Stone moroko (Pseudorasbora parva)

Synonyms: Leuciscus parvus, Pseudorasbora parva parva

Common Names: Topmouth gudgeon, false razbora

Description:

Stone moroko (Pseudorasbora parva), also called the topmouth gudgeon, is a small fish in the minnow and carp family, native to eastern Asia [1][2]. It has spread around the world by hitching along with farmed and ornamental fish and is considered one of the worst invasive fish in Europe [2]. MISIN tracks it as an early-warning target because it breeds explosively and is a healthy carrier of a deadly fish parasite ? the "rosette agent" ? that it can pass to native fish [2][3].

Identification:

Stone moroko is a small, slender fish, usually about 3 to 4 inches long (rarely over 4 inches), slightly flattened from side to side [2]. It is grey to olive on the back and silvery to yellowish-green on the sides and belly, with short fins, rounded fin edges, and fairly large scales [2]. The clearest clue is the mouth: the lower jaw turns up so the mouth points upward (the source of the name "topmouth"), and young fish have a dark stripe running along each side [2]. In the breeding season males darken and develop small bumps (tubercles) on the head [2].

Because it is not known to be widespread in North America, the most useful time to look is any time a small, unfamiliar upturned-mouth minnow turns up in a pond, water garden, or stocked water ? especially where ornamental or farmed fish have been released [2]. A suspected stone moroko should be reported right away (see Management).

At-a-glance cues:

1. Small (about 3?4 inch) slender minnow with large scales [2]
2. Mouth turns upward, with the lower jaw projecting (the "topmouth") [2]
3. Grey-olive back and silvery to yellowish-green sides; short, rounded fins [2]
4. Young fish have a dark stripe along each side [2]

Habitat:

Stone moroko is native to eastern Asia ? China, Japan, Korea, and the Amur River basin [1][2]. It was carried into Europe in the 1960s, accidentally mixed in with shipments of farmed carp, and then spread further through the ornamental fish trade, becoming established across much of the continent [2].

It lives in still or slow-moving fresh water ? ponds, lakes, ditches, water gardens, and the quiet stretches of rivers ? and does well in warm, weedy, nutrient-rich water [2]. It is hardy and tolerant of a wide range of conditions, which helps it take hold quickly wherever it is introduced [2].

Reproduction:

Stone moroko spreads almost entirely because of people moving fish and water. The main pathways are the accidental movement of the fish along with farmed or stocked fish and the ornamental and aquarium trade; it can also travel with bait or in water moved between ponds [2]. The simplest way it reaches a new pond is the release or escape of ornamental fish [2].

Once present, it breeds fast. It spawns repeatedly through spring and summer, the female laying eggs on hard surfaces such as stones, stems, and shells, and the male guards them until they hatch; the fish matures young, often within a year or two [2]. This rapid, repeated breeding lets small numbers build into large populations quickly, so the key is to keep it out of new waters in the first place [2].

Impact and Damage:

Stone moroko is not a danger to people, but it is one of the most concerning small invasive fish because of what it carries. It is a healthy carrier of the "rosette agent" (Sphaerothecum destruens), a parasite that infects many kinds of fish: the stone moroko itself stays healthy while passing the parasite to native and farmed fish through contact and through the water, where it can cause disease, poor condition, and population declines ? a threat even where the stone moroko itself is not abundant [3]. This parasite has been linked to declines of native European fish [3].

On top of the disease risk, stone moroko competes with native and farmed fish for food, space, and spawning sites and eats the eggs and young of other fish, which can reduce native populations and commercial fish production [2]. Because it combines fast breeding, broad tolerance, and a deadly parasite, it is treated as a high-priority early-detection target in North America [2][3].

Similar species:

Stone moroko is a small minnow that can be mistaken for several native minnows; the upturned mouth and the dark side stripe on young fish are the key things to check.

Fathead minnow (Pimephales promelas) is a common native minnow of similar size and habitat, but its mouth is small and points forward rather than turning up, and it has a stouter, more rounded body [2].

Bluntnose minnow (Pimephales notatus) is another native minnow often found in ponds and slow streams, but it has a rounded, blunt snout with a low, forward-facing mouth and a dark stripe ending in a spot at the tail, unlike the upturned mouth of stone moroko [2].

Common shiner (Luxilus cornutus) and similar native shiners are silvery minnows that share the habitat, but they are more slender and silvery with a normal terminal mouth, lacking the distinctive upturned jaw of stone moroko [2].

Monitoring and rapid response:

Signs to watch for:

1. A small, unfamiliar minnow with an upturned mouth in a pond, water garden, ditch, or stocked water [2]
2. Young fish with a dark stripe along each side [2]
3. A new small minnow appearing where ornamental or farmed fish have been released or escaped [2]

What you can do:

1. Never release aquarium or pond fish, and never dump aquarium or pond water into any lake, stream, ditch, or storm drain ? this is the main way the fish and its parasite spread [2].
2. Never move fish or water from one pond or waterbody to another, and do not use unknown small fish as bait [2].
3. If you think you have found stone moroko, do not release it ? keep it, photograph it, and report it right away so it can be confirmed quickly [2].
4. Clean, drain, and dry boats, nets, buckets, and gear before moving between waters [2].
5. This is an early-detection, rapid-response species: small enclosed waters have been cleared only by draining or treating the whole water, so finding and reporting it early ? before it spreads ? is by far the most effective step [2].

When to call a professional:

1. Any suspected stone moroko, since confirming and responding to a new occurrence is an agency-led, time-sensitive effort [2]
2. Established populations or fish in connected waters, where eradication requires draining or whole-water treatment by trained staff [2]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Global Biodiversity Information Facility (GBIF). Pseudorasbora parva (Temminck & Schlegel, 1846). gbif.org
2. CABI Compendium. Pseudorasbora parva (topmouth gudgeon). cabidigitallibrary.org
3. Gozlan RE, St-Hilaire S, Feist SW, Martin P, Kent ML (2005). "Disease threat to European fish." Nature 435:1046. semanticscholar.org

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