タイヘイヨウサケ属、オンコリンクス属、Oncorhynchus属
出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/10/04 00:16:08」(JST)
| Oncorhynchus Temporal range: Late Miocene - Recent |
|
|---|---|
| Sockeye salmon, Oncorhynchus nerka Female (above) and male in mating season |
|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Salmoniformes |
| Family: | Salmonidae |
| Subfamily: | Salmoninae |
| Genus: | Oncorhynchus Suckley, 1861 |
| Type species | |
| Oncorhynchus gorbuscha (Walbaum, 1792) |
|
| Species | |
|
See text |
|
Oncorhynchus is a genus of fish in the family Salmonidae; it contains the Pacific salmons and Pacific trouts. The name of the genus is derived from the Greek onkos ("hook") and rynchos ("nose"), in reference to the hooked jaws of males in the mating season (the "kype").
Salmon and trout with ranges generally in waters draining to the Pacific Ocean are members of the genus. Their range extends from Beringia southwards, roughly to Japan in the west and Mexico to the east. In North America, some subspecies of O. clarki are native to the landlocked Great Basin, while others are native to the Rio Grande and western tributaries of the Mississippi River Basin which drain to the Gulf of Mexico, rather than to the Pacific.
Unlike many trout species of the mainly European genus Salmo, many Oncorhynchus are anadromous (migratory) and semelparous (die after spawning). Some species of char (Salvelinus genus) are native to Pacific waters and are also referred to as trout.
Several late Miocene [about 7 million years ago (Mya)] trout-like fossils in Idaho, in the Clarkia Lake beds, appear to be of Oncorhynchus.[1] The presence of these species so far inland established Oncorhynchus was not only present in the Pacific drainages before the beginning of the Pliocene (5–6 Mya), but also that rainbow and cutthroat trout, and Pacific salmon lineages had diverged before the beginning of the Pliocene. Consequently, the split between Oncorhynchus and Salmo (Atlantic salmon) must have occurred well before the Pliocene. Suggested dates ions have gone back as far as the early Miocene (about 20 Mya).[2][3] One fossil species assigned to this genus, O. rastrosus, the sabertooth salmon (synonym Smilodonichthys), is a 9-foot (2.7 m)-long species known from Late Miocene to Pleistocene fossil.[4]
Speciation among Oncorhynchus has been examined for decades, and a family "tree" is not yet completely developed for the Pacific salmonids. Mitochondrial DNA (mtDNA) research has been completed on a variety of Pacific trout and salmonid species, but the results do not necessarily agree with fossil research, or molecular research. Chum, pink and sockeye salmon lineages are generally agreed to have diverged in the sequence after other species.[2] Montgomery (2000) discusses the pattern of the fossil record as compared to tectonic shifts in the plates of the Pacific Northwest of America. The (potential) divergence in Onchorhyncus lineages appear to follow the uprising of the Pacific Rim. The climatic and habitat changes that would follow such a geologic event are discussed, in the context of potential stressors leading to adaptation and speciation.[3]
One interesting case involving speciation with salmon is that of the kokanee (landlocked) sockeye. Kokanee sockeye evolve differently from anadromous sockeye—they reach the level of "biological species". Biological species—as opposed to morphological species—are defined by the capacity to maintain themselves in sympatry as independent genetic entities. This definition can be vexing because it apparently applies only to sympatry, and this limitation makes the definition difficult to apply. Examples in Washington, Canada, and elsewhere have two populations living in the same lake, but spawning in different substrates at different times, and eat different food sources.[5] There is no pressure to compete or interbreed (two responses when resources are short). These types of kokanee salmon show the principal attributes of a biological species: they are reproductively isolated and show strong resources partitioning.
