Astacus astacus
The Noble Crayfish
The Native Crayfish of the Northern Lakes
The Noble Crayfish (Astacus astacus) is the only freshwater crayfish native to Finland — a slow-growing, long-lived decapod crustacean (not a fish, but treated as one in Finnish angling tradition) that has shaped lakeside summer culture for centuries. Adults are a deep reddish-brown to dark green-brown above with a paler underside; the claws are robust, granulated, and — most importantly for identification — red on the underside. The rostrum bears two pairs of distinct ridges, and the tail segments lack the white-tipped claws and smooth blue-tinted carapace of the invasive Signal Crayfish (Pacifastacus leniusculus) it is so often confused with.
In Finnish it is rapu, and few foods are more woven into Finnish summer than the boiled crayfish of late July. The season opens at 12:00 on 21 July by national fisheries law, and the weeks that follow are the high ritual of rapujuhlat — crayfish parties on lakeside terraces with bibs, candles, dill-laden boiling broth, dark rye bread, butter on toast, and small, ice-cold glasses of schnapps drunk with ceremonial little songs between every claw. To catch your own, by trap or by hand from a boat at dusk, is one of the deep summer rites of the Finnish lakeland.
Ecologically, Noble Crayfish are nocturnal benthic omnivores of clear, cool, well-oxygenated freshwater. They burrow into clay banks, hide under stones and submerged timber by day, and forage at night for invertebrates, plant material, carrion and the occasional dead or weakened fish. Mating occurs in autumn; females then carry eggs in berry under the abdomen all winter, releasing tiny juveniles in late June. Wild Finnish stocks have been devastated since the 19th century by crayfish plague (Aphanomyces astaci) — a water-mould carried, asymptomatically, by the introduced Signal Crayfish — and the species is listed as Endangered (EN) on the Finnish Red List. Many southern lakes today hold only Signal Crayfish; recovering Noble Crayfish populations exist, but they are isolated, plague-vulnerable, and managed with care.
Seasonal Data
Activity patterns of the Noble Crayfish in Finnish waters — deep dormancy under ice, awakening only when bottom water clears 10 °C, peak trappability in the late-July to October window, then a sharp autumn taper as moulting suspends below 8 °C.
Diet Spectrum
Noble Crayfish are opportunistic omnivores with a strong seasonal shift — heavy invertebrate predation in summer, broader carrion-and-detritus scavenging in spring and autumn, and minimal feeding under the ice.
Growth by Age
Noble Crayfish grow slowly at 60°N+ — only four active months a year, moulting suspended below 8 °C. The 10 cm legal minimum is typically reached at 4–6 years; a 12 cm specimen is usually 6–8 years old, and the rare 15 cm trophy represents over a decade of patient summers. Long-lived (15–20+ years where unfished), they are an irreplaceable engine of recovering populations.
| Age (Years) | Length | Weight | Relative Size |
|---|---|---|---|
| 1 | 3 cm | 1 g | 18% |
| 2 | 5 cm | 4 g | 29% |
| 3 | 6.5 cm | 9 g | 38% |
| 4 | 8 cm | 18 g | 47% |
| 5 | 9.5 cm | 32 g | 56% |
| 6 | 10.5 cm | 45 g | 62% |
| 8 | 12.5 cm | 75 g | 74% |
| 10+ | 14.5 cm | 120 g | 85% |
Habitat Requirements
Water Temperature
Cool-water specialist. Moulting and growth suspended below 8 °C; lethal extended exposure above 24 °C.
Oxygen
Highly oxygen-demanding — excluded from eutrophic systems with summer hypoxia. Clean cool oligo-mesotrophic water is essential.
Structure
Obligate shelter-seeker. Adults hold tight to crevices and burrows by day, emerge to forage at night. Structure density caps population density.
Substrate
Hard or burrow-grade substrate is non-negotiable. Soft-mud lakes that suit Bream are unsuitable for Noble Crayfish.
Water Depth
Concentrates in the shallow shoreline zone where structure, oxygen and food converge. Rare below 10 m in Finnish lakes.
Water Type
Strict freshwater — does not tolerate Baltic brackish conditions (the invasive Signal Crayfish handles low salinity better).
