Influence of Summer Stream Temperatures on Black Spot Infestation of Juvenile Coho Salmon in the Oregon Coast Range

Abstract

High summer water temperatures can adversely affect stream salmonids in numerous ways. The direct effects of temperature associated with increased metabolic demand can be exacerbated by other factors, including decreased resistance to disease and increased susceptibility to parasites. We quantified the occurrence of black spot infestation caused by a neascus-type trematode (family Diplostomidae) of juvenile salmonids in Oregon's West Fork Smith River stream network during summer 2002 through fall 2003. The highest 7-d average of the daily maximum (ADM) temperatures was positively correlated with infestation rates in both years. We summarized the frequency and infestation severity of juvenile coho salmon Oncorhynchus kisutch by location within the network and summarized temperatures for 19 study reaches. Summer ADMs ranged from approximately 24°C near the watershed mouth to approximately 17°C in the upper reaches, while tributary ADMs ranged from approximately 16–18°C in the lower reaches to 12–17°C in the upper reaches. Temperatures were consistently higher in summer 2003 than in summer 2002. The presence of black spot infestation was more frequently noted in warm main-stem reaches than in adjacent cooler tributary reaches, and infestation rates were generally higher in 2003 than in 2002 in the same reach. Movements of coho salmon juveniles within the stream network also appeared to influence observed infestation rates.