Crayfish come ashore for a reason - it is a signal of specific changes in their habitat or physiological state. Most often, the mass appearance of crayfish on land is observed in breeding season (June-July), at lack of oxygen in water (for example, after prolonged heat) or when searching for new territory after the floods. If you notice crayfish on the open shore during the day, this is almost always a sign environmental stress in a reservoir: siltation, chemical pollution or a sharp drop in water level. Normally, healthy crayfish are active at night and hide under stones or driftwood, so their daytime appearance requires attention.
There is a popular myth that crayfish come ashore before the rain - supposedly they “anticipate” a change in the weather. In fact, their behavior is not related to atmospheric pressure, but to dissolved oxygen concentration, which falls as the water temperature rises. For example, on hot July days, even in clean lakes, the O₂ content can drop to a critical 2-3 mg/l (with a norm of 5-8 mg/l), forcing crayfish to migrate closer to the coastal zone, where the water is better saturated with oxygen due to waves and currents.
Biological reasons for crayfish coming to land
The main factors that force crayfish to leave the water are divided into: natural (life cycle related) and forced (due to worsening conditions). The first include:
- 🦀 Reproduction: Females come into shallow water to lay eggs on the undersides of leaves or rocks. This happens at night when the water temperature is 15-20°C.
- 🌊 Shedding: during shell replacement (2-3 times a year), crayfish seek shelter on the shore, where the risk of being eaten by predators is lower.
- 🏡 Migration of young animals: Young crayfish (2-3 cm in size) often crawl onto wet soil in search of food - detritus and small invertebrates.
Forced reasons are always associated with degradation of the reservoir:
- ⚠️ Hypoxia (oxygen starvation) due to algal blooms or the discharge of organic waste.
- 🧪 Pollution heavy metals (lead, cadmium) or pesticides - crayfish are trying to leave the toxic zone.
- 🔥 Fires on peatlands, leading to water acidification (pH below 5.5).
If you see crayfish on open sand during the day, this is a sure sign that something has happened in the reservoir. fish kill (mass death due to lack of oxygen). In such cases, crayfish come ashore to escape, but often die from dehydration.
What time of day are crayfish active on the shore?
In natural conditions crayfish activity peak falls on 2-4 am - at this time they feed and look for partners for reproduction. The daytime appearance of crayfish (from 10:00 to 16:00) is always abnormal and indicates:
| Time of day | Reason for exit | What to do |
|---|---|---|
| Night (22:00–4:00) | Natural behavior (feeding, breeding) | Don't disturb the crayfish - they will return to the water |
| Early morning (5:00–8:00) | Finding shelter after a night's activity | Check the water level - the reservoir may be getting shallow |
| Day (10:00–16:00) | Oxygen starvation or pollution | Notify your local Rosprirodnadzor office |
| Evening (18:00–21:00) | Preparing for night activity | Can be observed with a flashlight (do not point the light directly at the crayfish) |
Exception - autumn months (September-October), when crayfish can come ashore during the day as the water temperature drops below 10°C. In this case, they bask in the sun, like reptiles, to speed up their metabolism before wintering.
How to distinguish healthy cancers from sick ones by behavior
Healthy crayfish, even those that come ashore, retain quick response to movement and immediately hide under cover. The following signs indicate problems in a pond:
- 🦵 Lethargy: the cancer lies on its side and does not respond to touch - a sign of hydrogen sulfide poisoning.
- 🟠 Orange or white spots on the shell - fungal infection (Achlya), common in polluted water bodies.
- 🌀 Spinning in place - damage to the nervous system due to heavy metals.
- 💀 Mass death on the shore - a consequence of the discharge of wastewater or pesticides.
⚠️ Attention: if you find dead crayfish with greenish tint of the shell, this may indicate botulism - do not handle them with your bare hands and do not allow pets to come into contact with the carcasses.
To check water quality at home, you can use nitrate test strips (NO₃⁻) and ammonia (NH₄⁺). Exceeding the norm (more than 50 mg/l of nitrates or 0.5 mg/l of ammonia) confirms pollution that is dangerous for crayfish.
