Prey species and size choice of the molluscivorous fish, black carp (Mylopharyngodon piceus)

The black carp, Mylopharyngodon piceus , is used for biological control of freshwater mollusks in various parts of the World. Fish-borne zoonotic trematodes (FZTs) are a public health concern in Vietnam and we suggest using black carp in nursery ponds, which are important for FZT transmission, to control snails serving as first intermediate hosts. However, the use of large juvenile (>2 kg) black carp in nursery ponds could be problematic and we decided to determine consumption rates by black carp of various sizes and their choice of different sizes of selected snail species found in aquaculture ponds in northern Vietnam. Furthermore, shell strength of common snails was assessed. Average daily consumption as percentage of fish weight ranged from 8.12% for smaller fish (100–250 g) to 4.68% in the larger fish (610–1250 g). Bithynia fuchsiana , the intermediate host of Clonorchis sinensis , and some intestinal trematodes were readily consumed by even the smallest black carp tested. The proportion of Melanoides tuberculata , an important host for intestinal trematodes, declined with an increase in its shell height. The same was observed for two viviparid snail species, Angulyagra polyzonata and Sinotaia aeruginosa ; these species do not serve as first intermediate host for FZTs. Small black carp (100–250 g) consumed 50.4% of the second largest size class (26–30 mm) and 19% of the largest size class (>30 mm), while medium-sized black carp consumed 49.6% of the largest M. tuberculata and almost all snails of other size classes. Large black carp consumed 75% of the largest size class (>30 mm) of M. tuberculata . Crush resistance (log ₑ-transformed) increased linearly with shell size (log ₑ-transformed) in most species tested. Crush resistance was the lowest in B. fuchsiana , while there was an overlap between M. tuberculata and the viviparid snails. We concluded that black carp of smaller size preferentially fed on M. tuberculata .