakes of North America.

ICE FISHING IS A MAJOR AND DISTINCT DISCIPLINE in the northern range, and it works for the same reason the spring fishery does: under ice the whole water column is cold, so the fish are not confined to a deep band and can be reached through a hole.

DEPTH MAKES RELEASE DIFFICULT AND THAT SHAPES THE ETHICS. A lake trout brought quickly from substantial depth suffers barotrauma, and the record's spawning section explains why this species recovers from losses more slowly than almost any freshwater fish — late maturity, intermittent spawning, extreme longevity. Anglers intending to release should be thinking about the depth they fish before the method.

AND SEASONS, SLOT LIMITS AND LAKE-SPECIFIC ARRANGEMENTS ARE COMMON AND VARIABLE, set by the competent authority for each water.

Lake trout seasonality is the sharpest access cycle of any freshwater fish in this catalogue, and it is entirely a consequence of thermal stratification.

WHEN THE LAKE IS UNIFORMLY COLD, THE FISH ARE EVERYWHERE. After ice-out in spring and again in late autumn, the water column is close to uniform in temperature, and lake trout use the whole lake including the shallows and the surface. This is when they are accessible from shore, on light tackle and near the top — a genuinely different fish from the one that exists in July.

AS THE LAKE STRATIFIES THEY RETREAT TO A NARROW BAND. The record's habitat section sets out the squeeze: cold enough and oxygenated enough coincide only in part of the hypolimnion, and in many lakes that band is thin and deep. Through midsummer the fish are confined to it and everything shallower is empty.

WHICH MAKES THE SEASON A MATTER OF WHERE RATHER THAN WHETHER, and it explains why lake trout fishing changes method completely between May and July rather than merely becoming harder.

AUTUMN BRINGS THE SPAWNING MOVE TO SHALLOW ROCKY SHOALS, which concentrates fish in shallow water again and is when they are most vulnerable. Many jurisdictions close or restrict the fishery around it; whether any does is a matter for the competent authority.

WINTER UNDER ICE RETURNS THEM TO THE WHOLE WATER COLUMN, which is why the ice fishery works: the thermal barrier is gone, and fish can be reached through a hole at ordinary depths.

THE PATTERN INVERTS THE INTUITION FOR MOST ANGLERS. For a warm-water fish, summer is the accessible season; for lake trout, summer is the closed door and the cold months are the open one. Anybody carrying temperate coarse-fishing assumptions to a lake trout water will fish it backwards.

AND THE CLIMATE IMPLICATION IS DIRECT. A warming lake has a thinner and deeper habitable band for longer each year, which extends the inaccessible period and, more seriously, squeezes the habitat itself.