Surveillance of Catastrophic Fish Decline in Lake Qarun: Endurance of the Egyptian Sole (S. aegyptiaca Chabanaud, 1927) Against Isopod infestation

Lake Qarun in Egypt has experienced a catastrophic decline in fish populations, with isopod infections identified as one of the major contributing factors. A ten-season surveillance study was conducted to investigate disease patterns, fish species composition, cymothoid species identification, and parasite behavior across the eastern, middle, and western regions of the lake. The study examined clinical signs of disease, prevalence and intensity indices, and the relationships between fish species and both free-swimming (FSCs) and permanent cymothoid stages. A total of 1,901 Tilapia spp., 2,056 Solea aegyptiaca, and 7,650 Atherina spp. were captured, along with 823 FSCs. The isopod species infesting Lake Qarun was morphologically identified as Livoneca redmani. The prevalence of isopod infestation among Tilapia spp., S. aegyptiaca, and Atherina spp. was 43.46, 12.59, and 5.27%, respectively. Tilapia spp. and S. aegyptiaca exhibited both gill and skin infestations, whereas Atherina spp. showed only skin infestations. Double-gill pouch infestations were observed exclusively in Tilapia spp., with a branchial infestation intensity index ranging from 1.2 to 2.0, compared to 1.0 in S. aegyptiaca. Tilapia spp. demonstrated a strong association with the parasite, while S. aegyptiaca and Atherina spp. exhibited weaker associations. The decline of Tilapia spp. was correlated with a reduction in FSC numbers. The study revealed that Tilapia spp. serve as permanent hosts, whereas S. aegyptiaca and Atherina spp. act as transitory hosts. Notably, S. aegyptiaca could become a permanent host in the absence of Tilapia spp. A stock improvement program for S. aegyptiaca could help mitigate the decline in Lake Qarun’s fish populations by sustaining fish stocks capable of surviving L. redmani infestation, thus preserving economic value.