Studies on the conservation of the red-spotted masu salmon Oncorhynchus masou ishikawae in northeastern Kyushu, southern Japan

Chapter 1. Introduction The red-spotted masu salmon Oncorhynchus masou ishikawae is an endemic subspecies of O. masou in Japan, originally distributed in the Pacific coastal region of western Honshu Island, Shikoku Island, and in some rivers of northeastern Kyushu Island flowing into the Seto Inland Sea. This species is highly valued as a fishing resource for its beautiful appearance and delicate taste. In Kyushu, it is found as a fluvial form in mountain streams. This form is known to show a restricted movement within a stream, and such sedentary behavior has been suggested to cause a restricted gene flow between populations and to lead to local adaptations. Since the 1970s, stocking of hatchery-reared salmonids including red-spotted masu salmon has been intensively conducted throughout Japan to prevent their exhaustion caused by overfishing. However, recent studies have shown that such stocking into Japanese streams may result in introgression of poorly adapted genes into indigenous populations, and this may lead to breakdown of local adaptations. Therefore, stocking should not be performed for a long-term conservation of wild salmonids, but necessary for their short-term utilization. To achieve both conservation and utilization of salmonids in mountain streams, 'zoning' is considered to be effective, where both hatchery-reared fish stocking and fishing regulations are properly applied to water zones classified according to the condition of natural reproduction and the presence or absence of indigenous salmonids populations. This study was conducted to clarify the natural reproduction and indigenous status of red-spotted masu salmon in order to implement 'zoning' into the Ogata River and its tributary, the Kohbaru Stream (16.4 km in length), in the upper Ono River, Taketa City, Oita Prefecture, northeastern Kyushu, southern Japan. Chapter 2. Longitudinal changes in fish assemblage Longitudinal changes in fish assemblage were investigated in the study area, in October 2003, and February and August 2004. A total of 18,015 individuals (14 species/subspecies representing 7 families) were recorded at 15 stations, by observers using snorkels. A major fish community transition was apparent from headwater stations with a simple assemblage dominated by red-spotted masu salmon and Chinese minnow Phoxinus oxycephalus, to downstream stations with more complex assemblages dominated by cyprinid fishes, such as Japanese dace Tribolodon hakonensis and dark chub Candidia temminckii. Multivariate analyses separated the 15 stations into three groups: headwater stations with a salmon/minnow -dominated assemblage (415-820 m altitude), a transition zone with a dace/chub/minnow-dominated assemblage (260-397 m), and downstream stations with a cyprinid-dominated assemblage (232-255 m). The fish assemblage structures were considered to be influenced by longitudinal environmental gradients, being significantly correlated with six environmental variables (topographic type and altitude, and stream gradient, mean wetted width, discharge and minimum daily mean water temperature) showing monotonic changes over the length of the stream investigated. On the other hand, cyprinid population densities did not show such changes, but declined within the transition zone downstream from erosion-control dams, suggesting that such densities were affected by smaller spatial scale factors, including dam-induced habitat degradation. Chapter 3. Longitudinal distribution of newly emerged fry of red-spotted masu salmon The longitudinal distribution of newly emerged fry and post-fry of red-spotted masu salmon was investigated by snorkeling in 14 consecutive sections of the study area. In two surveys, conducted at intervals of 11-32 days from January to April 2005, the densities of newly emerged fry were greatest in the four most upstream sections but remained very low in the six most downstream sections, the skewed distribution being stable. The densities of post-fry individuals were also greatest in the four most upstream sections, showing significant seasonal correlations (October 2003, February and August 2004). Furthermore, the density of newly emerged fry from January to April 2005 was significantly correlated with that of post-fry individuals in August 2004. On the bases of these and earlier reports, it was concluded that red-spotted masu salmon fry in mountain streams of Kyushu Island stay near the spawning redd, at least until they attain the parr stage. Chapter 4. Habitat use by newly emerged fry of red-spotted masu salmon The habitat use of newly-emerged fry of red-spotted masu salmon was studied on a channel-unit scale in the Ogata River, from January to March in 2007-2009. Fry abundance was investigated by snorkeling along both banks and the habitat components (channel-unit type, water depth, velocity, vegetation cover, width-depth ratio and distance from spawning redds) assessed in 56 consecutive sections of the lower distribution area of the species in the river. The number of fry per section was positively associated with channel-unit type near the riffles but negatively related to water depth near the banks and distance from spawning redds. Fry were strongly associated with channel-units near the riffles in their lower distribution area, probably because of the profitable microhabitat (low water-velocity habitats adjacent to a fast current, thereby ensuring high energy efficiency, were common in the upper distribution area but were restricted to the marginal area of riffles in the lower area). It is also suggested that fry moved downstream from profitable channel units which lacked profitable microhabitat. Therefore, for maintaining sustainable reproduction and effective stock enhancement of the species in the lower distribution area, it is important to create profitable microhabitats within channel-units near the riffles, thereby balancing the abundance of fry with the carrying capacity of suitable habitats. Chapter 5. Genetic structure of the red-spotted masu salmon The genetic structure of two fluvial forms of the red-spotted masu salmon, amago (ordinary form) and iwame (markless form), collected from two headwater streams, the Kohbaru Stream and and its tributary, the Hakiai Stream, was examined using mtDNA to identify whether they were indigenous to the streams. The Hakiai Stream consists of two tributaries, Mennotsura-dani and Manryo-dani. The iwame population occurs only in Mennotsura-dani, and since 1970, fishing has been prohibited for the iwame and amago populations in most locations of Mennotsura-dani and Manryo-dani, respectively. One haplotype (Hap-1) was shared by three isolated populations examined: it was fixed (100%) in the iwame population from Mennotsura-dani and dominant (90.5%) in the amago population from the upper Kohbaru Stream, but was infrequent (10.0%) in the amago population from Manryo-dani. Hap-1 was also frequent in the samples of amago fry caught in the Kohbaru Stream. Based on these results and the long persistence of the iwame population in Mennotsura-dani, both the iwame population in Mennotsura-dani and the amago population in the upper Kohbaru Stream are considered to be indigenous, whereas the amago population in Manryo-dani appears to have been mostly replaced by hatchery-reared fish informally stocked in the upper location. Chapter 6. Conclusion From the results stated in the previous chapters, the current status of red-spotted masu salmon in the study area is considered as follows: (i) they may be adversely influenced by dam-induced habitat degradations within and below the transition zone (Chapter 2), (ii) their natural reproduction is active above the transition zone while inactive within and below the zone (Chapter 3), and (iii) their indigenous population exists in the uppermost Kohbaru Stream (Chapter 5). Based on these results, the Oita Prefectural Government proposed a 'zoning' plan consisting of four parts: (i) prohibition of stocking and fishing in the uppermost location of the Kohbaru Stream, (ii) removal of the upstream boundary of a fishing-prohibited area in the Hakiai Stream, (iii) approval of stocking and fishing below the fishing-prohibited area of the two streams, and (iv) creating suitable habitats for newly-emerged fry within and below the transition zone in order to maintain sustainable reproduction and effective stock enhancement of the species (Chapter 4). This plan was agreed by a committee whose members were the staff of the Onogawa Fisheries Cooperative, those of the Taketa Municipal Government, representatives of a local fishing group, and local residents of Taketa City. Since 2009, the conservation of red-spotted masu salmon in these streams has been successfully implemented.