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Ruffe fishing: the little perch that hunts in the dark

On the underside of its head sit flat, round mucus pits — and with them, German Wikipedia writes, it senses movement in the water. FishBase adds: it feeds at night and finds its prey through its lateral line system; the membranous walls of its head canals make it especially sensitive to direction. According to English Wikipedia, its sensory cells stay sharp into old age, whereas in the European perch they grow weaker as the fish matures. This article is about the ruffe, *Gymnocephalus cernua* — also pope in English; German Kaulbarsch, French grémille (also perche goujonnière), Italian acerina, Spanish acerina (Wikidata), Portuguese acerina eurasiana, Dutch pos, Danish and Norwegian hork, Swedish gärs, Finnish kiiski, Russian обыкновенный ёрш (ёрш), Turkish Trakya levreği, Croatian balavac, Slovenian okun, Japanese ラフ (rafu) and Arabic راف (Wikidata). For Greek and Albanian we found no documented common name; there the scientific one stands. It is small — but a predator in perch format, and FishBase lists it as a game fish.

The “ball” among perches

It was described by Carl Linnaeus in 1758 as *Perca cernua* — so as a perch right from the start. Today it sits in the genus *Gymnocephalus*, according to FishBase from the Greek gymnos, naked, and kephale, head: its head bears no scales. German Wikipedia explains the German name Kaulbarsch with its stocky build — “Kaul-” is said to be a dialect form of “Kugel”, ball. Two other languages have given it a less flattering name: the Croatian balavac recalls bale, slime, and a Swedish variant, according to Wikidata, is snorgärs — a ruffe feels in the hand just like its name. Croatian Wikipedia describes its skin as slimy and covered with sharp, grey scales.

How to recognise it

The surest feature is the dorsal fin: in the ruffe it is not divided (German Wikipedia) — 12 to 16 spines at the front, 11 to 15 soft rays behind, all in one continuous piece. The European perch, by contrast, has two separate dorsal fins. On the gill cover sits a long, strong spine, with several short ones on the preopercle — so anyone unhooking it handles it carefully. The back is olive-brown to grey-green with dark, irregular blotches, the flanks are yellowish, the breast has a reddish sheen and the belly is whitish to pale green (German Wikipedia). FishBase gives body depth — 24 to 27 % of standard length — as the feature that separates it from its relatives in the genus.

  • Size: usually 12 cm (FishBase), according to German Wikipedia 12 to 15 cm; the heaviest published weight is 400 g. According to FishBase it reaches maturity at around 10.5 cm.
  • Age: according to FishBase and German Wikipedia, females live up to about 10 years, males about 7.
  • Habits: FishBase describes it as solitary, forming no schools; German Wikipedia speaks of small groups. Where it lives alongside the European perch, both sources say it occupies the deeper water layers.

Where it lives

According to FishBase it lives in fresh and brackish water, at depths of 2 to 85 m, at 10 to 20 °C: in nutrient-rich lakes, in lowland and hill-country rivers, in the estuaries of large rivers, in brackish lakes up to about 10 to 12 ‰ salinity and in reservoirs. It prefers still or slow-flowing water with a soft, plant-free bottom, or deep spots over sand and gravel. The more nutrient-rich a water becomes, the more common it gets, and according to FishBase it tolerates a fair amount of pollution. According to German Wikipedia it is native north of the Pyrenees and the Alps — in the catchments of the North Sea, the Baltic, the White Sea, the Barents Sea, the Black Sea and the Caspian — as well as in England and in Siberia east to the Kolyma. German Wikipedia says it is most common in large estuaries and along brackish coasts such as the Baltic.

What it eats — and when

According to FishBase it eats zooplankton, chironomid larvae, worms and amphipods; German Wikipedia also names fish eggs and fry. In coastal lakes and tidal estuaries, FishBase says, it lives free-swimming and also hunts fish. Timing is what matters: it feeds at night and, thanks to its lateral line, can hunt in darkness, according to German Wikipedia. On the water that means: it searches for food on the bottom, and dusk and night are its time. If you want to target it, offer something small close to the bottom — and check the rules for your water first.

