Tench

Description The tench or ‘doctor fish’ (Tinca tinca) is an alien freshwater and brackish water species of the Cyprinidae family, introduced into the Prespa Lakes in the 1980’s. Tench are a heavy-built, thick-set fish with a small barbell at each corner of the mouth. The colour ranges from deep blackish-olive to pale golden tan, with a bright reddish eye. The body is slimy, with small scales being covered by a thick layer of mucus. Very large specimens may reach 800 mm.   Biology The tench is most often found in still waters with a clay or muddy substrate and abundant vegetation. This species is rare in clear waters across stony ground, and is absent altogether from fast-flowing streams. It tolerates water with low oxygen concentrations, being found in waters where even carp cannot survive. Tench feed mostly at night with a preference for fauna like chironomids (insects) on the bottom of eutrophic waters and snails and pea clams in well vegetated waters. It can live up to 20 years. It spawns for the first time at 2-6 years and 70-250 mm SL, females a year later than males. The pelvic rays аре more robust, longer and extending beyond the anus in the male. It spawns in May-October, in Central Europe usually in June-July, at temperatures above 19⁰C, mostly at 22-24⁰C and as many as 300 000 eggs may be produced. Several males follow each female, which releases eggs in several portions above vegetation. Females may spawn 1-9 times each year, every 11-15 days, if suitably warm weather persists. A high embryonic mortality is observed when the temperature fluctuates strongly. The larvae and juveniles are restricted to dense vegetation. They are tolerant to low oxygen concentrations and salinities of up to 12 ‰. They feed upon detritus, benthic animals and plant material. The adults often feed predominantly on molluscs. Breeding takes place in shallow water usually among aquatic plants where the sticky green eggs can be deposited. Growth is rapid, and fish may reach a weight of 0.11 kg within the first year.   Distribution They are found throughout Eurasia from Western Europe including the British Isles east into Asia as far as the Ob and Yenisei Rivers. They are also present in Lake Baikal and normally inhabit slow-moving freshwater habitats, particularly lakes and lowland rivers. They were only recently introduced into the Prespa Lakes.   Fishing significance No. It has very low population number.   Impacts Impacts specific to tench are difficult to find, as this species is often lumped together with others in the Cyprinidae family, such as koi and common carp. The ability of tench to survive in a degraded environment causes some confusion, as it is unclear whether they contribute to this degradation or simply inhabit a niche that native fish cannot occupy. Some experimental studies showed that they can increase periphyton (algal) biomass through selective predation on gastropods (clams, snails), which keep periphyton under control through grazing. This ‘trickle-down’ effect could have negative impacts on aquatic communities if it occurs to a significant extent in the wild. There is no evidence that they affect other fish directly, however, a number of studies have implicated them in water quality decline.   Management Possible ameliorative fishing.