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Figure 50. Tench, AUM 15556, 194 millimeter SL, from Bonner County, Idaho.
The Tench (fig. 50) is a medium-sized fish, typically attaining 60-80 cm TL and weighing 2-3 kg (Moyle, 2002) with a maximum size of about 84 cm and 8 kg (Berg, 1964; Wheeler, 1978). The body is thick, and the caudal peduncle is deep and short. The skin is covered with a thick layer of mucous and is extremely slimy. Scales are small, elongated horizontally, and deeply embedded in the skin, leaving only 1/4-1/3 of the scale exposed (Berg, 1964). Lateral-line scales number 70-120. The head is triangular, generally averaging just <30 percent of SL. The snout is relatively long and the interorbital distance is broad. The mouth is terminal, oblique, and small, with a single barbel at the end of each maxilla (corner of jaw). Pharyngeal teeth are in a single row, typically with either four or five on each side (usually 0,5-5,0 or 0,4-5,0; fig. 51). In juveniles, the pharyngeal teeth may be wedge-shaped and slightly hooked at the tip. There are about 10-16 gill rakers on the first arch, although anterior rakers may be difficult to distinguish from “throat tubercles” (Coad, 2005). The rakers are moderately long. The dorsal fin is rounded and has ii-iv (6-9) rays. The anal fin is also rounded and has iii-iv (5-9) rays. The caudal fin is somewhat truncate (squared). Meristics for Tench are given in appendix B.
Figure 51. Pharyngeal teeth (0,5-4,0) of Tench, AUM
The body is dark green on the back and greenishbrown to dark brown on the sides (fig. 52). The belly typically has a yellowish tinge. Body coloration may vary with habitat, ranging from almost black in shallow, muddy lakes to much lighter in running water or clear water lakes (Berg, 1964). The fins are dark gray to dark brown to black. Eyes are orange-red. An ornamental variety (“Golden Tench”) is orange-yellow or reddish.
Sexual dimorphism is evident but not pronounced. In mature males, the second unbranched ray of the pelvic fins is thickened and is accompanied by a muscular protuberance from the flank. Additionally, these fin rays are longer, reaching past the vent and extending to the anal fin (Berg, 1964; Wheeler, 1978; Coad, 2005).
Figure 52. Tench taken by anglers from two sites in Europe. (A) specimen from Williss Lake, a small
Tench can be distinguished from native cyprinids by the combination of a single barbel at the end of each maxilla and the presence of deeply embedded, fine scales (generally >100 in lateral line). The dorsal fin is short and lacks a heavy spine-like ray, which distinguishes it from Common Carp, Goldfish, and Crucian Carp. Additionally, the Tench has only one pair of barbels (as opposed to the Common Carp, which has two pair), more lateral-line scales (100 or more in Tench, 40 or fewer in Common Carp) and a squared caudal fin (as opposed to a forked one in Common Carp).
An ornamental variety popular in water gardens (called the “Golden Tench”) is orange-yellow or reddish. Tench have been crossed with a number of Old World cyprinids, including Goldfish, Common Carp, Ide, Bighead Carp, and Rudd (Victorovsky, 1966; Schwartz, 1972, 1981). There are no known hybrids with native North American cyprinids.
Males generally mature in their third year, a year before females (Moroz, 1968; Wheeler, 1969; Moyle, 2002; Yilmaz, 2002). Spawning is initiated when water temperatures reach 18 °C (Moroz, 1968; Wheeler, 1969; O'Maoileidigh and Bracken, 1989; Neophitou, 1993; Moyle, 2002). The species aggregates for spawning in shallow, densely vegetated areas where the adhesive eggs are broadcast over vegetation (Rosa, 1958; Wheeler, 1969; Moyle, 2002; Wydoski and Whitney, 2003). The Tench is a batch spawner (Moroz, 1968) and will spawn over artificial substrates resembling vegetation (Sanchez-Herrera and others, 1998). The species is reported to reproduce readily in artificial habitats, for example, in some reservoirs in California (Shapovalov, 1944). Fecundity can be high. In Russia, fecundity varied from 280,000-827,000 eggs (Berg, 1964). In the Danube, fecundity varied from 25,800-357,210 eggs (mean 135,200 eggs; Moroz, 1968). In Poland, fecundity ranged from 18,400-416,100 eggs per female (Pimpicka, 1991). Fecundity increases with body mass and length (Moroz, 1968; Pimpicka, 1991). According to Coad (2005), the maximum fecundity is 1,241,200 eggs in eastern Europe. Eggs are small (1-1.2 mm), yellow or green, and hatch in 3-8 days at 22-24 °C (Rosa, 1958; Wheeler, 1969).
Within its native range, the Tench grows to 7.5 kg (Berg, 1964). Moyle (2002) reported that individuals reach sizes of 60-80 cm TL and 2-3 kg in California. Wild Tench may live up to 11 years (Neophitou, 1993).
