Threatened Fishes of the World: Oncorhynchus masou formosanus (Jordan & Oshima, 1919) (Salmonidae)

0) land-locked salmon in the northern hemisphere. The mitochondrial DNA analysis Numachi et al. 1990) suggests that Oncorhynchus masoumigrated from Japan Sea through Tsushima Strait into Taiwan areas some 10 to 80 millions years ago. Upon the retreat of glaciers, some members remained in the upper streams of Tachia River and through geographical isolation evolved from an anadromous into a landlocked species. This species is considered a glacier relic and is recognized as a sister subspecies to Japanese salmon, Oncorhynchus masou . Before 1940s it was widely distributed in tributary streams (namely Chichiawan, Wuling, Yousheng, Nanhu, Hofan, and Suchiehlan creeks) with elevation higher than 1500 m of the upstream areas of Tachia River in central Taiwan. Between 1950s to 1970s, the range was reduced to Suchiehlan, Wuling, Chichiawan and an unnamed tributary of Chichiawan creeks. Now they can only be found in lower stretch of Wuling Creek (within the boundary of a newly established Shei-Pa National Park), Chichiawan and an unnamed tributary of Chichiawan creeks. The majority of the population is confined to a seven-kilometer stretch of Chichiawan Creek within the Wuling area. Abundance: Total number is less than 1000 according to a field survey conducted in 1991. Restocking of wild population through captive breeding program has been going on since 1986. Habitat and ecology: The majority of this fish prefers to stay in backwater pools and plunge pools created by silt trapping dams. This species can not tolerate water temperature above 17C. Microhabitat use is season- and size- dependent. During summer time, adult fish prefer to stay in areas upstream where higher diversity of microhabitats can be found far away from silt trapping dam. Young individuals prefer slow flow and wider sec tion of the stream where natural shelters are abundant. During winter time due to reduction of water level, adult fish move to plunge pool underneath trappin g dams while young fish aggregate in lateral scour pools formed by drifted woods, tree roots and boulder along the creek. Stomach content analysis reveals tha t aquatic insects (Trichoptera, Ephemeroptera, Disperta) accounts for 74% of food while terrestrial insects (Hymenoptera, Coleoptera, Hemiptera, Diptera ) accounts for 22% of food. Some Arachonidae, Diploapoda, Crustacea, and Anura are also found in the stomachs. This species is considered an insectivorous fish. Reproduction: The spawners are considered hiding lithophils, prefer to build nests with gravels ranging in diameters from 0.2 to 25.6 cm. Females build the nests by u sing tail and sideway body movement to clear silt away from bottom and create elliptical shaped nests. The average size of nest range is about 0.3 m2 (0.2‐1.8 m2) with a long axis 0.7 m (0.2‐1.2 m) and a short axis 0.5 m (0.2‐0.85 m). Water depth is 0.25 m (0.1‐0.6 m). Water current speed at the surface of nest is 0.09 ms 1 (not to exceed 0.6 ms 1) and at the bottom of nest is 0.06 ms 1 (not to exceed 0.45 ms 1). However, due to skewed sex ratio in the population (1 female: 0.65 male), mating between 1 female and two or three males have been observed. Eggs are laid from mid- to late October and within 20‐30 days later they reach eyed-embryo state. The water temperature in the spawning ground is 11‐15C. In general it takes 30‐60 days (after fertilization) to hatch (around late November). From tagging experiment it was determined that two-year-old fish are the dominant spawners. The life expectancy is 4‐5 years. Threat: The Der Jee Dam was built in the middle section of Tachai River in 1974 to control flooding and provide irrigation in the low land areas of central Taiwan. In order to prolong the life span of thi s dam, a series of silt trapping dams were built along the upper reach of Tachai River where most of natural habitats of this fish can be found. Within its existing habitat, albeit very small (with only a stretch of about 7 km), there are 11 silt trapping dams along the stream which further result in habitat fragmentation and isolation. Furthermore, illegal logging and uncontrolled land development such as operation of orchards and vegetable farms at high elevation, coupling with h urricanes, leads to extensive soil erosion of slopes along tributaries. This leads to severe siltation of spawning and nursery grounds. Illegal logging also exposed s tream bank areas to direct sunlight and raise water temperature in preferred shallow habitats. This leads to significant reduction of microhabitats. Conservation action: This fish was first declared as a ‘natural treasure’ by Japanese government in 1938 when Taiwan was under Japan’s colonial rule. The protection decree specifically d eclared: (1) prohibition of any introduction of exotic fish species into natural habitats of this fish; (2) fishing moratorium from 1 May to 31 December; (3) no logging and change of landscape within 300 m radius of Chichiawan Stream. After departure of Japanese in 1945 this fish received very little attention and in the ensuring y ears the population declined dramatically due to illegal fishing and deterioration of its natural habitats. It was not until 1976 the wild population started g aining notice of concerned biologists. First attempt was made to use artificial propagation to restock the dwindling population. This fish was first declared as an endan gered species in 1984 by the Taiwanese government after the enactment of endangered species act. Since then many scientists have participated in the field study of th is species. The establishment of the Shei-Pa National Park legally gives park rangers authority to protect the species and its natural habitats. Conservation recommendation: The removal of all silt trapping dams should be the first priority in the recovery plan. The removal of these dams will increase genetic diversities of on ce segregated populations. However, this action will inevitably shorten the life span of the Der Jee Dam with great economic loss and is not likely to be considered a v iable option to various government agencies. It is recommended that a good number of fish produced from breeding programs should be located in hatchery to maintain a spare breeding stock in case disaster happened to its natural population. The breeding program should also mix spawners from different populations to enha nce genetic diversity of restocked populations. The alternative plan is to buy back vegetable farms and orchards and to conduct an extensive and a long term refore station project to reduce soil erosion. The successful soil conservation can lead to removal of silt trapping dams within a decade which in turn will allow genetic exch anges among fragmented populations.