The Sultanate of Oman’s aquaculture industry is expanding with an on-going assessment of potential new fish species for culture. The king soldier bream, Argyrops spinifer (Forsskål) (Sparidae), is one such species that is under consideration. During a routine health assessment of specimens caught in the Sea of Oman throughout the period November 2009 to March 2011, a number of gill polyopisthocotylean monogeneans were recovered.

Ammonia is a common aquatic pollutant and is toxic to fish. The main nitrogenous compound excreted by fish is also ammonia, which may reach toxic concentrations in high-density fish culture, reducing growth and productivity. In this study, changes in gill structure and mucus cell distribution were examined when juvenile Nile tilapia, Oreochromis niloticus, were exposed to chronic and acute concentrations of ammonia. For chronic exposure, the tilapia were exposed to daily increments in ammonia concentration rising from 0 mg L-1 ammonia on Day 0 to 5 mg L-1 by Day 8; the experiment was terminated on Day 9. There were obvious histopathological changes to the gills over this period including hypertrophy and fusion and of the secondary lamellae. The mucus cell number in gill histology sections were observed to increase until Day 6 then decrease thereafter until the termination of the trial. For the acute exposure, juvenile tilapia were exposed to 5 mg L-1 ammonia for 5 hours. Hourly changes to the gills were monitored, with notable changes to gill filaments and an observed increase in the mucus cell number over the duration of the experiment.

The Omani abalone, Haliotis mariae, is the only species of abalone found in Omani waters. Given the rarity of this species and the high price it can fetch on the market (US$ 150 kg-1 dry weight), the wild abalone fishery in the Dhofar region has been regarded as a valuable income source for the past decade. The present study was undertaken set to investigate the mortality of abalone held at the Mirbat Research Center, through bacteriological and histopathological based investigations and challenge tests. Only the adult wild abalone that had been kept for a year in the hatchery, visually, appeared to be clear of disease symptoms. Infected individuals typically were swollen around the mouth, had fluid tinged with blood, bubbles in the intestines, and, very weak adhesive strength. The foot area (muscle) of diseased animals was noticeably very soft and individuals that were seen lying upside down on the bottom of the tank subsequently died. On dissection, the intestinal organs released bubbles and a foul smelling odour. Identification of the isolated bacteria using various identification methods indicated that individuals were infected with Staphylococcus sciuri. Histopathology of infected individuals revealed spongiosis of the tissues with evident bacterial infection. Neither of these histopathological conditions were seen in healthy abalone. The study concludes that the bacterium Staphylococcus sciuri may be the likely cause of abalone mortalities.

In 2016, Barramundi, Lates calcaifer juveniles imported from Thailand to the aquaculture station at Sultan Qaboos University exhibited remarkable external clinical signs of losing scales, severe muscle necrosis, and ascites. Fin rot was also observed in the diseased fish. The mortality reached more than 90 % and it was assumed that certain bacterial species were associated with the diseased fish. The objective of the present study was to investigate the infection route of disease, revealing the causative agent and finding the most effective antibiotic treatment. The suspected pathogen vehicle was mosquito larvae in the outlet of the culture tank as bacterium was isolated inside the mosquito larvae. Primary, traditional phenotypic tests and the vitic test confirmed that the bacteria were pathogenic Aeromonas sobria and Lactococcus garvieae. Eight of the most commonly used antibiotics in the aquaculture industry was used for antibiotic susceptibility test. It showed that that Gentamycin was the most effective antibiotic while the most effective environmentally friendly source was henna, Lawsonia inermis, at a concentration of 10%.

Killifish inhabit shallow freshwaters where they tolerate varied salinity regimes, resist low oxygen, and are known for their ability to control mosquito larvae populations. Environmental conditions strongly influence parasite transmission. There is a lack of studies on pathogenic killifish infections in Oman, despite their consumption by local populations. This study highlights the significance of understanding parasitic infections in killifish due to their potential impact on ecosystem health, water quality, and risks to other organisms and human health. The aim of this research is to investigate the infection prevalence of metacercariae Clinostomum in Omani killifish, Aphaniops stoliczkanus and A. kruppi , along salinity gradients and other water parameters. Furthermore, the infection intensity and incidence in relation to fish size, gender, and fish organs will be studied. Five sites were sampled using trap nets from the Al Amerat region to the mangrove area in Al Qurum. Infection prevalence decreased progressively toward the sea: 75%, 56%, 10%, 0%, and 0% at sites A through E, respectively. Water analysis revealed higher total dissolved solids and turbidity at sites A, B, C, and D. Larger killifish and females showed greater susceptibility to Clinostomum spp. infection, with the trunk region being the most commonly infected body part (36% of parasites). These findings contribute valuable baseline data for future monitoring efforts in Omani freshwater ecosystems and underscore the importance of considering environmental parameters in fish health assessments and potential biological control applications. Further research incorporating seasonal variation, intermediate host distribution, and molecular characterization of parasites would strengthen our understanding of this ecologically important host-parasite system.

Clinostomum spp. are digenetic trematodes characterized by high genetic diversity and complex life cycles involving three hosts: snails, fish, and birds. These parasites signifi-cantly impact freshwater ecosystems, particularly when infecting fish as the second in-termediate host. Despite their ecological importance, no studies have examined the morpho-molecular characteristics of Clinostomum spp. from freshwater fish species in Oman, limiting our understanding of their life cycles and potential threats to the local aquatic environment. The present study investigated Clinostomum spp. infecting killifish (Aphaniops sp.) from four locations in the Muscat region of the Sultanate of Oman (Al Amirat, Al Khoud, Darsait, and Al Bahayes) using integrated morphological and mo-lecular approaches. Metacercariae were morphologically characterized through organ measurements and molecularly identified using mitochondrial cytochrome c oxidase subunit I (COI) sequences. Morphometric analysis revealed that Clinostomum parasites from Omani freshwater bodies were consistently smaller than the established species C. complanatum and C. marginatum reported from other regions. DNA sequence analysis indicated that the Omani specimens were closely related to, but distinct from, Turkish C. complanatum, suggesting they may represent a potentially novel species or subspecies.