Marine leeches (Zeylanicobdella arugamensis) are ectoparasites that can infest various cultured marine fish (e.g; groupers, snappers, seabass and pomfret). When these parasites latch onto its host, their bites causes wounds that will allow secondary infections of pathogenic bacteria and fungus leading to unsightly fish and mortality of cultured populations. BP2 is a marine leech trapping device developed to reduce the number of marine leech infestations in cage cultured marine fishes without the usage of chemicals, that is easy to apply and reduces handling stress to fish. This trial was conducted to determine the efficacy of the commercially produced devices in attracting leeches. Percentage of leech prevalence and mean intensity on the fish over time was also recorded. The trial ran for a total of 13 days with 5 days interval of 3 cycles.
Background numbers for lesions, wounds, haemorrhages, prevalence and mean intensity of leeches on each grouper were recorded by sampling 3 fishes out of each of the 8 grouper cages. To find out the effectiveness of the BP2 device, BP2 was submerged inside 4 grouper cages. BP2 was removed from the water after 5 days. The number of leeches and cocoons on the BP2 were recorded. These steps were repeated every 5 days for 2 more cycles.
The BP2 managed to harvest a total of 1,578 leeches and 52,330 cocoons in just 3 cycles of application (Table 1). After 2 cycles of BP2, the prevalence reduced from 100% (M.I = 144 leeches/body) to 83% (M.I = 9 leeches/body). And in the 3rd round, the prevalence increased to 94% (M.I = 200 leeches/body) (Table 2). Here, the M.I increased presumably due to newly hatched leeches as we noticed that the leeches collected were mostly juveniles (Figure 1).
As the marine leeches are ever present in the sea, it is important to continue applying the BP2 in the cages every 5 days to ensure that the leeches’ lifecycle is disrupted, and the fishes are protected.