Genki Sahashi

Migration costs drive convergence of threshold traits for migratory tactics

Partial migration of some, but not all, members of a population is a common form of migration. We evaluated how migration costs influence which members migrate in 10 populations of two salmonid species. The migratory patterns of both species were evaluated based on the size at maturity for resident males, which is the threshold trait that determines the migratory tactics used within a population. In both species, this size was smaller in males located further from the sea, where migration costs are presumably higher. Moreover, the threshold sizes at maturity in males were correlated between both species. Our results suggest that migration costs are a significant convergent selective force on migratory tactics and life-history traits in nature.

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

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

Iteroparity of stream resident masu salmon Oncorhynchus masou

The degree of iteroparity in stream-resident forms of masu salmon Oncorhynchus masou was examined using mark-recapture studies in natural streams. In a partially migratory population, at least 10% of resident males survived after maturation and repeatedly matured for up to 5 years. In the landlocked amago salmon subspecies, the post-maturation survival rate was at least 7% and repeat maturation was observed for up to 3 years for both males and females.

Spatial expansion and increased population density of masu salmon parr independent of river restoration

Spatial expansion and increased population abundance of masu salmon Oncorhynchus masou have been recently reported following habitat restoration in some rivers, but no studies have examined these types of changes under natural conditions. We found spatial expansion and increased population density of masu salmon parr in rivers of eastern Hokkaido where no habitat restoration had occurred during the period examined. Changes in the distribution and population densities of parr are often regarded as effects of river habitat destruction and restoration, yet such changes should be evaluated while taking into account the effects of other ocean environmental conditions as well.