Tench (Tinca tinca)

Synonyms: Cyprinus tinca

Common Names: Doctor fish, green tench, golden tench

Description:

Tench (Tinca tinca) is a stout, dark-green fish in the minnow and carp family, native to Europe and Asia [1][2]. It was brought to the United States in the 1870s as a food and sport fish and has since turned up in dozens of states, with established populations in a few [2]. MISIN tracks it because it stirs up muddy bottoms, clouds the water, and competes with native fish, and because it is hardy, long-lived, and hard to remove once it takes hold [2][3].

Identification:

Tench is a thick-set, deep-bodied fish that looks somewhat like a carp, olive-green to nearly black on the back and golden-bronze below [2]. The easiest clues are its bright red-orange eyes, a single small barbel (whisker-like feeler) at each corner of the mouth, and very small scales set in a thick, slippery coat of slime ? the slick skin is why it is sometimes called the "doctor fish" [2]. Its fins are dark, rounded, and lack sharp spines [2]. Tench can grow large, well over a foot long [2].

Tench are most likely to be seen in the warm months, when they are active in weedy, muddy shallows and spawn in late spring and summer; in winter they burrow into the mud and go dormant [2]. Anglers sometimes catch an unfamiliar dark, slimy carp-like fish ? checking for the red eyes and tiny corner barbels confirms it [2].

At-a-glance cues:

1. Stout, dark olive-green body with a golden-bronze belly and a thick, slimy coat [2]
2. Bright red-orange eyes [2]
3. A single small barbel at each corner of the mouth [2]
4. Very small scales and dark, rounded, spineless fins [2]

Habitat:

Tench is native to Europe and western and northern Asia, and was imported to the United States in the 1870s as a food and sport fish [1][2]. It has since been recorded in about 38 states, though many of those populations died out; it remains established in a handful, including parts of California, Colorado, Connecticut, Washington, and Idaho, and continues to turn up in new waters [2].

It lives in shallow, slow-moving or still water ? ponds, lakes, and slow rivers ? especially where the bottom is muddy and aquatic plants are thick [2]. Tench is very tough: it tolerates warm water, low oxygen, and degraded, murky conditions that many native fish cannot, and it can burrow into the mud to ride out stress and winter [2].

Reproduction:

Tench reach new waters because of people. The main routes are stocking the fish for food or sport, escapes from fish farms and holding ponds (including during floods), the ornamental trade, and illegal releases; from there it can move downstream into connected waters [2]. Because of its potential to spread and harm native fish, tench is banned in a number of places [2].

Tench mature at about three to four years old and spawn in warm, shallow, weedy water in late spring and summer, often releasing eggs in several batches that stick to underwater plants [2]. The fish is long-lived, often reaching 10 to 15 years or more, so even a small introduced group can persist and build up over time [2]. Because new populations start from released or escaped fish, the most important step is keeping tench out of new waters [2].

Impact and Damage:

Tench is not a danger to people; its impact is on the water and on native fish. As it feeds along the bottom it stirs up sediment, which can cloud the water and lower water quality [2][3]. It competes with native and game fish for bottom-dwelling food, and it can carry fish parasites that may spread to other fish [2][3].

Tench can also set off knock-on effects in the food web. Because it eats snails and other small grazers, removing those grazers can let more algae grow on plants and surfaces ? an effect demonstrated in pond and enclosure experiments ? which can shift the balance of a shallow lake [3][4]. Its overall impact in North American waters is still not fully documented, but tench is treated as a serious concern because it is hardy, long-lived, tolerant of poor conditions, and very difficult to remove once established [2][3].

Similar species:

Tench is mistaken for a few other heavy-bodied, dark fish ? the red eyes, tiny scales, and slimy skin set it apart.

Common carp (Cyprinus carpio) is the most common mix-up because both are stout, olive-toned cyprinids, but carp have larger scales and two pairs of longer barbels at the mouth, and they lack the bright red eyes and slick, small-scaled skin of tench [2].

Goldfish (Carassius auratus) can look similar when drab and olive, but goldfish have no barbels at all and larger scales, and they lack the tench's red-orange eyes and heavy slime coat [2].

White sucker (Catostomus commersonii) is a native bottom fish of similar size and dark color, but it has a downward-facing, fleshy sucker mouth with no barbels and lacks the red eyes of tench [2].

Monitoring and rapid response:

Signs to watch for:

1. A stout, dark olive-green, very slimy fish with bright red-orange eyes in weedy, muddy ponds, lakes, or slow rivers [2]
2. A small barbel at each corner of the mouth and very small scales [2]
3. An unfamiliar carp-like fish caught while fishing, especially in the warm months [2]

What you can do:

1. Never release tench, never use it as bait, and never stock it ? dispose of any tench you catch in the trash, not back in the water [2].
2. Never move fish or water between waterbodies, and do not dump pond, aquaculture, or aquarium fish into natural waters [2].
3. Learn to tell tench from carp and goldfish by the red eyes, tiny corner barbels, and slimy, small-scaled skin, so you do not move it by mistake [2].
4. Report any tench you catch or see, especially in a new lake or river, so agencies can respond quickly [2].
5. Removing tench from open, connected waters is an agency-led effort with no easy fix; eradication has generally required treating whole enclosed waters [3].

When to call a professional:

1. Tench found in a lake, river, or any water connected to other waterbodies, where it cannot be contained [2][3]
2. Any control involving draining or whole-water treatment, which requires permits and trained staff [3]
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. Integrated Taxonomic Information System (ITIS). Tinca tinca (Linnaeus, 1758) ? TSN 163348. itis.gov
2. U.S. Geological Survey, Nonindigenous Aquatic Species Database. Tinca tinca (tench) ? Species Profile. nas.er.usgs.gov
3. IUCN Invasive Species Specialist Group, Global Invasive Species Database. Tinca tinca. iucngisd.org
4. Beklioglu M, Moss B (1998). "The effects of tench (Tinca tinca (L.)) and sticklebacks (Gasterosteus aculeatus L.) on planktonic and benthic communities in mesocosms in a shallow lake." Aquatic Ecology 32:229?240. doi.org/10.1023/A:1009946505397

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