Egypt's mariculture industry relies heavily on the seabass sector, a major marine fish species in Mediterranean aquaculture. However, industries face significant challenges such as disease outbreaks, aquatic pollution, and poor water quality. This investigation focused on the parasitism of seabass (Dicentrarchus labrax) in the Damietta Governorate of Egypt, during the period from August 2022 to July 2023. A total of 200 seabass specimens, weighing 25–200 g was collected through random sampling. The fish samples underwent a series of assessments, including clinical evaluations, parasitological investigations, and histopathological analyses. Concurrently with fish collection, various water parameters have been documented, including temperature, salinity, pH, and heavy metal concentration. Parasitological examination revealed a high prevalence (75%) of ectoparasitic protozoa belonging to Trichodina spp. The ectoparasitic copepod Caligus spp. had a total prevalence of 15%, whereas a single Isopoda species had an infestation rate of 2%. The chi-square trend analysis revealed a significant positive relationship between Trichodina prevalence and seasonal changes (P < 0.001). The water parameters tested showed a significant negative relationship between Trichodina prevalence and water temperature (r = -0.99), with a non-significant negative relationship with water salinity (r = -0.75). The heavy metal levels across seasons were higher than the permissible limits. Histopathological changes in infected tissues indicated various types and degrees of lesions, and Scanning Electron Microscopy (SEM) observations highlighted the parasite-host relationship due to parasitic infestations. Overall, abiotic stressors could be responsible for the greater prevalence of parasitic infestations in fish, negatively affecting tissue structure and fish health. Therefore, the strict monitoring of fish farms and biosecurity practices must be implemented to guarantee fish welfare and sustainability.

Fusarium oxysporum, was isolated from a naturally infected captive-reared Labeo rohita in Tripura, India. The infected fish exhibited clinical signs of fungal growth with haemorrhage on the ventral body surface and erosion of the caudal fin. Morphological identification of fungal colony, including microscopic observation of 5-day-old culture, revealed the presence of multi-septate macroconidia and one-septate microconidia. Growth requirement study revealed that the fungus grew best at 30°C and pH 6.0. In vitro studies revealed that the isolate produced extracellular enzymes such as caseinase, lipase, starch hydrolase and DNase, which contributed to its pathogenicity. The partial 18S gene sequence was assigned NCBI accession number PQ013739.1 and confirmed its identity as F. oxysporum. Pathogenicity test by intraperitoneal injection of fungal spore suspension in healthy L. rohita fingerlings with fungal growth at the site of injection; LD50 value obtained by probit analysis was 4.85 × 107 spores/ml. Antifungal susceptibility testing indicated that none of amphotericin B, ketoconazole, itraconazole, clotrimazole, and nystatin produced zones of inhibition against the fungal isolate. In contrast, the fungus exhibited sensitivity to sodium chloride and copper sulfate, with reduced radial growth at salinity levels exceeding 0.5% and CuSO4 concentrations above 25 ppm. Our findings confirm that the F. oxysporum strain is virulent to L. rohita, and highlights the potential utility of salinity and copper sulfate-based management strategies for control of this fungus in aquaculture.