Does any fish scale of a fish have the same number of marks? A case study for two <scp>Mugilidae</scp> species

This study evaluates the difference in growth marks in scales from nine body areas of two Mugilidae species from the Gulf of Mexico: Mugil curema and Mugil cephalus. It addresses whether the different body areas show more (or fewer) marks, and which area(s) would be more useful in fish biology studies relying on mark analysis. We hypothesized no differences in the number of marks, neither across body areas nor between sexes. We counted the marks in 3704 and 6374 scales from 117 and 200 adult specimens of M. cephalus and M. curema, respectively. Although we did not count the marks on damaged or regenerated scales, we obtained their percentage of occurrence. We decided to discard the effect of the sampling location on the number of marks using a Kruskal-Wallis test (p = 0.073). The number of scale marks in the different body areas was modelled as a function of sex and body area using hierarchical Bayesian zero-inflated Poisson models. The highest percentages of regenerated or damaged scales were found in the anterior areas of M. curema and the ventral-anterior area of M. cephalus. We found a differential effect of both sex and body area on the number of marks. For M. cephalus, only the sex did not have a probable effect. The number of marks decreased from the anterior-to-posterior areas along the middle area, whereas the ventral and dorsal areas showed the opposite trend. Therefore, the central area was the least variable. For M. curema, males had more marks than females, the ventral area exhibited more marks than the others, whereas the middle area was less variable. Based on these results, we suggest that scales from the middle-central area are more suitable for age studies for both species.