Tench

Tench are among the most sharply seasonal fish in European fresh water, and the reason is a metabolic one that shows up more starkly in them than in the species they share a lake with.

A FISH'S METABOLIC RATE IS SET BY THE WATER. Being ectothermic, a tench's whole physiology — digestion, activity, the rate at which it can process food — scales with temperature. What varies between species is how steeply. Roach remain functional in cold water and go on feeding through it; tench do not. Below roughly ten degrees a tench's digestive capacity falls away to the point where feeding stops being worth the effort, and the fish becomes genuinely hard to catch rather than merely less obliging. Winter tench are a specialist's problem, not a matter of trying harder.

SO THE YEAR HAS A SHAPE. As water warms in spring the fish become active and begin feeding hard, and they do so while also building eggs — a period of high demand and rising capability at the same time. That combination is why the weeks before spawning are traditionally the most productive of the year, and it is worth being honest about what that means: the period an angler would choose is the period the fish is carrying next season's recruitment.

THE DAWN FEEDING SPELL IS NOT FOLKLORE AND IT IS NOT ABOUT LIGHT. Tench water is typically shallow, weedy and rich. Through a warm day, submerged plants photosynthesise and oxygen is abundant; overnight they respire instead, and dissolved oxygen falls to its daily minimum in the small hours. It then recovers rapidly once light returns. Add overnight cooling, which raises the water's capacity to hold oxygen, and the hour after first light is the point at which a warm, weedy, productive lake is best able to support a large fish doing hard work. The fish are feeding then because that is when the water lets them.

WHICH ALSO EXPLAINS WHY THE SPELL CLOSES. As the day warms and the fish have fed, both the driver and the appetite go, and a lake that produced fish at five o'clock is empty at nine — with nothing having changed except the water.

Tench reproduction is unusually late and unusually cautious, and both features are responses to the same problem: the fish needs more warmth than its range reliably provides.

THE TIMING IS LATE. Tench spawn well into summer and require sustained warm water to do it — considerably warmer than roach, bream or perch in the same lake. That is a real physiological requirement rather than a preference, and it puts the species at the edge of what a northern European summer delivers.

SO THE STRATEGY HEDGES. Tench spawn FRACTIONALLY: rather than releasing everything in one event as roach do, the female releases eggs in batches over several weeks. Sticky eggs are scattered over submerged weed each time. The effect is to spread the year's reproductive investment across multiple sets of conditions, so that a cold week, a sudden weather change or a poor plankton bloom removes one batch rather than the whole year.

THAT IS A DIFFERENT ANSWER TO THE SAME PROBLEM ROACH SOLVE BY SYNCHRONISING. Roach compress everything into a few days to swamp predators and accept the risk of a badly timed cold snap. Tench spread the risk and accept that they never swamp anything. Both are legitimate; which is better depends on how variable the environment is, and tench live at the cold edge of theirs.

AND IT STILL FAILS IN COOL SUMMERS. Where a summer does not deliver sustained warmth, tench spawning may fail entirely, which is why recruitment in the north of the range is patchy and why some populations are effectively maintained by stocking rather than by breeding. A lake with plenty of large tench and no small ones is a lake where the adults are surviving and the reproduction is not working.

WHICH CONNECTS DIRECTLY TO THE FISHING. The pre-spawning period is traditionally the most productive of the year, and the reason is that fish are feeding hard to build the reserves that fractional spawning will spend over the following weeks. That is the same statement as saying that the best fishing coincides with the fish carrying the year's recruitment, and on a species whose recruitment is this marginal it is worth knowing.

Tench are one of the few coarse species where the honest answer to 'how big do they get' has changed materially within the span of a fishing lifetime, and the reason is instructive.

THE BASELINE IS SLOW AND STEADY. Tench grow slowly, mature late, and live a long time. They are warm-water fish that feed heavily through a fairly short summer window and effectively shut down through winter, so the growing season is a matter of months rather than a year, and a large tench has accumulated its weight over many of them.

WHAT CHANGED WAS THE WATERS, NOT THE FISH. Through the later twentieth century, tench in Britain increasingly found themselves in large, rich, deep, lightly-fished gravel pits — an artificial habitat that did not exist before mineral extraction created it, and one with far more food and far fewer competitors than the traditional weedy estate lake. Tench in those waters grew to sizes that older literature treats as barely credible, and the shift is a habitat effect rather than a genetic one.

SO THE SIZE EXPECTATION HAS TO NAME THE WATER. In a small, dense, traditional tench pond a two-pound fish is ordinary and a four-pounder is a good one. In a large gravel pit holding a sparse population, the same fish is unremarkable and the specimens are far heavier. Neither population is behaving abnormally.

FEMALES ARE THE LARGE FISH AND IT IS EASY TO CHECK. Tench are strongly sexually dimorphic: males have thickened, spoon-shaped pelvic fins with a heavy muscular pad at the base, females have small flat pelvic fins. Every very large tench is female. That is worth knowing before wondering why the biggest fish in a water are all built the same way.

AND SPAWNING CONDITION MATTERS MORE THAN IN MOST SPECIES. Tench spawn late, in genuinely warm water, and a female in early summer may be carrying a very large ovary indeed. The heaviest recorded weights for individual fish cluster in the weeks before spawning, and the same fish in autumn will weigh appreciably less. A weight taken in June and a weight taken in September describe the same animal in different states.

Tench occupy the part of a stillwater that most other species avoid, and every feature of the animal is an adaptation to it.

THE HABITAT IS WARM, SHALLOW, WEEDY, SOFT-BOTTOMED STILL OR SLOW WATER. Tench are fish of estate lakes, ponds, meres, canals, drains, gravel pits and the slow reaches and backwaters of lowland rivers. What they want is silt to root in, dense submerged and marginal vegetation to move through, and warmth. They are not a fish of clean gravel, current or open water, and where they occur in rivers it is in the slack margins rather than the flow.

OXYGEN TOLERANCE IS THE DEFINING ADAPTATION AND IT IS EXTREME. Tench tolerate dissolved oxygen levels that would kill most coarse fish, which lets them occupy the hot shallow silted margins in high summer, and it lets them survive in small waters that periodically deoxygenate. That tolerance is the reason a tench can be the only substantial fish in a farm pond, and it is why they are so often found in the weediest, most stagnant-looking corner of a lake.

THE WEED IS NOT INCIDENTAL, IT IS THE STRUCTURE. Tench feed among and beneath dense vegetation on invertebrates in the silt and on the plants themselves, and their whole body plan — small scales embedded in a thick slime coat, rounded fins, powerful low-speed manoeuvrability — is built for pushing through weed rather than for open-water speed. A cleared, weed-free water loses tench habitat even if nothing else changes.

WHICH IS WHY THE FEEDING SIGNS ARE THE HABITAT SIGNS. Tench working silt release fine bubbles in patches, and pinprick bubbling over a soft bottom at first light is the most reliable indication of the species there is. Reading the ground for tench means reading the bottom composition and the weed rather than the depth.

WINTER IS A DIFFERENT ANIMAL. Tench become largely inactive in cold water, hold in deeper or more sheltered water, and feed very little — which is a habitat shift as much as a behavioural one, and it is why tench waters seem empty for months.

AND THE ARTIFICIAL WATERS MATTER MORE THAN THE NATURAL ONES IN BRITAIN. Most tench fishing takes place in gravel pits and constructed stillwaters, and the largest fish come from them. Those environments — deep, clear, steep-sided, developing weed slowly over decades — are not what the species evolved in, and tench in them behave differently from tench in a shallow silted estate lake.