Oncorhynchus

Oncorhynchus is a genus of fish in the family Salmonidae; it contains the Pacific salmon and Pacific trout. 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 native ranges in waters draining to the Pacific Ocean are members of the genus. Their range extends from Beringia southwards, roughly to Taiwan in the west and Mexico to the east. In North America, some subspecies of O. clarki are native in the Rocky Mountains and 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. Several species of Oncorhynchus have been introduced into non-native waters around the globe establishing self-sustaining wild populations. The six Pacific salmons of Oncorhynchus are anadromous (migratory) and semelparous (die after spawning). Migration can be affected by parasites. Infected individuals can become weak and probably have shortened lifespans. Infection with parasites creates an effect known as culling whereby fish that are infected are less likely to complete the migration. Anadromous forms of Oncorhynchus mykiss known as steelhead are iteroparous. The Coastal cutthroat trout form of Oncorhynchus clarki is considered semi-anadromous as it spends short periods of time in marine environments. Several late Miocene trout-like fossils in Idaho, in the Clarkia Lake beds, appear to be of Oncorhynchus. 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 have gone back as far as the early Miocene (about 20 Mya). 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 fossils. 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. 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. 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 State, Canada, and elsewhere have two populations living in the same lake, but spawning in different substrates at different times, and eat different food sources. 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. A general decline in overall Pacific salmon populations began in the mid 19th century. As the result of western expansion and development in the U.S., experts estimate salmon populations in the Columbia River basin had been reduced to less than 20% of their pre-1850 levels by 1933. In 2008, Lackey estimated that Pacific salmon stocks in the Pacific Northwest were less than 10% of their pre-1850 numbers. Many of the remaining salmon runs are dominated by hatchery raised salmon, not wild salmon. Many isolated subspecies of the Pacific trouts, particularly those of Oncorhynchus mykiss rainbow trout andOncorhynchus clarki cutthroat trout have declined in their native ranges. Many local populations or distinct population segments of anadromous forms of steelhead have declined in their native ranges. The resulting declines have resulted in a number of populations of Oncorhynchus species or subspecies being listed as either endangered, threatened or as 'Species of Special Concern' by state, federal or international authorities. Two Oncorhynchus clarki subspecies are considered extinct. Declines are attributed to a wide variety of causes—over fishing, habitat loss and degradation, artificial propagation, stocking, and hybridization with or competition with introduced, non-native species. For example, the Yellowfin cutthroat trout (Oncorhynchus clarki macdonaldi) is extinct as a result of the introduction of non-native rainbow trout into its native waters. Declines in the abundance of wild salmon due to over fishing placed greater pressure on hatcheries to increase production and restore the wild salmon stock to supply fisheries. 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. 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. 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. 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.