Effects of temperature on disease progression and swimming stamina in Ichthyophonus‐infected rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract

Rainbow trout, Oncorhynchus mykiss, were infected with Ichthyophonus sp. and held at 10 degrees C, 15 degrees C and 20 degrees C for 28 days to monitor mortality and disease progression. Infected fish demonstrated more rapid onset of disease, higher parasite load, more severe host tissue reaction and reduced mean-day-to-death at higher temperature. In a second experiment, Ichthyophonus-infected fish were reared at 15 degrees C for 16 weeks then subjected to forced swimming at 10 degrees C, 15 degrees C and 20 degrees C. Stamina improved significantly with increased temperature in uninfected fish; however, this was not observed for infected fish. The difference in performance between infected and uninfected fish became significant at 15 degrees C (P = 0.02) and highly significant at 20 degrees C (P = 0.005). These results have implications for changes in the ecology of fish diseases in the face of global warming and demonstrate the effects of higher temperature on the progression and severity of ichthyophoniasis as well as on swimming stamina, a critical fitness trait of salmonids. This study helps explain field observations showing the recent emergence of clinical ichthyophoniasis in Yukon River Chinook salmon later in their spawning migration when water temperatures were high, as well as the apparent failure of a substantial percentage of infected fish to successfully reach their natal spawning areas.

Effect of Ichthyophonus on blood plasma chemistry of spawning Chinook salmon and their resulting offspring in a Yukon River tributary

“…In-river conditions in the Yukon River have changed over the past 30 yr, with June water temperatures having increased by approximately 2.5°C (Kocan et al 2004, Kahler et al 2007). Temperature changes influence disease processes in poikilotherms (Finn & Nielson 1971), by causing higher parasite loads, faster die-offs, and reduced swimming performance with increased temperature (Kocan et al 2009). Stress lowers the ability of fish to maintain homeostasis and carry out actions crucial for endurance, growth, and reproduction (Schreck 1982).…”

“…As Chinook salmon abundance fluctuates, prevalence of the disease also changes (Kocan et al 2004, Zuray et al 2012, as is typical of a parasite cycle where increased numbers of individuals in an area can increase the incidence of disease and vice versa (Altizer et al 2006). Evidence suggests that Ichthyophonus may cause pre-spawning mortality in Chinook salmon (Kocan et al 2004(Kocan et al , 2009. Since 2003, the prevalence of Ichthyophonus in Chinook salmon has declined continuously and appears to be correlated with a greater than 50% decline in the population abundance of Chinook salmon in the Yukon River (Zuray et al 2012).…”

“…Infection with Ichthyo phonus reduces oxygen uptake and exercise capabilities (Tierney & Farrell 2004), and fish appear to compensate by increasing heart mass and cardiac output . This results in lower stamina and swim speed (Kocan et al 2009). Due to these effects, Ichthyophonus prevalence can be higher in lower parts of the Yukon River compared with the spawning grounds as fish die before reaching their final destination .…”

Summer temperature regimes in southcentral Alaska streams: watershed drivers of variation and potential implications for Pacific salmon

“…MWMT in excess of 20°C during 1 or more years at 13 sites may have affected upstream migrations of adult salmon, five species of which move up the region's streams during summer. These impacts may include delayed migration (Quinn et al 1997;Salinger and Anderson 2006), increased vulnerability to disease (Fryer and Pilcher 1974;Kocan et al 2004Kocan et al , 2009, and reduced swimming performance (Brett 1995;Lee et al 2003). In extreme cases, warming conditions coupled with low water may lead to mass salmon die-offs, as have been observed in Cook Inlet and elsewhere in Alaska (Murphy 1985;Woolsey 2013;Viechnicki 2013;Georgette 2014;Doogan 2015).…”

“…Timing of life history events, like spawning, emergence, the onset of exogenous feeding, and smolting, are adapted to prevailing environmental conditions and are largely driven by temperature (Brannon 1987;Quinn 2005). High temperatures can block migration corridors (Quinn et al 1997;Salinger and Anderson 2006), increase disease virulence (Fryer and Pilcher 1974;Kocan et al 2009), and cause stress or outright death (Richter and Kolmes 2005). As such, warming temperature has the potential to alter the suitability of water bodies for salmon populations.…”