Evaluation of suitable body size for tagging and releasing of yellowfin seabream (Acanthopagrus latus)

The data obtained by tag-recapture method are commonly used to evaluate the fish population dynamics and the effect of stock enhancement. However, to ensure the accuracy of the recapture data, it is necessary to choose the appropriate fish size for tagging. Acanthopagrus latus was employed to conduct two indoor experiments. In the first experiment, the effects of T-bar anchor tags on growth, survival and tag retention of A. latus from four size groups (average body length: 5 cm, 7 cm, 11 cm, and 14 cm, respectively) were examined and the size-dependent effects on survival and tag retention were tested. In the second experiment, different mix ratios of tagged fish to untagged fish from two size groups (average body length: 7 cm and 14 cm, respectively) were set for simulated catching and the catching results were resampled. The differences in catching results between groups were compared, the differences between catching result and resampling result were also compared. The results show that: ① there was no significant difference in the specific growth rate between 7 cm, 11 cm and 14 cm groups. ② the survival rate was size-dependent. All the tagged fish of 5 cm group died in the first week after tagging. The survival rate of other size groups was 77.5%, 92.5% and 100.0%, respectively. The relational expression of logistic regression between body length and the probability of survival was: P=exp(0.099X−6.900)/1+exp(0.099X−6.900). ③ the tag retention rate was high (97.5% in 7 cm group, 100.0% in 11 cm and 14 cm group) but seemed to be unrelated with fish size. ④ there was no significant effect of fish size on the catching result and the resampling result but significant effect of mix ratio of tagged to untagged fish on it. In summary, it is recommended that study objectives and cost should be considered when choosing the appropriate fish size for tagging stocking fishes like A. latus. If the probability of survival needs to be >50.0%, the tagged individuals with body length should be >7 cm. If >75.0%, the body length should be >8 cm. If >95.0%, the body length should be >10 cm. And it is worth further study to know about the appropriate proportion of tagged fish for tagging and releasing.