According to the Committee on the Status of Endangered Wildlife in Canada, Pacific Salmon are collectively considered an endangered species; meaning the various species are facing “imminent extirpation or extinction.” Only 55.63% of total population levels of Pacific salmon in 2006 are remaining today and there has been no change in extinction risk since 2005.[6] Only 3% of the total population in 2006 is still viable, or able to reproduce, in 2012 meaning a very likely risk of extinction in the near future.[6] Because Pacific salmon (Oncorhynchus sp.) are important economic and cultural resources, their viability (or ability to survive) in response to future climatic change is of great concern. Global warming and climate change will soon cause warmer temperatures resulting in more precipitation falling as rain than as snow thus diminishing snowpacks and altering with the timing of stream flows. As more rain than snow occurs, there will be an increase in river flows, as there will be more water released from previously ice and/or snowpacks. This increase in temperature will also cause water temperature to rise overall. Not all of these anticipated effects are necessarily detrimental to the habitat of pacific salmon but they do have severe implications on the salmon population. However, as global temperature increases there will be a higher frequency of severe floods which will result in increased egg mortality since salmon eggs are laid in the gravel at the bottom of streams/lakes/oceans and disturbance of the stream bed will kill the salmon eggs.[7] This flooding will also greatly disrupt the cycles for fall and winter spawning because Pacific salmon spend 2–3 years feeding in the North Pacific and then migrate to their home lake-stream system where they spawn and die.[8] If the flooding changes the levels of the streams and lakes, then this will change migration patterns for salmon. Climate change will also cause winter snow to retreat sooner in the spring, possibly harming salmon whose life cycles and migration are timed with this snow retreat and linked to nearshore plankton blooms since now the retreat will be sooner but the plankton bloom will be around the same time.[9] If this is the case, then salmon migrating back with the snow retreat will have limited food resources due to the divergence between the retreat and plankton bloom. Salmon return to their lake-stream nurseries in great numbers, in densities around 5,000 to 30,000 salmon per km2 and if snow retreats sooner in the spring the migration patterns of these densities of salmon could be changed and less salmon would be able to return to their nurseries to spawn.[10] During the summer, summer flows will be lower and decrease the stream/lake/ocean system into smaller and less diverse areas.[9] Computer modeling indicates, “with near 100% likelihood” that snowmelt timed earlier in the year will drop summer stream flows in basins with significant snowmelt by the year 2050. This will threaten the existence of cold-water fish, especially Pacific salmon.[11] With increased global temperature, the issue of possibly decreased growth rates arises because warmer water temperature may affect the food supply required to support the increased metabolic demands of captive-raised salmon.[9] A decrease in size at maturity has been shown in stocks over the past 42 years, coinciding with an increase in sea surface temperature in the ocean. If growth rates decrease, it could cause excess pre-spawning mortality because upstream migrations are arduous and smaller salmon have less energy stores to make the migration. Decreased growth rates due to higher global temperature means smaller salmon with less energy stores that have decreased ability to survive and less likely to reproduce therefore endangering the Pacific salmon population. Recommended water temperature for Pacific salmon are about 7-16 degrees Celsius and the upper lethal temperatures are between 25-26 degrees Celsius.[12] With rising global temperatures due to climate change, the upper lethal temperature level for Pacific salmon are quickly approaching. If this level is reached and salmon go extinct, human and indigenous animal food resources will be lost and the economy built around salmon fishing destroyed.
Declines in the abundance of wild salmon due to overfishing placed greater pressure on hatcheries to increase production and restore the wild salmon stock to supply fisheries.[13][14] The problem is that hatcheries can never truly replicate the environment of wild salmon, an issue which often results in physiological and behavioral differences between wild salmon and those reared in hatcheries. These differences are often the product of genetic changes associated with inbreeding, artificial selection, and natural selection as well as different environmental pressures acting on hatchery fish than wild populations.[15] Due to the size selective nature of fishing favoring larger fish, a reduction in average size of the adult salmon has been observed over time. The smaller salmon make a greater proportion of the remaining individuals continuing the population, and problems arise when these hatchery-reared fish are introduced into the wild populations. Unlike wild salmon, larger salmon are selected for in hatcheries and are typically much larger than wild salmon.[12] The result is that hatchery-produced salmon tend to out-compete wild salmon for space, food, and other resources. Some salmon species in hatcheries exhibit predatory behavior toward wild salmon because they grow to be much larger. Regardless of whether predation is observed, natural social interactions are disturbed by the release of large numbers of hatchery-reared salmon where wild populations are low because salmon in hatcheries naturally have a higher propensity towards aggressive behavior.[16] Overall, natural salmon populations are put at risk when hatchery-reared salmon populations are introduced due to competition for resources, predation by larger individuals, and negative social interactions that upset the natural order observed in wild salmon populations. As a result, wild salmon populations are steadily dropping as the pressure to continue breeding salmon in hatcheries increases. Conservation efforts that work to place limitations on hatcheries to increase the wild salmon populations are hindered by financial pressures because hatcheries effectively support many states economically by accounting for over 70% of the salmon harvested for recreational and commercial purposes.[17]
Pacific salmon are harvested throughout the world as a source of food in countries ranging from the United States to South Korea.[18] Over the past century, pacific salmon have been extensively fished through both recreational fishing and commercial fishing. In fact, since the 1970s there has been a nearly threefold increase in catch of pacific salmon.[12] As this catch has increased, a selection of reduced body size has been observed. In Japanese Chum Salmon, for example, between 1970 and 1988 there has been a continuous decrease in body weight averaging between 11 and 32 percent.[19] In part, this decline in body weight has been related to the size selective effect of fishing gear used in the harvesting of salmon populations.[13] Salmon of larger body weight are more apt to be caught during fishing efforts, causing lower body weight to be a beneficial character trait for survival. Thus, pacific salmon have become continuously smaller in body size. However, studies have also shown that for pacific salmon a larger mean size at the time of reproduction increases the survival of offspring.[20] The life history of salmon favors delayed reproduction because fecundity increased with body size.[18] Consequently, the smaller body size of salmon results in a negative impact on population growth by decreasing the survivability of progeny, and thus decreasing the growth rate of populations. This reduction of productivity in pacific salmon is, in part, seeded in the overfishing and has causes a reduction in population sizes throughout pacific salmon species.
