Fall–winter collection of two salmonid species: seasonal changes in population densities in four tributaries of the Kushiro river system

Abstract

The environmental conditions faced by stream-dwelling fish in many regions are highly variable during fall and winter, and may include very low water temperatures or ice formation (Brown et al. 2011). Such extremes can result in increased overwinter mortality (Hurst 2007). To minimize this, stream-dwelling fish have developed phenology responses to cope with the variable conditions seen during winter (Huusko et al. 2007; Shuter et al. 2012). In the absence of direct behavioral observation, seasonal changes in population density provide indirect evidence of behavioral repertoires in fish response. Moreover, the seasonal changes in population density may influence population processes via a density-dependent effect (Bailey et al. 2010). Numerous studies have evaluated the seasonal change in population densities in stream-dwelling salmonids between spring and fall (e.g., Niemela et al. 2001). However, few studies have investigated seasonal changes in population density during fall and winter, especially at the community level. Morita et al. (2011) examined the annual seasonal change in the densities of three salmonid species (white-spotted charr Salvelinus leucomaenis leucomaenis, masu salmon Oncorhynchus masou, and rainbow trout Oncorhynchus mykiss) in the Shoro river system. The authors noted that the density of white-spotted charr decreased in both tributaries between fall and winter, whereas the densities of masu salmon and rainbow trout increased in the warmer of the two streams during this period. Unfortunately, however, the study lacked spatial replication as the authors were only able to compare seasonal changes in population densities between two tributaries. Thus, the relationship between salmonid densities and water temperature in winter remains poorly understood. To address this, we evaluated the seasonal changes in the population density and ratio of two salmonid species, white-spotted charr and masu salmon, between fall and winter in four tributaries of the Kushiro river system, eastern Hokkaido, Japan. White-spotted charr (hereafter charr) and masu salmon (hereafter salmon) are the dominant species within the Kushiro river system and generally occur sympatrically (Hariu 2010). The field study was conducted in four tributaries: Hitominai Stream, Oriyomappu Stream, Ooshima Stream, and Minamitawa Stream (see Sahashi and Morita 2013: fig. 1). The sampling reaches included the typical biotopes of the two species in each tributary (see abiotic characteristics of the four tributaries in the electronic supplementary material of Sahashi and Morita 2013). The length of the study reaches was 80 m in Hitominai Stream, Ooshima Stream, and Minamitawa Stream. The length of the reaches at Oriyomappu Stream was 55 m in fall and 27.5 m in winter. In each tributary study reach, we deployed fixed data loggers (Stow-Away TidbiT; Onset Computer Corp., Bourne, Massachusetts, USA) on the stream bed to measure water temperature at hourly intervals between 1 June and 4 September 2010, and 1 November and 1 February 2011. G. Sahashi Division of Marine Bioresource and Environmental Science, Graduate School of Fisheries Sciences, Hokkaido University, Hakodate 041-8611, Japan