The Eurasian ruffe exhibits a coloration and markings similar to those of the walleye. With an olive-brown to golden-brown color on its back and paler sides, it showcases yellowish-white undersides. Typically measuring around 25 cm (9.8 in) in length, the ruffe can be highly aggressive despite its relatively small size. It possesses a large, spiny dorsal fin that acts as a deterrent to predators. The distinctive features of the ruffe include its continuous dorsal fin and slightly downturned mouth.
Natively, the Eurasian ruffe can be found in the basins of the Caspian, Black, Aral, Baltic, and North Sea, as well as in Great Britain, parts of Scandinavia, and certain regions of the Arctic Ocean basin. However, the ruffe has been introduced to other parts of the world, particularly the Great Lakes in North America. Unfortunately, this introduction has resulted in detrimental consequences, as the ruffe has outpaced other species in reproduction and is rapidly invading new habitats.
In its native Eurasian range, the ruffe primarily feeds on zoobenthos, which includes chironomids, small aquatic bugs, and larvae. These food sources are predominantly found in the benthic zone of the water column. Remarkably, when the ruffe was introduced to the Great Lakes, it maintained its diet, exacerbating the competition for resources among native species.
The reproductive capacity of the Eurasian ruffe is remarkably high. Typically maturing in two to three years, some ruffe populations in warmer waters can reproduce as early as the first year of life. A single female ruffe can lay an astonishing 130,000 to 200,000 eggs annually. During the spawning season, ruffe migrate from deep, dark waters to warmer shallow areas. The primary spawning period for ruffe occurs from the middle of April through June.
The life cycle of the Eurasian ruffe begins as an egg, like many other fish species. The size of the eggs varies depending on the female's size, ranging from 0.34 to 1.3 mm (0.013 to 0.051 in) in diameter. If a female lays a second batch of eggs in the same season, they tend to be smaller than the initial batch. Hatching occurs within 5-12 days at temperatures between 10-15 °C (50-59 °F). After hatching, the embryos enter the embryonic/juvenile stage, where they grow to approximately 4.5 to 5.0 mm (0.18 to 0.20 in) in length within a week. The young ruffe then become active swimmers and feeders. They do not form schools at this early stage.
The ruffe gradually matures over a span of 2-3 years, reaching full maturity at around 20 cm (7.9 in), with a maximum length of 29 cm (11 in). Growth is more pronounced in clear, brackish waters. Generally, both female and male ruffe have a lifespan of 7 to 11 years.
Ecological Effects: The introduction of the ruffe into Lake Superior has had severe ecological consequences. This invasive fish species not only competes with other fish for food supply but also reproduces at an accelerated rate. As a result, the ruffe's population growth diminishes the availability of food resources for native fish species. The ruffe's adaptability to various habitats and temperatures, coupled with its exceptional ability to detect water vibrations through specialized organs called neuromasts, gives it a competitive edge over other fish. If left unchecked, the ruffe has the potential to disrupt the delicate ecosystem of the Great Lakes, particularly Lake Superior.
Control Measures: Scientists and fishery managers have been actively searching for effective methods to control the ruffe population in the Great Lakes. Initially, efforts focused on increasing the populations of natural predators of the ruffe, such as Walleye and Northern Pike, to limit its proliferation.
Other control methods under consideration include chemical and poison control. Large schools of ruffe can be targeted for poisoning; however, if even a few individuals survive, they can quickly repopulate. Chemicals can be tailored to specific fish species, with the chemical lampricide TFM being effective against ruffe while sparing other fish. It is crucial to prevent the movement and repopulation of surviving ruffe individuals to mitigate further spread. Researchers are investigating the use of pheromones, particularly the ruffe's alarm pheromone, which has the potential to repel the species from natural mating areas and contribute to population decline.
Apart from its invasion in the Great Lakes, the ruffe has also become abundant in Loch Lomond, Scotland. It is believed that the fish was introduced to the loch by pike anglers using live bait. The increase in ruffe population poses a threat to the endemic whitefish population known as powan (Coregonus lavaretus) as the ruffe preys on their eggs. The ruffe has also become a primary food source for predators such as the great cormorant, grey heron, and northern pike in the area.