Cradle or plague pit? Illuminated cages increase the transmission risk of parasites from copepods to coregonids

Cestode-infested copepods (Cyclops abyssorum) are strongly attracted to illuminated cages used for rearing juvenile whitefish (Coregonus sp.) in Lake Fuschlsee, Austria. In the submersed cages, artificial light at night concentrated zooplanktonic organisms about 40-fold compared to normal abundances outside cages, thus ensuring a sufficient food supply for planktivorous fish. However, inside the cage the average prevalence of Triaenophorus spp. and Proteocephalus spp. within C. abyssorum was 5.4 and 5.1 times higher than in the pelagic zone of the lake. In absolute numbers 3.83 Ind. L− 1 within the cage hosted a procercoid compared to 0.04 Ind. L− 1 in the open water. After three months in the cage, 90% of the coregonids harboured on average 5 plerocercoids of Triaenophorus crassus and 100% were parasitized by Proteocephalus exiguus. Total length and weight of whitefish with T. crassus plerocercoids were lower than of those without the parasite. Rearing fish for stocking purposes in illuminated cages increases the risk of cestode accumulation in fish populations. We discourage from using this method in lakes with cestode-problems or suggest to at least release fish before procercoids of Triaenophorus spp. occur in the zooplankton to prevent massive infestations inside the cage.

► Illuminated cages strongly accumulate cestode-infested copepods.
► The risk of an infestation for reared fish increases with time.
► Juvenile whitefish infested with Triaenophorus crassus are smaller and lighter.
► To minimize the risk of Triaenophorus spp. infections, fish should be released early.