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Today on Animal Fact Files we’re discussing the tench. This fish is also known as the doctor fish. It has a covering of mucus over its scales that helps prevent injury. When this was first observed, it was believed that other fish would rub up against the doctor fish in order to gather some healing mucus for themselves and that the mucus, in turn, would heal the other fish. This concept doesn't really hold water, and the name has since fallen out of favor for this reason, but the tench is still listed as the doctor fish from time to time.

The tench is Classified in the carp family. It's thick-bodied with barbels on the corners of its thick lips– a feature commonly seen in other carp species, although the barbels in the tench aren't as pronounced. This fish has small scales embedded in its skin and red eyes, although these appear dark brown to black in different color varieties. Tench are usually green-toned with a dark olive back and pale yellow to cream belly, however, there is also a dark variant that appears almost black as well as a golden variant that's sometimes referred to as the banana tench!

Tench are native in freshwater systems of Europe and western Asia, but they've been introduced on most continents around the world. They prefer shallow, warm, still waters with lots of dense aquatic vegetation. This amount of underwater foliage specifically benefits their young. Tench spawn in late spring to early summer, and the males may compete for females. They also chase females to initiate mating. A female tench releases thousands of sticky, green eggs in that dense vegetation and the male quickly fertilizes them. The eggs are nearly indiscernible in this habitat. Spawning occurs every few days for a few months with a single female producing hundreds of thousands of eggs. These take about a week to hatch.

Like their eggs, tench young attach to vegetation and live off their yolk for their first week of life. They eventually start eating algae as they patrol safely inside the protective plants they're born inside. As they grow, they begin to eat larger prey. Adult tench are almost entirely carnivorous and tend to take animals from the muddy bottom of a body of water. These fish eat snails, mussels, and other invertebrates, fish larvae, and more. In their natural range, this helps keep the ecosystem in balance, but in places they've been introduced, they may outcompete native fish species who also eat benthic animals. Introduced tench may also eat so many algae-eating invertebrates that algae blooms occur which reduces the amount of available oxygen in the water. This can be a big issue for native species even though it's not really an issue for a tench.

Not unlike a lot of their family members, tench can live outside of water if they're kept moist. In one example, a tench was caught and mailed to a facility for identification only to arrive in its parcel still alive! After identification, the fish was released back into the wild! Although this is beneficial for tench who may face droughts, it also facilitates introduction to non-native habitats. On the flip side, this makes them easy to keep fresh and sell. Tench are eaten for their meat and also used as sport fish. Their natural predators include larger fish such as perch as well as predatory birds.

Tench spend winters buried in mud at the bottom of the water column. Females live longer and grow larger than the males. These fish average 8 inches, 20 centimeters, in length, but can reach over 2 feet, 61 centimeters, at maximum size. It takes male tench 2-3 years to reach reproductive maturity but females can take upwards of 7 years before they're ready to reproduce. Thankfully, they can live for over a decade if they can avoid threats throughout their lifetime.

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