Temperature response and salt tolerance of the opportunistic pathogen Saprolegnia parasitica: Implications for the broad whitefish subsistence fishery

We examined the effects of temperature and salt concentration on growth of the freshwater oomycete Saprolegnia parasitica that has recently (since 2013) been found to infect an important subsistence fish (in Iñupiaq, Aanaakłiq; broad whitefish, Coregonus nasus) on the Colville River in Nuiqsut, Alaska. Using two confirmed isolates (one from the Colville River and another from a southern British Columbia aquaculture facility), we tested the following hypotheses: (1) the isolate from Alaska will grow at a greater rate than the isolate from British Columbia at lower temperatures, (2) the isolate from British Columbia will grow at a greater rate at higher temperatures than the Alaska isolate, and (3) increasing salinity will reduce the growth rate of both isolates similarly at all temperatures. In addition, we used local observations—subsistence fishers and observations associated with scientific monitoring—to assist in interpreting the potential implications of our experimental results in the context of these environmental observations. In the habitat relevant to this study, water temperature ranges between <0°C and 18°C, and salinity ranges between 0 and 30 parts per thousand due to a seasonal (and occasional west wind-driven) saltwater intrusions. No statistically significant differences were detected in growth rate or salt tolerance between the two isolates at the temperatures and salinities tested; high temperature (24°C) and low salt concentration are associated with the highest growth rate for both isolates. From our lab study, one might conclude that the peak host colonization would occur during the seasonal period of warmest water temperature; however, the observations by local fishers and biologists show this not to be the case. We conclude that, at this time, we do not have evidence that peak warm water is the primary cause of an increased incidence of infection by this freshwater mold. Although indirect and lag analysis of temperature and timing of infection were not part of this study, we note that there is a greater role of complex interactions among biotic and abiotic factors (including temperature) that may predispose some individuals in the population to become infected during spawning season.