The five fish are being built by Professor Huosheng Hu and his robotics team at the School of Computer Science and Electronic Engineering, University of Essex. He hopes to release them into the water by the end of next year.
The fish, which cost around £20,000 to make, will measure 1.5 metres (1.6 yards) in length (roughly the size of a seal) and swim at a maximum speed of about one metre (1.1 yards) per second.
He said: "I am incredibly excited about this project. We are designing these fish very carefully to ensure that they will be able to detect changes in environmental conditions in the port and pick up on early signs of pollution spreading, for example by locating a small leak in a vessel.
SF fans may recall that, in his 2002 story Slow Life, science fiction author Michael Swanwick writes about robot fish who help explore distant worlds:
The Mitsubishi turbot wriggled, as if alive. With one fluid motion, it surged forward, plunged, and was gone.
(Read more about the Mitsubishi turbofish)
Also, in the movie Blade Runner, Deckerd is investigating what looks like a kind of scale. When he asks around on Animoid Row, he is told that it is a manufactured object. Take a look at the top of the scene taken from the movie (below); it clearly shows a mechanical, artificial fish.
(Robotic fish from Animoid Row in Blade Runner)
In the book from which the movie is taken - Philip K. Dick's Do Androids Dream of Electric Sheep?, one of the androids wears a fish scale coat. Artificial, of course. In the future world imagined by Dick, and artistically filled out by Ridley Scott in the movie, all of the animals on Earth had died, or were dying. Only robotic animals remained, as pets.
Update: See also this robotic fish depicted in the series Red Dwarf - thanks Yossi.