Allometric growth patterns in longfin yellowtail <scp><i>Seriola rivoliana</i></scp> (Valenciennes 1833) larvae and juvenile under hatchery conditions

Survival rates of longfin yellowtail, Seriola rivoliana, during the early life stages, remain critically low, presenting a considerable challenge to large-scale production. The allometric growth patterns of S. rivoliana were analysed under hatchery conditions from 0 to 30 days post-hatching (DPH). A total of 620 specimens were examined, including the yolk sac (n = 90), preflexion (n = 240), flexion (n = 90), postflexion (n = 150) and early juvenile stages (n = 50). The total length (TL) was used as the independent variable, and six body segments (head length, eye diameter, mouth length, body depth, trunk length and tail length) were analysed as dependent variables. Newly hatched larvae measured 2.62 ± 0.02 mm TL, exhibiting a closed mouth and anus with a large yolk sac containing a single oil globule. Morphological development progressed gradually with the onset of exogenous feeding, increased external pigmentation and swim bladder inflation during the first 10 DPH. Body thickening and notochord flexion were characterised in subsequent stages. The growth in TL followed an exponential pattern relative to age (TL = 2.316 e^0.083DPH; r² = 0.99), with a specific growth rate of 8.3% per day. Biphasic allometric growth patterns were identified across all measured traits, with inflection points between 5.22 and 8.69 mm TL. Paired t-tests revealed significant differences (p < 0.05) in growth coefficients (b values) before and after the inflection points, confirming the existence of distinct ontogenetic phases. These findings provide insights into early developmental strategies and contribute to the optimisation of larval rearing protocols for this species.