Allometric Growth and Condition Factors Throughout an Annual Life History of Landlocked Ayu Plecoglossus altivelis altivelis in Haidzuka Dam Reservoir

Ayu Plecoglossus altivelis altivelis is a commercially and recreationally important freshwater fish in Japan. This species is categorised into two ecological forms: amphidromous migrating between rivers and sea and landlocked migrating between rivers and lakes or dam reservoirs. Despite its ecological and economical values, growth analysis has been developed limitedly in the early life history of particularly the amphidromous form, and the growth potential of the landlocked form throughout an annual life history remains almost unknown. This study investigated the allometric growth and condition factors along with their variation among months and life stages of artificial landlocked ayu in Haidzuka Dam reservoir and its connected Tabusa River. A total of 281 individuals were collected monthly from October 2017 to September 2018, and their total length (TL) and body weight (BW) ranged from 1.11 to 14.98 cm and 0.001 to 32.148 g, respectively. Although the allometric coefficient (b) of the length–weight relationship varied from 2.71 to 3.44 among months, it indicated overall positive allometric growth (b = 3.83) calculated from the equation BW = 0.0013TL^3.83. The growth pattern altered significantly among life stages from positive allometric for larvae, juveniles, and youngs to negative allometric for adults. The calculated asymptotic length and growth coefficient of the logistic growth model were 17.71 cm and 3.91 year⁻¹, respectively. Significant differences were found in allometric (K_A) and Fulton's (K_F) condition factors among months, while there was no difference in relative condition factor and relative weight (W_R). Life stage-specific analysis also revealed that the adult stage was nutritionally healthier compared to other life stages based on K_A and K_F. Only K_F showed significant correlations with both TL and BW, making it an ideal condition factor to predict the wellbeing of ayu in this reservoir. Moreover, the Wilcoxon signed-rank test for W_R showed no significant divergence from 100, suggesting a balanced habitat in this reservoir for ayu. These findings will be crucial for further studies and to develop appropriate strategies for the sustainable management of artificial landlocked ayu in dam reservoirs.