Allis shad

An allis shad's habitat is not a place but a sequence, and the sequence is what the species has been losing.

ADULTS LIVE AT SEA, feeding on zooplankton and small fish in coastal and shelf waters, where they spend the great majority of their lives. They are effectively a marine fish for three to six years.

SPAWNING REQUIRES CLEAN GRAVEL IN A RIVER, in moderate flow, and frequently a long way upstream. This is not a fish that spawns in the tidal reach and goes home: allis shad historically ascended major Atlantic rivers for a great many kilometres to reach suitable gravel.

THE ESTUARY IS THE TRANSITION AND IT MATTERS IN ITS OWN RIGHT. Adults gather in estuaries in spring before ascending, and juveniles use them on the way down in their first year, acclimatising to salt water as they go.

SO THE HABITAT IS THE WHOLE RIVER, FROM MOUTH TO SPAWNING GRAVEL, and it has to be continuous. A shad cannot leap. Where a weir, a barrage or a dam interrupts the route, everything above it becomes inaccessible, and the population that used it disappears.

THAT IS THE ENTIRE EXPLANATION OF THE SPECIES' DECLINE across western Europe. Rivers were engineered for navigation, milling and power through the nineteenth and twentieth centuries, and allis shad populations collapsed in step. The remaining strongholds are in French Atlantic rivers — the Loire, the Garonne, the Gironde system — and even those have contracted.

WHICH MAKES RECOVERY A FISH-PASSAGE QUESTION RATHER THAN A FISHING ONE. Where passes have been installed at obstacles, shad have used them. The species responds to connectivity being restored, which is a more hopeful finding than the decline suggests, and it is why the species is protected across much of its range while the engineering catches up.

Allis shad share the twaite shad's physical fragility and add a conservation position that makes the handling question largely academic in Britain, and it is worth being clear about both.

THE SCALES ARE DECIDUOUS AND THAT IS THE MECHANISM. A shad's scales attach loosely and detach at the lightest contact — hand, net, stone, sleeve. Beneath them is skin and the mucus barrier that keeps water out and infection away, so a fish that has been handled has lost a patch of its waterproofing and its immune defence at once. The damage is invisible at the bank and it kills days later.

THE FISH IS ALSO SPENDING EVERYTHING IT HAS. An allis shad in a river is on a spawning migration, feeding little or not at all, running on reserves, and committed to reaching gravel a considerable distance upstream. It has no capacity to absorb an additional cost. A fish taken on a river takes a lure by aggression or reflex rather than by hunger, which is worth remembering: it is not feeding, it is reacting.

SO THE HANDLING RULE IS ABSOLUTE RATHER THAN GRADUATED. Unhook in the water. Do not lift the fish out. Never lay a shad on a bank or on dry stones. Soft, knotless, wet net if a net is used at all. Barbless hooks so that in-water unhooking is realistic.

AND THE LEGAL POSITION COMES FIRST IN PRACTICE. The species is protected across much of its range, populations have contracted sharply where rivers were dammed, and in Britain allis shad is at best marginal — the substantial British shad runs are twaite. What is permitted differs by river and by country, closed areas and periods apply where the runs occur, and this is exactly the kind of rule that changes. Check with the authority for that river.

THE DEEPER POINT IS ABOUT WHERE THE SPECIES' VULNERABILITY LIES. Allis shad need to reach spawning gravel far upstream, which is why weirs and dams removed them from river after river across Europe. Handling is the smallest of the pressures on this fish, and treating it carefully is a matter of not adding to a list that is already long rather than of solving anything.

Allis shad size is worth knowing for two reasons, and the second one is not obvious: on this species the size is part of the identification.

ADULTS RETURNING TO SPAWN ARE COMMONLY FORTY TO FIFTY-FIVE CENTIMETRES AND ONE TO TWO KILOS. That is the ordinary run fish and it is what somebody fishing a French Atlantic river in spring encounters.

OVER SIXTY CENTIMETRES AND TWO AND A HALF KILOS IS LARGE, and reported maxima approach eighty centimetres. Fish of that size are rare and they are a feature of the larger Atlantic rivers rather than of anything in Britain.

WHICH IS WHERE THE IDENTIFICATION COMES IN. Allis and twaite shad are separated diagnostically by gill raker count — allis carries a very large number of fine closely-set rakers on the first gill arch and twaite carries substantially fewer and coarser ones — and nothing visible from outside settles it as reliably. The spot pattern that everybody uses is supporting rather than diagnostic, because spots fade and both species vary.

SO SIZE IS A USEFUL PRIOR RATHER THAN AN ANSWER. Allis run larger on average than twaite, and adults commonly exceed the size any twaite reaches. A very large shad from a river with both is worth checking rather than assuming, and a shad from a British river is much more likely to be twaite whatever its size, because the substantial British runs are twaite.

AND THE TWO HYBRIDISE WHERE THEY MEET, which is the honest complication. An intermediate fish exists, it cannot be named confidently by any field character including the raker count in some cases, and a page that offered a clean binary would be misleading in the most useful-sounding way available.

THE PRACTICAL CONCLUSION for anybody holding a shad on a river bank is that they should probably not be holding it at all — the fish is protected in much of its range, it is running to spawn, its scales are coming off in their hand, and the identification can be made from a photograph taken with the fish in the water rather better than from a fish on the grass.

The allis shad year is very simple to state and the simplicity is the species' whole problem: it spends most of the year at sea, and everything that decides whether there will be a next generation happens in a few weeks in fresh water.

AT SEA FOR MOST OF THE YEAR, feeding on zooplankton and small fish, filtering with the fine gill rakers that also identify it. That is the growing phase and it happens where nobody sees it.

THE RUN IS IN SPRING AND IT IS TEMPERATURE-DRIVEN. Fish enter estuaries and ascend rivers as water temperature rises, with the run concentrated into a few weeks. The timing shifts between years and between rivers with the spring, which is why a date-based expectation is unreliable and a temperature-based one is not.

SPAWNING HAPPENS AT NIGHT, OVER GRAVEL, AND IT IS AUDIBLE. Pairs roll at the surface in moderate flow, noisily and visibly enough that a spawning site can be located by listening from the bank. Eggs are semi-buoyant and settle into the gravel interstices, which is why the gravel has to be clean rather than silted.

AND THE FISH HAS TO GET A LONG WAY UPSTREAM TO FIND IT. That single requirement is the species' entire vulnerability. Allis shad spawn a considerable distance from the sea, and a weir, a dam or a poorly designed passage anywhere below the spawning gravel removes the population — not reduces it, removes it, because a fish that cannot reach the gravel does not spawn somewhere else instead. That is why the species contracted sharply across Europe as rivers were dammed and why the remaining major populations are in French Atlantic rivers with passable lower reaches.

AFTER SPAWNING, THE SURVIVORS DROP BACK. Unlike a salmon, allis shad is not obligately semelparous and some fish return to sea and repeat the run, though the migration is costly and mortality is high.

WHAT THIS MEANS FOR ANYBODY INTERESTED IN THE SPECIES' RECOVERY is that the useful interventions are all structural: passage at barriers, clean gravel, and unobstructed access to the upper river. Handling care is worth doing and is not the variable. That is an uncomfortable thing for an angling catalogue to say and it is the accurate one.