Signal crayfish

Signal crayfish reproduction is the mechanism behind their success as an invader, and describing it is describing the invasion.

MATING IS IN AUTUMN, after which the female carries the fertilised eggs attached beneath her abdomen through the winter — the berried state — brooding them for months before they hatch in spring.

SHE CARRIES AND PROTECTS THE YOUNG after hatching too, with juveniles remaining attached beneath her for a period before dispersing. That parental investment produces far better survival than a broadcast strategy, from a comparatively modest number of eggs.

THEY MATURE FAST AND BREED YOUNG relative to the native white-clawed crayfish they displace, and they grow faster and larger. Faster maturation plus higher survival is the arithmetic of invasion.

BURROWING IS PART OF THE LIFE CYCLE AND PART OF THE DAMAGE. Signal crayfish excavate extensive burrows in banks, which they use for shelter and overwintering, and the resulting bank collapse and siltation is a substantial physical impact on rivers independent of anything they eat.

THEY CARRY CRAYFISH PLAGUE WITHOUT SUCCUMBING TO IT, and the disease is lethal to native European crayfish. So the reproductive success of the invader also spreads a pathogen that removes the competitor — which is why native crayfish decline faster than competition alone would predict.

AND THE PRACTICAL CONSEQUENCE FOR ANYBODY HANDLING THEM is that moving live signal crayfish, or moving damp equipment between waters, spreads both the animal and the plague. Trapping is licensed or prohibited in many jurisdictions for exactly this reason, and this record does not state what any current rule permits.

Signal crayfish are measured either as carapace length or as total length depending on the source, and for this species the more useful comparison is not with a limit but with the native it displaces.

ORDINARY ANIMALS run five to eight centimetres of carapace and twelve to sixteen centimetres in total.

A LARGE ONE exceeds eighteen centimetres total.

THE EXCEPTIONAL END approaches twenty-five centimetres in established populations, and males grow larger than females with disproportionately heavy claws.

THE COMPARISON THAT MATTERS. The native white-clawed crayfish rarely exceeds about twelve centimetres in total. A signal crayfish is therefore substantially larger at maturity, grows faster, matures earlier and is more aggressive — and that size and growth advantage is a large part of why it displaces the native even before the disease it carries is taken into account.

GROWTH IS BY MOULTING, several times a year when young and less often thereafter, and maturity comes in two to three years with a lifespan around twenty.

THERE IS NO MINIMUM SIZE, and there is no conservation reason for one. This is the single record in this catalogue where taking more is not a concern and where the management objective is removal rather than sustainable yield.

BUT SIZE DOES NOT MAKE IDENTIFICATION SAFE. A small signal crayfish and an adult native overlap in size, and identification must be made on the pale patch at the claw hinge and the red claw undersides rather than on how big the animal is. Getting it wrong in one direction destroys a protected native; getting it wrong in the other returns an invasive to the water. Where there is any doubt at all, the correct action is to leave the animal and report the sighting to the relevant authority.

This record inverts the standing advice on FishingHQ, and the inversion is the most important thing on the page.

THE ANIMAL ITSELF IS STRAIGHTFORWARD. The claws pinch hard enough to break skin on a large male, and the grip is from behind across the back of the carapace where neither claw reaches — the same hold as for a lobster. Put it down rather than pulling against a grip.

A SIGNAL CRAYFISH MUST NOT BE RETURNED TO THE WATER ALIVE, and in several jurisdictions doing so is an offence. It must not be moved alive between waters under any circumstances. If you are not able or willing to dispatch what you catch, you should not be trapping for them.

AND THE HANDLING THAT MATTERS MOST HAS NOTHING TO DO WITH THE ANIMAL. Crayfish plague is a water mould carried on damp equipment — boots, waders, nets, keepnets, boat hulls, landing mats — and it survives long enough in wet gear to travel between catchments in a car boot. It is fatal to native European crayfish and it does not need a crayfish to move it. That means the biosecurity duty falls on every angler who has fished a water holding signals, whether or not they have ever seen one.

CHECK, CLEAN, DRAIN AND DRY everything between waters, and dry it thoroughly rather than briefly — the mould survives damp far longer than most people assume. This is the single most effective thing an individual can do about the spread of this species, and it costs nothing but time.

TRAPPING ITSELF IS CONTROLLED. In many jurisdictions it requires prior consent from the relevant authority, with specified trap dimensions designed to prevent otters and water voles from entering and drowning. Those specifications are legal requirements rather than good practice, and traps that do not meet them kill protected mammals.

