White spot disease, caused by the parasitic Ichthyophthirius multifiliis, induces mortality exceeding 80% in intensive aquaculture systems, resulting in global annual losses of >$1 billion. Current chemical controls (e.g., formaldehyde) face environmental persistence and drug resistance challenges. This study developed a synthetic isoquinoline derivative, BHTCA (s)-2-tert-butoxycarbonyl-7-hydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid), as a biodegradable alternative against I. multifiliis in grass carp (Ctenopharyngodon idella). In vitro assays revealed stage-selective efficacy: theronts exhibited higher susceptibility (EC₅₀ = 0.10 mg/L at 4 h) than tomonts (EC₅₀ = 0.40 mg/L at 24 h), with BHTCA disrupting ciliary structures and compromising cyst integrity. In vivo trials (0.6 mg/L) reduced parasite loads by 78.1% and host mortality by 66.7% versus untreated controls. Acute toxicity tests confirmed a high therapeutic index (LC₅₀ = 16.75 mg/L; >167× effective concentration). With demonstrated efficacy, low production cost, and minimal eco-toxicity, BHTCA presents a sustainable strategy for Ichthyophthiriasis management in aquaculture.

Understanding the early life history of the endangered largemouth bronze gudgeon (C. guichenoti) is crucial for optimizing artificial rearing and conservation strategies. To provide a unified scientific basis for these efforts, this study characterized the ontogeny of C. guichenoti larvae from hatching to 30 days post-hatch (dph) by integrating analyses of allometric growth, digestive histology, and enzyme activities. Morphometric analysis revealed that total length increased exponentially with age. Positive allometric growth was observed in swimming organs (pectoral, dorsal, and caudal fins) and eye diameter, indicating a priority in developing locomotion and sensory capabilities for survival in lotic environments. Histological examination showed that the digestive tract became patent by 4 dph, and functional structures were established by 5 dph, coinciding with yolk sac absorption and the initiation of exogenous feeding. The digestive system reached near-adult maturity by 20 dph. Enzyme activity profiles showed that trypsin activity increased continuously, peaking at 30 dph, while amylase and alkaline phosphatase peaked at 20 dph. Lipase activity remained low until a slight increase at 30 dph. These results indicate that C. guichenoti larvae adopt a “locomotion-first” strategy, with the digestive system becoming functional at 5 dph and maturing by 20 dph. These findings collectively define the critical windows for artificial propagation, specifically guiding the timing of initial feeding and feed formulation.