Evaluation of external anchor tags for estimation of growth and survival in Tilapia (Oreochromis shiranus chilwae Trewavas)

Abstract

The effects of tagging with Floy FD-68B T-bar anchor tags on estimates of growth in Tilapia (Oreochromis shiranus chilwaei were investigated in a pond and in a field experiment. In the pond experiment, mean length increments of tagged and marked fish were compared. In the field experiment growth of tagged and "untouched" individual wild fish were compared by measuring scale circuli spacing (Circ.), which is correlated to instantaneous growth rate. Length increments of tagged and untaggedlmarked fish were not significantly different in either experiment. In the pond experiment, the total mortality rate in the small tagged fish was significantly higher than in the marked fish. The recoveries of tagged fish in the pond experiment increased with fish size. Recoveries exceeded 80% at lengths over 13 em TL. The ratios of tagged to marked recoveries were 1.02 and 0.74 forlarge and small fish respectively. The study shows that tagging of Tilapia with Floy anchor tags does not in general alter the growth rates of the fish