Spread of proliferative kidney disease in fish along stream networks: A spatial metacommunity framework

Proliferative kidney disease (PKD) is a high-mortality pathology that critically affects freshwater salmonid populations. Infection is caused by the endoparasitic myxozoan Tetracapsuloides bryosalmonae, which exploits freshwater bryozoans as primary hosts. Incidence and severity of PKD have recently increased, largely owing to rising water temperatures linked to climate change, causing a decline in fish catches and local extinctions in many river systems. Here, building on a recently proposed local model of PKD transmission, a spatially explicit metacommunity framework is developed to study the spatial effects of the disease spread in idealised stream networks. At the local community scale, the model accounts for demographic and epidemiological dynamics of bryozoan and fish populations. At the network scale, the model couples the dynamics of each local community through hydrological transport of parasite spores and fish movement. The model also explicitly accounts for heterogeneity in habitat characteristics and hydrological conditions along a river network. Network effects are investigated by running simulation experiments on synthetic river network replicas derived from Optimal Channel Networks, spanning trees known to reproduce all the mutually connected topological and metric features of real rivers. Network connectivity can produce heterogeneous patterns of PKD prevalence even when the underlying spatial distributions of fish and bryozoans are homogeneous. Prevalence is generally higher at the downstream sites: if fish mobility is neglected, the spatial distribution of prevalence follows that of the upstream drainage area; otherwise, prevalence patterns are correlated with the proximity to the outlet. Downstream invasion speed of PKD is generally high, due to the fast dynamics of hydrological spore transport. For the tested values, effects of water temperature on prevalence heterogeneity are minor. However, climate change may increase invasion speed in both downstream and upstream directions. PKD can establish in bryozoan-free river reaches, on the condition that the infection be sustained by upstream or downstream hot-spots. These results further our understanding of the drivers of fish distribution in riverine ecosystems and may provide the basis for the development of intervention and management tools, especially facing climate change.