There has been evidence that the sockeye salmon are affected by thermal conditions and their responses to temperature are relatively strong and tend to vary from region to region. Canada has also used the Species at Risk Act to recognize the importance of biological diversity when it come to the conservation of the salmon population.[21] This means that multiple species of salmon would be looked at when it comes to conservation as well as multiple areas that each species live in. COSEWIC, a Canadian organization for the conservation of species, has named the Interior Fraser River Coho, the Cultus Lake Sockeye, and the Sakinaw Lake Sockeye to all be endangered. In British Columbia sockeye salmon in four different watersheds were certified by the Marine Stewardship Council or MSC as sustainable fisheries in July 2010 and the certification is good for a period of five years.[22] In 2011 MSC also certified the Pink Salmon Fishery and as of 2012 The Chum Salmon Fisheries started their review under the MSC to become certified as a sustainable fishery.[23]
The US government has been working developing a nation wide policy for the salmon populations.The Pacific Salmon Stronghold Conservation Act was re-submitted to congress and if passed will create geographic strongholds for salmon populations.[24] Other policies include the Wild Salmon Policy which was enacted in 2005 and its number one focus is the conservation of salmon off of the coasts. Even localized policies have begun, with one in Oregon which focuses on the southernmost watershed and was approved January 2013.[25] In the Alaskan efforts, there is evidence of eight known regional groups of survival. It is also seen that the emigration of smolts (young salmon) from freshwater to other areas such as marine areas have shown significant consequences on the survival of different salmon groups. The Alaska Department of Fish and Game first received MSC, Marine Stewardship Council, Certification in sustainable seafood back in 2000.[23] Each certification is good for a period of five years, with yearly check ups to ensure that the fishery remains sustainable. It was renewed again in 2007, but in 2012 The ADFG left the program. The Annette Island Reserve salmon fishery is under the control of the Metlakatla Indian community and as such was not included in the previous assessments of the Alaskan fisheries. It received it’s sustainability certification in June 2011.[23] The Wild Salmon Center is a non profit organization that works on promoting conservation efforts for the Salmon worldwide and in the United States has secured some of its nine protected locations equaling 8 million acres for protected watershed areas for the west coast salmon.[24] Other efforts of the Wild Salmon Center include combating illegal fishing, maintaining sustainable fisheries, and creating local watersheds as new habitats.[26]
Poaching is a threat to Oncorhynchus salmon populations in Russia. It is estimated that illegal catching if salmon is 1.5 times more than the reported catch. The Wild Salmon Center is working with Russian authorities to try and help improve traceability systems so that markets and distinguish between legal sustainable salmon and the illegal salmon.[27] The Wild Salmon Center has secured some of its protected locations for the salmon populations. In efforts with the WWF, the Wild Salmon Center was also able to have a sockeye salmon fishery certified as completely sustainable in 2012.[28] The Iturup Island Pink and Chum Salmon Fishery was first certified in 2009 and was the first Russian salmon fishery to receive certification in sustainability by MSC.[23] Other fisheries that were certified by MSC include the Northeast Sakhalin Island Pink Salmon, certified in June 2012, and the Ozernaya River Sockeye Salmon, certified in September 2012. The Aniva Bay Pink Salmon and the Sakhalin Island Pink salmon are both under review by the MSC.[23]
Some of the species in this genus are highly variable and a plethora of now-obsolete taxa have been described. The following list follows the most recent treatment in Fishbase. Currently, 15 species in this genus are recognized:[29]
In addition to the above, the following subspecies have been named:
| Wikimedia Commons has media related to Oncorhynchus. |
| リンク元 | 「Oncorhynchus属」「オンコリンクス属」「タイヘイヨウサケ属」 |
| 拡張検索 | 「Oncorhynchus nerka」「Oncorhynchus keta」「Oncorhynchus tshawytscha」「Oncorhynchus mykiss」「Oncorhynchus kisutch」 |