The use of scale increment as a means of indicating fish growth and growth impairment

Growth impairments were studied for three juvenile fish species: the red drum (Sciaenops ocellatus), the orange-spotted grouper (Epinephelus coioides) and the marine medaka (Oryzia melastigma). Fish were subjected to long-term (i) food deprivation, (ii) exposure to benzo[a]pyrene, (iii) hypoxia, respectively. Fish growth (fork length and body weight) and scale increments (circulus number and radius length) were measured. Results of Pearson correlation analysis showed a significant positive correlation between scale increment and growth for the red drum (fast growing) and the marine medaka (small sized) (r = 0.741–0.882, p < 0.001). For the slow growing orange-spotted grouper, a significant correlation was found only between body weight and circulus number (r = 0.406, p < 0.05). Overall, scale circulus number is more consistently linked to fish growth than that of radius length. Importantly, our findings demonstrate that daily circulus deposition on scales of fast-growing red drum is feasible to indicate daily growth of the fish. Moreover, scale increment exhibited the strongest correlation with fish growth as compared to that of serum T3 level and muscle RNA:DNA ratio. Our data support the use of scale increment as a means of indicating fish growth and growth impairment, particularly for the fast growing fish.