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 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.

The hypothesis that roach can use red dorsal fin as an important signal for school mates is verified in these experiments.

Roach were trained in aquarium to enter the corridor of 75 cm length, 25 cm width and 20 cm height made of 2 mm transparent plastics. The inner walls of the corridor were treated with the sandpaper to be frosted. The corridor was divided into the three sections of 25, 25 and 25 cm length. Roach were trained (1 week) to feed from the small feeder in the center of the third section. After feeding, fish could get out from the corridor get his sides or go back. 5 fish of about 49-51 mm standard length with red dorsal and other red fins were used. Both walls of the third  section could go up to replace them. These walls were frosted, when roach fed, and had artificial models of fish, when they were tested on schooling preferences. The third experimental section was divided into the three measuring areas: 10 cm area near to the first wall, 5 cm neutral area and 10 cm area near to the second wall.

Bodies of the fish models were made of high quality sticking foil of silver color, WTP Inc. # 801. Dorsal, caudal and anal fins were made of neutral semitrasparent sticking film (see Fig. 1). No pectoral and pelvic fins were imitated. In half of the models, dorsal fins were made of sticking foil of red color, WTP Inc. # 405. All models were equipped with the 3D sticking eyes with black pupils on the silverish background.

Figure 1. High quality artificial fish like models with bodies made of silver sticking foil. Dorsal, caudal and anal fins are made of semitransparent sticking film. In half of the models dorsal fins are made of red sticking foil.

In the center of the first replaceable wall, 6 fish models with the semitrasparent fins were sticked to imitate an oval like small school. The standard size of all models was 5 cm. In the center of the second replaceable wall, 6 fish models with the red dorsal fins were sticked in the same order.

To test schooling preferences, one of the trained fish (each of 5 fish) entered the corridor, passed him to the third section and rushed to the feeder. However, not finding habitual feed, the fish was forced to draw its attention to the left or right school imitations (panels). The first choice, that is entering the left or right areas nearest to the schools, and the time spent (within 2 min period) in these areas were fixated for statistics.

In the most laboratory tests as expected, roach moved first to the red-marked individuals and spent more time near them (37 tests in total, sign test and Wilcoxon signed-rank test, p < 0,05). The same results were obtained in the nature with the same panels used with the two-flank net guide corridor: 41 tests in total, naïve roach of 3-5 cm standard length, first choice sign test only.