Characteristics and risk assessment of cryptocaryoniasis in large yellow croaker (Larimichthys crocea) at different densities in industrialized aquaculture

With the introduction and advancement of sustainable development in aquaculture, traditional fish farming methods are gradually transitioning to advanced industrial farming models. To explore the occurrence patterns and risks of cryptocaryoniasis in industrialized high-density aquaculture, we used a non-lethal dose of Cryptocaryon irritans theronts (400 theronts/m(3)) to infect Larimichthys crocea at different rearing densities [low-density group (0.088 kg/m(3)), medium-density group (0.88 kg/m(3)), and high-density group (8.80 kg/m(3))]. We explored the relationship between L. crocea density and the growth rate of the C. irritans population by observing the differences in growth, feeding, and survival rates of the L. crocea in each group and the changes in water quality and the number of C. irritans tomonts during the rearing process. We also evaluated the risks associated with the occurrence of cryptocaryoniasis in aquaculture. The results showed significant differences in the relative levels of the first round of infection among different density groups of L. crocea, which affected the outbreak speed of cryptocaryoniasis and the maximum relative tomont number (RTN) of L. crocea. Specifically, the outbreak speed of cryptocaryoniasis from fast to slow was as follows: medium-density group>low-density group>high-density group. The RTN when all L. crocea had died in each group was 12.82 (medium-density group), 49.7 (low-density group), and 106.58 (high-density group). The occurrence of cryptocaryoniasis reduced the feeding and growth rates of L. crocea in all groups, and the effect was most pronounced in the second week and the late stages of infection. The occurrence of cryptocaryoniasis caused abnormal swimming behavior in all three density groups. The main manifestations of C. irritans infection in L. crocea included rubbing the snout against the barrel wall, body imbalance, dispersed swimming, and delayed responses. The deterioration of water quality was most pronounced in the high-density group, but the outbreak speed of cryptocaryoniasis was the slowest in the highdensity group, suggesting that no significant correlation existed between the deterioration of water quality and the outbreak speed of cryptocaryoniasis. The outbreak speed of cryptocaryoniasis varies depending on the fishrearing density, and the progression and risk of cryptocaryoniasis are not proportionally correlated with the rearing density of L. crocea.