DETERMINATION OF AGE AND GROWTH OF CATLA CATLA (HAM.) FROM OPERCULAR BONES

Powel (1981) mentions that the validity of any skeletal methods for the study of growth depends in the occurrence of isometric growth between skeletal structure and body length of fish. The determination of age and growth of fish from opercular bones is well established in fishes of temperate waters and has been found to be more satisfactory than other methods e.g. from scales, vertebrae, spines, or hard parts (Le Cren 1947, Frost and Kipling 1959, Shafi and Maitland 1971, Mann 1973). The present study deals with the role of opercular bones of C. catla in determining the age and growth of the species. Opercular bones were collected from 194 specimens of C. catla ranging from 21 cm to 80 cm in total length. The fishes were collected from the pond of BCSIR Laboratories, Rajshahi. Total length of fishes were measured by means of wooden length measuring board graduated into cm. The length of the opercular bone was taken by a small scale divided into cm. Length and age were fitted to the Von Bertalanffy growth curve by “Ford-Walford Plot”. Treatment of opercular bones: Frost and Kipling (1959) method was applied for treatment of opercular bones. The left opercular bone was removed from each specimen with a scalpel and immediately put into hot water and cleaned easily with a cloth. The age rings on the opercular bones were visible drying. The opercular bones were numbered and stored in envelops. The age rings on the opercular bones can be seen with nacked eye but a more reliable count of them could be made holding the opercular bone against a narrow, sharp light source in a dark room. The maximum distance between the centre of the opercular bone and its margin was taken as the length of the opercular bone. The relationship between bone length (Y) and toatl length (X) of C. catla was established. The age groups were considered as 0+ (less than one year), 1+ (less than two years), 2+ (less than two years) and so on. The yearly increase in length for each of the age groups was found by subtracting the mean length of each agegroup from the next older age-groups. The total length of the fish varied from 21 to 80 cm. No complete age ring was visible in the opercular bone of 0+ age group. Age ring was first visible in the fishes of 1+ age group. In a single fish of the sample, measuring 80 cm in length, seven (7) complete rings were counted; which seems that in the total collection that specimen was the oldest one. Age group 2+ was the most dominant one with a total number of 64 specimens. In the opercular bone of a single fish of 80 cm length a total number of 7 complete age rings were the oldest specimen in the sample. This findings agrees with Shafi et al. (1977) who found mean length for a 8 years old C. catla from Kaptai lake as 12.40 cm. They obtained no ring formed on scale after the completion of first year of life. However, this is not true for opercular bone. The formation of first annual ring on opercular bone after completion on one year of life is not related with spawning. Shafi et al. (1977) have found that the first ring on scale appears during spawning time at two years of age.