More or less uniformly colored yellow, orange and red dorsal
fins without additional marks occur in many freshwater fish, first of all in
cyprinid fish such as roach, Rutilus
rutilus, rudd, Scardinius
erythrophthalmus, and more. In roach, for example, the rear part of dorsal
fin is constantly red, this fin can be folded and rise. Theoretically, roach can fold its red dorsal fin with the approaching of natural
predators (like perch, Perca fluviatilis,
or pike, Esox lucius), becoming
cryptic, and rise it using as red signal to contact with schooling and sexual
experiment, the hypothesis that perch can eliminate roach with the bright red
dorsal fins was verified.
experiment was carried out in an enclosure fenced off the shore by three nets
2x2x2 m of 40 cm height. The maximum depth in the enclosure with pure sandy
bottom was 30 cm. From the side of open water, dense thickets of pondweed (Potamogeton) were
located. 25-30 roach (4-5 cm standard length) and 3 perch (10 cm each) were
released into the enclosure. After adaptation (1 day and 1 night), roach formed
an actively moving school and fed from the bottom. 3 perch were in pondweeds and sometimes,
through 5-15 min, attacked the roach
three artificially red-marked roach were added to the natural roach.
Experimental fish were caught separately by net and marked with the assistance
of needle with the bright red thread (1 cm) tied to the base of the dorsal fin. These fish quickly joined an available school
of natural roach performing synchronized actions. However, after two hours of observations these red-marked roach were eaten by perch.
an experiment was closed because of ethical demands and an evidence of its results.
Previous experiments with artificial
fish models have shown that roach can use their bright red dorsal fins in
schooling and other forms of social behaviour. However, this color signal does not evolve into the trait because of the lack of clear
sexual dimorphism in roach (selection of males) beeing at the same
time under the strong predation pressure.
more information, please read Roach prefer red-marked school mates