Dark beginnings

000017Extensive studies of fish communities in three Czech reservoirs (Orlik, Zlutice, Rimov) revealed remarkable diel horizontal migration between the inshore and offshore areas. Fish were dispersed within a reservoir during daytime and aggregated in the inshore zone during nighttime. This can be deduced from the comparison of the day and night beach seining and hydroacoustic survey, and which is further supported by gillnetting and diet analysis (see poster presentation Říha et al., 2007). However, the phenomenon of diel inshore/offshore migration is not completely understood. Main questions are: what proportion of population undergoes such migration, are there differences in migration patterns among species, and are migration patterns same for both sexes and for all size/age classes. To found the answers, we decided to focus our research not on population as a whole (direct catches, hydroacoustics), but on single individuals. Means of acoustic tracking equipment seemed to be appropriate for following fish movements on diel scale.

The study was carried out on the Rimov Reservoir, 210 ha large water supply reservoir located in the south of the Czech Republic. Ten individuals of common bream Abramis brama, one of the most common species of the reservoir, were tagged during June 2007. The implantation was done by a veterinarian experienced in fish tagging. Five individuals were captured in the littoral using beach seine and five were caught in the open water by purse seine. The standard lengths and weights of bream ranged from 300 to 450 mm and from 666 to 1130 g, respectively. The tags used were CT–82–AR–2 with a dry weight of 10 g (Sonotronics, Tuscon, Arisona; the rule of 2% weight of a tag regarding fish weight was followed). The tracking system (receiver USR–96, directional hydrophone DH4, Sonotronics) was borrowed from University of Constance, Germany (Prof. Reiner Eckmann). The bream positions were measured fortnightly during both day and night from August till the beginning of December when the reservoir froze. One positioning was realized in January under ice and one during spawning time.

From the first tracking, nine out of ten tagged bream were localized during each tracking session. Seven out of these nine bream died soon after tagging most probably as positions of these fish were within a diameter of 80 m during all eleven months of tracking. Two survivors originated both from the catches in the open water of the reservoir. One bream moved within a 500m section and the other covered a section four-times larger. Neither of these two bream utilized beachy littoral, however the one was found further from the shore during most of the tracking. Both bream migrated to the deepest part of the reservoir at the dam to overwinter.

As the dataset is based only on two bream we decided not to process the tracking in a serious way ended with any kind of publication. The most probable reasons of death of seven bream were high temperature during tag implantation and too big tag for compressed bream body.

The study was realized within a project Horizontal acoustic surveys and fish behaviour in the open water supported by the Czech Science Foundation in 2007 – 2010.

Říha, M., Prchalová, M., Kubečka, J., Čech, M., Peterka, J., Vašek, M., Frouzová, J., Hohausová, E., Draštík, V., Jůza, T., Kratochvíl, M., Tušer, M., Jarolím, O.: Using telemetry in study of fish inshore/offshore migration in reservoirs – 7th Conference on fish telemetry, 17 – 21 June 2007, Silkeborg, Denmark (poster + book of abstract pp. 110)