Juvenile Pacific salmon make unidirectional downstream migration during which they are infected by various types of diseases. One of the important diseases affecting the survival of juvenile salmon is trematode. Trematode life cycle is bizarre, requiring two additional hosts (snails and large predators such as birds and mammals) to complete their life cycle. The effect of trematodes on the mortality of the host fish is also complex. The mortality strongly depends on the number of infections, but the trematodes do not propagate after infecting the fish. Furthermore, resulting fish mortalities are thought to take place mostly after the fish migrate into an estuary. These complexities obscure the dynamics of juvenile salmon and trematode interactions. In this study, we modeled the number of trematode infections with a system of partial differential equations.
Results/Conclusions

We found that increased fish density actually reduces the number of infections per individual fish. This occurs because as the number of fish increases, more trematodes bind to fish, reducing the density of trematodes in the water. The reduced trematode density, in turn, reduces the infection rate. The result suggests that the schooling behavior of the fish may be an effective way of reducing the infections, which in turn reduce the mortality rate. Controlling the release rate of juvenile salmon from hatcheries, we may also be able to reduce the number of infections. We also found that the location of trematode infection along the river is an important factor affecting the number of trematode infections when the fish reaches the estuary. This type of information can potentially help manage the diseases.