WHAT NOT TO ASSUME. That trapping will clear a water. It will not — no established population has been eradicated by trapping anywhere, because traps select for large animals and leave the juveniles that rebuild the population. Trapping is a harvest and a monitoring tool, not a control measure, and describing it as control overstates what anybody can achieve.

AND ESTABLISH THE CURRENT POSITION BEFORE ANY TRAP GOES IN. The consent regime, permitted gear and reporting requirements differ between countries and they change.

Signal crayfish are unusual in this catalogue because their habitat description is also a description of habitat damage: the animal modifies the water it occupies, and the modification is the reason it matters.

THE REQUIREMENT IS FRESH WATER WITH A BANK IT CAN DIG. Signal crayfish occupy rivers, streams, canals, lakes and reservoirs across a very wide range of conditions, and what they need is not a particular water type but a substrate that will hold a burrow — soft or friable banks, undercut margins, silt and clay — together with hard cover in the channel: stones, cobbles, tree roots, woody debris. They are nocturnal and shelter-dependent, and by day a signal crayfish is in a burrow or under something.

THE BURROWING IS EXTENSIVE AND IT CHANGES THE RIVER. Signal crayfish dig systems of burrows into banks at high density, and the cumulative effect is bank collapse, increased fine sediment input and altered channel form. That sediment then fills the gravel interstices that salmonid eggs, invertebrates and burbot-type species depend on. So the habitat account for this species is inseparable from its impact: it does not simply live in a river, it re-engineers it.

TOLERANCE IS EXTREMELY BROAD, which is the other half of the invasion story. Signal crayfish handle a wide temperature range, moderate pollution, low oxygen better than the native crayfish they displace, and both flowing and still water. They colonise canals as readily as upland streams, and they can move overland between waters for short distances in damp conditions. Very little in the way of physical habitat excludes them.

WHICH IS WHY THE HABITAT STORY IS ALSO A BIOSECURITY STORY. The species carries crayfish plague, to which it is largely resistant and to which the native white-clawed crayfish is not, so the presence of signals in a catchment is functionally the absence of natives. Movement of the animal, of water, and of damp equipment between catchments is the transmission route, and cleaning and drying gear between waters is the control.

AND WHAT MAY BE DONE ABOUT ANY OF THIS IS A LEGAL QUESTION WITH A LOCAL ANSWER. Trapping, keeping, moving and releasing crayfish are variously restricted or prohibited depending on jurisdiction, and in several places handling them at all requires authorisation. FishingHQ does not state what any current rule is; the competent authority does.

Crayfish seasonality is driven by temperature acting on a cold-blooded animal that has to shed its shell to grow, and the moult is the part that surprises people.

ACTIVITY TRACKS WATER TEMPERATURE ALMOST DIRECTLY. Signal crayfish are largely dormant in cold water, becoming progressively more active as the water warms through spring, reaching peak activity through the warm months, and slowing again in autumn. In winter they remain in burrows and feed very little. Anything that depends on the animal moving — trapping, encountering them at night, seeing them at all — collapses in the cold and is not an indication of abundance.

MOULTING PRODUCES SHARP, TEMPORARY ABSENCES. A crayfish grows by shedding its exoskeleton, and for a period around each moult it is soft, defenceless, hidden and not feeding. Moulting is synchronised enough within a population that a water can go from busy to apparently empty within days and back again a fortnight later. That is a moult, not a decline, and it is the single commonest misreading of crayfish activity.

MATING IS AN AUTUMN EVENT AND THE FEMALES THEN CARRY THE EGGS ALL WINTER. After mating in autumn the female attaches the eggs beneath her abdomen and broods them through the cold months, with the young hatching and releasing in spring. Berried females are therefore present through winter, and they are less active and less trappable while brooding.

MALES AND FEMALES ARE NOT EQUALLY CATCHABLE THROUGH THE YEAR because of that, and a seasonal sample taken at one point in the cycle gives a badly skewed picture of the population's sex ratio and size structure.

GROWTH IS CONCENTRATED IN THE WARM MONTHS for the same reason as activity, so a year's growth happens in a handful of months and the animal effectively pauses for the rest.

AND NONE OF THIS ADDRESSES WHAT IS PERMITTED. Trapping crayfish, keeping them, moving them and returning them to the water are variously licensed, restricted or prohibited depending on jurisdiction, and in several places any handling requires authorisation and returning a captured signal crayfish alive is specifically forbidden. Those rules exist for biosecurity reasons unrelated to the seasonal cycle above. The competent authority is the only source for them.