The Tench prefers shallow, vegetated areas of lakes and ponds, lower reaches of slow-moving rivers, adjacent backwaters, and oxbow lakes (Berg, 1964; Moroz, 1968; Wheeler, 1969; O'Maoileidigh and Bracken, 1989; Wydoski and Whitney, 2003). Larvae and juveniles inhabit areas characterized by dense aquatic vegetation (Copp, 1997). During summer months, the Tench seeks cooler temperatures by concentrating in deep holes and shaded areas (Moyle, 2002). It tends to be inactive during the day, resting in favored habitats such as dense aquatic vegetation, and forages for food after dusk (Rosa, 1958; Wieser, 1991; Perrow and others, 1996). Seeley (1886, p. 140) states: “It is timid, but crafty, and comes to the quiet surface of the water only in the evening.”
Tench populations are sensitive to loss of vegetated habitat and predation pressure. Channelization that reduces access to vegetated habitats (such as floodplains) has been associated with a significant reduction in juvenile Tench densities (Jurajda, 1995). In a survey of >600 Swedish lakes across a wide geographical area, population densities were much higher in ponds and lakes without piscivores (Bronmark and others, 1995). Additionally, population declines were documented in a Polish reservoir that became eutrophic (Bninska, 1991).
The Tench is tolerant of low levels of dissolved oxygen, and has been shown to survive oxygen concentrations <1.0 mg/L (Wheeler, 1969; Zhadin and Gerd, 1963; O'Maoileidigh and Bracken, 1989; Moyle, 2002). Rosa (1958) reported that the species survived in waters with a low oxygen level that would quickly kill other fishes. The 24-hr lethal oxygen concentration was experimentally derived as 1.2 mg/L (Kraiem and Pattee, 1980). Larvae can survive in waters with pH of 5-10, but a range of 7-9 is optimal (Hamackova and others, 1998).
The Tench is a remarkably hardy fish. Historical accounts from England claim fishmongers would bring individuals to market and return the unsold fish back into water to live (presumably until their next trip to market; Seeley 1886). Live Tench have also reportedly been shipped during winter in boxes of weeds (Rosa, 1958). Individuals from northern Europe can withstand temperatures near freezing; however, optimal temperature for growth was reported between 12-30 °C (Moyle, 2002). Rosa (1958) reported that the species burrows into the mud during winter for protection against the cold where it remains in a motionless condition similar to hibernation. The 24-hr upper lethal temperature for individuals acclimated to 20 °C was determined to be 32.3 °C (Kraiem and Pattee, 1980). The species is also reported to be tolerant to low levels of salinity. Wheeler (1969) reported that the species lives in estuarine conditions in salinities up to 10 ppt in Europe. Moyle (2002) reported survival in salinities as high as 12 ppt. Rosa (1958) reported that the species is found in brackish waters in the Baltic Sea and Tasmania, and calculated a 24-hr lethal salinity of 15.4 ppt.
The Tench appears to be a relatively unselective, generalist predator of invertebrates (especially benthic organisms) in both native and introduced regions. In Ireland, Tench <21 cm (in their fourth year or younger) fed primarily on plankton, whereas larger and older fish consumed mollusks, chironomids, other insects, and plankton (O'Maoileidigh and Bracken, 1989). In Spain, cladocerans, gastropods, small crustaceans, and insect larvae (especially chironomids) and adults were preferred prey items (Perez-Bote and others, 1998; Gonzalez and others, 2000). No differences in feeding habits were noted between the sexes (Perez-Bote and others, 1998). In Australia, nonindigenous Tench consumed chironomids, pulmonate gastropods, cladocerans, amphipods, ephemeropterans, and odonates (Rosa, 1958). In England and France, Copp and Mann (1993) studied food habits of Tench and found that young larvae primarily preyed on cladocerans, whereas older larvae and 0+ juveniles primarily consumed cladocerans and copepods. Giles and others (1990) reported that a population of Tench in the United Kingdom ate a wide variety of benthic invertebrates, including bivalves, amphipods, isopods, trichopterans, and other insects. Perrow and others (1996) reported that Tench fed primarily on chironomids. Prejs (1973) documented trichopterans, chironomids, mollusks, odonates, and ephemeropterans in the diet of Tench from a Polish lake. Bronmark (1994) demonstrated that Tench could dramatically reduce the biomass of snails and bivalves in enclosures and that it also fed heavily on insects and zooplankton. Moyle (2002) reported that the diet of small Tench (6-12 cm TL) consisted of aquatic insect larvae, especially mayflies, damselflies, midges, and caddisflies. Larger fish consumed whatever invertebrates were available, including snails and oligochaetes (Moyle, 2002).
The Tench is native to much of Europe and parts of western Asia (Berg, 1964; Coad, 2005).
The Tench was originally imported into North America from Germany by the U.S. Fish Commission in 1877 and distributed to at least 36 states by 1896. Subsequent stocks probably had other origins. For example, at least one other introduced stock was imported from Italy (Shapovalov, 1944). Intentional stocking and accidental escapes have increased its distribution in the U.S. The Tench has been reported in at least 39 states, but most introductions were unsuccessful, and the species has maintained reproducing populations in only a few areas (Fuller and others, 1999). Although the species was collected in the Richelieu River in the New York/Quebec area following a recent escape in about 2000, it is not yet known to be reproducing in that drainage.
Plate 12. Distribution of Tench in the United States. See Methods for details regarding data
Englishmen angling for Tench and other coarse fish in Bridget Lough, Ireland, 2004. (Photo by Leo G. Nico.)
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