What to do if crayfish come ashore en masse
If you witness abnormal behavior of crayfish, follow this algorithm:
☑️ Actions in case of mass emergence of crayfish on the shore
For urgent help with cancer you can:
- Organize shadow shelters on the shore (boards, stones) to reduce the evaporation of moisture from their bodies.
- If the reservoir becomes shallow - water the bank water from buckets, creating wet areas for migration.
- In case of chemical contamination - collect crayfish in a bucket of clean water and release them into another body of water (with the permission of environmentalists).
⚠️ Attention: Never transplant crayfish into other people's waters without testing the water! This may lead to the spread of invasive species (e.g. mantis crab) or diseases.
Myths about crayfish and their coming ashore
Common misconceptions that prevent you from correctly interpreting the behavior of crayfish:
Myth
“Crayfish come out before the rain”: In fact, crayfish react not to atmospheric pressure, but to drop in oxygen levels, which often coincides with hot weather before a thunderstorm. Their “prediction” of rain is simply a coincidence of weather conditions.
- 🌧️ "Crayfish predicts rain" - there is no evidence that they feel a change in pressure. Their activity is related to water temperature.
- 🌙 "Crayfish only come out during the full moon" — lunar phases do not affect their behavior, but at night they are actually easier to notice.
- 🧂 "Crayfish come out of salt water" — freshwater crayfish die in a salty environment, their exit is associated with fresh water.
The only myth that has any basis is the connection between massive release of crayfish and earthquakes. In 2011, Japanese scientists recorded abnormal behavior of crayfish 2-3 days before the earthquake in Miyagi Prefecture. However, the mechanism of this connection has not been fully studied, and crayfish cannot be used as a “seismic indicator”.
How to observe crayfish without harming them
If you want to study crayfish behavior, follow the rules ethical supervision:
- 🔦 Use red flashlight — crayfish do not perceive it as a danger (unlike white light).
- 📏Keep your distance 1.5-2 meters away from the water so as not to frighten the crayfish with the shadow.
- 🎥 To take photos, use macro lens - do not pick up crayfish unless necessary.
- 🧤 If you need to take a crayfish, put it on wet gloves (dry hands damage their gills).
The best time to observe is June-Julywhen crayfish are most active. During this period, males organize “mating tournaments” in shallow water, and females carry eggs on pleopods (abdominal legs). To avoid missing this spectacle, check the coastal rocks and driftwood 30-40 minutes after sunset.
Mass release of crayfish during the day is environmental signal, not a normal phenomenon. If you see more than 10 crayfish on the open bank, the pond needs to be checked.
FAQ: Frequently asked questions about crayfish on the shore
Is it possible to eat crayfish that come ashore during the day?
No! Cancers that come out during the day are most likely stressed or poisoned. Their meat may contain toxins (eg. saxitoxin from blue-green algae), dangerous to humans. Even after heat treatment, the risk of poisoning remains.
Why do crayfish come ashore after rain?
After a rainfall, a lot of organic matter (leaves, grass) enters the reservoir, which begins to rot, consuming oxygen. Crayfish come ashore not because of the rain itself, but because of the subsequent hypoxia. In addition, rainwater can erode banks, destroying crayfish burrows.
How long can a crayfish live on the shore?
With air humidity above 80% and a temperature of 15-20°C, the crayfish will live on the shore until 2-3 days, if its gills remain wet. In dry, hot weather (above 25°C), it dies within 4-6 hours from dehydration. Crayfish cannot breathe atmospheric air - their gills only work in water.
Is it true that crayfish come ashore before they die?
Yes, but only if there is a body of water completely deprived of oxygen. Normally, crayfish die in shelters. If you see crayfish lying on their back with their claws extended, this is a sign of poisoning (for example, chlorine or ammonia). Healthy crayfish, even on the shore, maintain a protective posture: the claws are clenched, the tail is tucked.
Can crayfish survive in an aquarium after being taken from the shore?
Theoretically, yes, but in practice, less than 30% of “wild” crayfish survive. Reasons:
- Stress from transportation (need aerated water with a temperature of 18-20°C).
- Incompatibility with other inhabitants of the aquarium (crayfish eat small fish and plants).
- Need for shelters (without them, the cancer will die from light or aggression of its relatives).
To keep crayfish, you need an aquarium of at least 100 liters with pebble soil, driftwood and live plants (for example, elodea).