Crayfish Trapping in Finland
Noble crayfish (rapu, Astacus astacus) are not caught on a rod — they are taken in baited traps (mertaa) lifted by hand or boat after sundown. The Finnish season is tightly regulated: trapping is legal only from July 21 at noon through October 31, with a 10 cm minimum (carapace plus tail), and the warm August nights are the heart of the ritual. Most catches happen on the second or third lift after dusk, and a good evening is as much about the sauna and dill-boiled crayfish afterward as about the haul itself.
Cylinder Mesh Trap (Merta)
Late Jul - OctThe standard Finnish trap: a cylindrical wire-mesh cage with one or two funnel entrances, baited with perch or roach pieces (raw fish heads work well in deeper water). Set in 1.5–4 m on rocky or gravel bottoms near reed edges, drop-offs, and stone shorelines. Soak 4–12 hours overnight; lift before noon to release undersize females and any berried hens.
Collapsible Basket Trap (Sukellusmerta)
Aug - OctFolding plastic-coated baskets are easier to transport and stack on small boats, popular for guided rapujuhlat trips on Saimaa and Pirkanmaa lakes. Bait with oily fish (smoked fish skin lasts longer than fresh), set deeper (3–6 m) on cooler late-season nights when crayfish move off the shoreline. Mark each trap with a buoy carrying the user's name and contact details, as required by law.
Torch & Hand Catching (Soihtupyynti)
Aug nightsOn calm, dark August nights you can wade clear, shallow shorelines (under 1 m) with a strong headlamp and lift crayfish by hand from behind the carapace. Works only on wadeable rocky bottoms in lakes with healthy populations and minimal weed. Slow and traditional — a few dozen animals on a good night is realistic, and any sub-10 cm catch goes straight back.
Hosted Rapujuhlat (Guided Trap Night)
Aug - SepMost foreign visitors join a host-led evening: the operator provides licensed traps, bait, boat transport, and the cooking. You set traps at dusk, sauna while they soak, and lift at 22–23:00. Required papers — kalastonhoitomaksu (state fee) plus the local water-area permit (often via Eräluvat or a kalatalousalue) — are arranged by the host. Daily personal quota and trap count vary by water body, so always confirm with your host.
Where to Trap Noble Crayfish in Finland
Productive noble-crayfish waters are concentrated in southern and central Finland; many western and northern lakes now hold signal crayfish (Pacifastacus leniusculus) instead, which are a separate, regulated species.
Saimaa System (Etelä-Karjala / Etelä-Savo)
61.0667°N, 28.1833°ERestored noble population
Parts of the greater Saimaa basin, particularly smaller side-lakes and protected bays, have been actively restocked with noble crayfish since the 1990s after crayfish-plague collapses. Clear, rocky shorelines and stable summer temperatures suit the species well. Several kalatalousalueet sell area-specific permits, and local hosts run organized rapujuhlat from Lappeenranta and Mikkeli.
Pirkanmaa Lake Chain
61.4833°N, 23.7667°EClassic central-Finland waters
The Längelmävesi-Roine-Hauhonselkä chain north of Tampere holds some of Finland's most stable noble populations, with gravel and boulder shorelines that the species favours. Permits are managed through regional fishing-license cooperatives; signal crayfish are absent from the upper chain, which keeps plague risk low. Best lifts come on warm, low-wind August nights.
Puula & Etelä-Savo Lakes
61.6833°N, 27.2667°EStronghold for trophy nobles
Lake Puula and surrounding mid-sized lakes in Etelä-Savo are known among Finnish crayfishers for above-average individuals, occasionally exceeding 14 cm. The catchment is partly closed off from invasive signal populations, which has helped natives recover. Permits are tightly limited and often booked through local landowner associations or guided operators.
Itä-Uusimaa Forest Lakes
60.4000°N, 25.9000°ESmall closed-catchment lakes
East of Porvoo, a network of small forest lakes with no surface connection to plague-affected waters has been used as noble-crayfish refuges and broodstock sources. Populations are modest and permits are scarce — usually only available via private landowner agreements or local hosts. A good entry point for visitors based around Helsinki who want a short drive to a legitimate noble water.