It is prey itself, too. According to Italian Wikipedia it is eaten by trout, eels, salmon and pike, and according to English Wikipedia, in Scotland, also by cormorants and grey herons. FishBase notes tersely: bait for pike. Whether and how bait fish are allowed on your water is governed by local fishing law — it differs from country to country, sometimes from region to region.

Spawning in shallow water

According to FishBase it spawns at 3 m depth or shallower on a wide range of bottoms. The eggs are white to yellow and stick as strands to stones and plants; according to FishBase the female lays them in two or more batches, usually about 30 days apart in summer, the first being the larger. The larvae spend little or no time in open water and settle on the bottom early; below 10 °C and above 20 °C they survive poorly. According to English Wikipedia they hatch after 5 to 12 days at 10 to 15 °C.

The perch that travelled in ballast water

In its home range it is an inconspicuous neighbour; the IUCN lists it as not threatened (Least Concern; assessed on 28 May 2022, according to FishBase). Elsewhere it is a problem. According to English Wikipedia it was discovered in the North American Great Lakes in the 1980s, probably brought in with ships' ballast water near Duluth (Minnesota); it was the first invasive species classed as a nuisance by the US programme against non-native aquatic nuisance species, and today it is the most common fish in the St. Louis River watershed, where it competes with the yellow perch for food. In Scotland's Loch Lomond it was found in 1982, probably brought in as live bait; there it eats the eggs of the powan, a whitefish species. According to English Wikipedia it has also been introduced to parts of France, northern Italy and Greece; FishBase notes that several countries have reported adverse ecological impact after its introduction. The lesson for anglers: never carry bait fish from one water to another.

Five contradictions, left standing

  • How long does it get? FishBase: at most 29 cm. German, Italian and Croatian Wikipedia: 25 cm. English Wikipedia gives 25 cm in its introduction and 29 cm further down.
  • How many eggs? German Wikipedia: 50,000 to 100,000 per female, depending on size. English Wikipedia: 130,000 to 200,000 a year.
  • When does it spawn? German Wikipedia: March to May. English: mid-April to about June. Croatian: April or May, when the water reaches 10 to 15 °C. Italian: March to August.
  • When is it mature? English Wikipedia: at 2 to 3 years, in warm water already in its first year. Croatian: at three years. FishBase gives no age but a length: around 10.5 cm.
  • Loner or group? FishBase: solitary, no schools. German Wikipedia: in small groups. Croatian: in small schools.

Three names that lead to the wrong fish

  • Slovenian “okun”, Russian “окунь”. Almost the same word, two fish: in Slovenian Wikipedia okun is the ruffe — the Russian окунь (okun) is the European perch. In Russian the ruffe is called ёрш.
  • Turkish “Trakya levreği”. Literally the “Thracian levrek” — and in Turkish levrek otherwise means the European sea bass, to which this series dedicates its own group. The ruffe has nothing to do with that sea fish.
  • “Donaukaulbarsch”. This German name — in English Danube ruffe — refers to a separate species, *Gymnocephalus baloni*, which according to German Wikipedia lives in the lower Danube and was only recognised as distinct in 1974. It is deeper-bodied, and its blotches are always vertical. Anyone who catches a “ruffe” on the lower Danube should take a second look.

That leaves the question every article in this series starts with: when. Habitat, food and habits are above, and they are fixed. What changes every day is everything else — light, water level, temperature, wind — and whether an evening on the water is worth it is decided right there. That is what StrikeAhead calculates: it takes the conditions on your water and tells you when the trip is worth it. And because you don't know an unfamiliar water, its drop-offs and its rules from day one, StrikeAhead Pathfinder tells you who to go out with: curated guides and skippers, personally vetted before listing — licence and insurance —, with verified catches instead of promo photos, no paid ranking and, for you as an angler, no booking fee. The app tells you when, Pathfinder with whom.