Oxytetracycline (OTC) is commonly used for marking otoliths because it binds with proteins in the blood and it is incorporated in nearly forming and mineralizing bone and cartilage. Otoliths are composed of calcium carbonate in the aragonite form, and they are never desorbed, so they provide a more complete growth record of the fish than do other structures. Otoliths grow by rhythmic deposition of aragonite crystals with a protein matrix with daily and annual. For each species, the method of estimating age by counting annuli must be validated to prove that one whole annulus is equal to one year of growth. There are several ways to validate age but tagging can provide more accurate information on annulus deposition. Our primary main objective was assessing forming OTC mark on the otolith tissue. Second, we compared marks produced by the OTC during the experiment.

The study site was in the Ghare Sou in the Gorgan at northeast of Iran. A total of 250common carp (Cyprinus carpio) fingerlings (2-4 g, 10-12 mm fork length) were used in this study during 2009 - 2013. The OTC was dissolved in the water, and fish added for 6 hours.Fifty fish were exposed to a three times marking regime.

For mark evaluation, otoliths were removed, dried and mounted on glass slides. The otoliths were studied under a compound microscope that was equipped with a lamp that supplied the UV radiation. Otoliths were examined by two readers. The distance between marked rings and circles measured in the Gorgan and Oman Fisheries Research and laboratories.

The common carp marked in this study exhibited good survival during and after marking with negligible mortality. Fortunately, the results show that OTC rings formed on otoliths and are detected clearly. This study is the first attempt to tag otoliths with OTC in Iran. The final report of this study will be published soon. It is hoped that in future the same studies will be completed for other species.