Oncorhynchus sp. nov. 'Bavispe Trout'

TAXONOMIC NOTES The Yaqui Trout is the only native salmonid in the Río Yaqui watershed. There is some evidence that the Yaqui trout in the Bavispe, Papigochi and the Tutuaca sub-basins are differentiated from each other phentotypically. Rainbow trout are raised as food in grow-out facilities in the Yaqui basin, and have escaped into the wild. Hybridization with the Yaqui trout has occurred but is of localized occurrence. JUSTIFICATION The Yaqui Trout occurs in a number of locations throughout the Yaqui and Guzmán basins at elevations ranging from 1,600-2,200 m above sea level. Extent of occurrence (EOO) for this species is estimated to be 7,339 km² and area of occupancy (AOO) is estimated to be 120 km². Population size is unknown, but presumably exceeds 10,000. Population trend is suspected to be stable or declining at a rate that precludes this species from being listed under a threatened category under criterion A. Subpopulations are not considered fragmented, given a higher thermal tolerance when compared to close congeners, and the ability to disperse through thermal barriers in lower elevation, mainstem rivers. Localized threats include deforestation and livestock overgrazing which have resulted in increased runoff, siltation, and higher stream temperatures, hybridization with rainbow trout, and mild fishing pressure. However, many of the localities where this species occurs are isolated and difficult to access. Therefore, this species is assessed as Near Threatened (NT), because it meets the thresholds for a threatened category under criteria B1 and B2 and there is evidence of decline in the area, extent and quality of habitat, but it occurs in greater than 10 localities and subpopulations are not fragmented. GEOGRAPHIC RANGE INFORMATION Yaqui Trout are common in the Bavispe and Tutuaca sub-basins, and in tributaries of the Río Tomochi (Papigochi sub-basin). The Bavispe has more than a dozen arroyos with strong and stable populations with Arroyo Yenquin and Arroyo Nutria being among the best. Habitat extends from headwaters to about 1,600 m asl. In general, the watershed appears to be among the best in Chihuahua/Sonora from an overall management perspective. Extent of occurrence (EOO) for this species is estimated to be 7,339 km² and area of occupancy is estimated to be 120 km², based on known collection localities. Threats are localized and expected to affect localities independently. Therefore, the number of locations where this species occurs exceeds 10. POPULATION INFORMATION Total population size is unknown but presumably exceeds 10,000. Population trend is suspected to be stable or slowly declining, principally because the watershed is sparsely settled and there is not a lot of fishing pressure on the species. A "duplicate" subpopulation exists in the Guzman basin to the east, having been transplanted there from the Bavispe sub-basin in the early 1900s. Subpopulations are not considered fragmented, given an potentially higher thermal tolerance for warm water and capabilities to disperse through and even live within lower elevation mainstem rivers. HABITAT AND ECOLOGY INFORMATION This species typically occupies small, cold water tributaries with northerly or eastward flow. Major tributaries are often thermal barriers to dispersal for close congeners. However, because the Yaqui trout may exhibit some tolerance to warmer waters, it is occasionally found in mainstem rivers such as rios Tomochi and Gavilan at lower altitudes (1,600 m asl). Basic life history and basic ecology are uncertain and require additional research. THREATS INFORMATION The principal threat to Yaqui Trout is logging and associated runoff. While there is no known instance of clear cutting in the area, extensive logging in the mid-1900s prompted Aldo Starker Leopold to remark that the Río Gavilan had undergone extensive degradation, including sawdust in runoff and flash flooding. Removal of streamside trees also contributes to warming of the stream waters. Trout generally require higher dissolved oxygen than other native fishes, and are therefore particularly susceptible to warming waters. Some pastures are overgrazed which also contributes to flash flooding of streams and rivers and can interrupt spawning and decrease visibility in the streams. Hybridization with non-native Rainbow Trout is a concern, and could eventually lead to widespread introgression. Currently, hybrids are uncommon and are localized near grow-out facilities for Rainbow Trout. Future genetic introgression with rainbow trout is expected, given government initiatives that promote the development of hatcheries within the region. USE AND TRADE INFORMATION There is no known trade of the taxon. It is occasionally targeted in subsistence fisheries. However, fishing pressure at these localities is low because human development is sparse and localities are sometimes remote. CONSERVATION ACTIONS INFORMATION There are currently no species-specific conservation actions in place. Good forest management (e.g. selective logging) is recommended. Reductions in cattle stocking densities would help mitigate the effects of rapid runoff and higher silt loads in the runoff.