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 mates.

In this experiment, the hypothesis that perch can eliminate roach with the bright red dorsal fins was verified.

An 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 perhaps unsuccessfully.

Then 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.

Thereafter, 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.

For more information, please read Roach prefer red-marked school mates