Oncorhynchus sp. nov. 'San Lorenzo Trout'

TAXONOMIC NOTES JUSTIFICATION This species is restricted to cold, high elevation headwater streams in the Remedios sub-basin of the San Lorenzo drainage. Its extent of occurrence (EOO) and area of occupancy (AOO) are both restricted. Continual declines in EOO and AOO are inferred based on the most serious threats, which include widespread introgression with rainbow trout, and declines in the quality, area, and extent of habitat resulting from deforestation and livestock overgrazing. Subpopulations are severely fragmented with limited to no genetic exchange due to thermal barriers outside of headwater habitat. Localized threats are likely to affect stream subpopulations independently and the number of locations where this species occurs is estimated to be six. Therefore, this species is assessed as Endangered (EN) under criterion B1ab(i,ii,iii)+2ab(i,ii,iii). GEOGRAPHIC RANGE INFORMATION Known populations reside within the Remedios sub-basin of the San Lorenzo. Río Truchas would appear to have the most significant population. Arroyo la Sidra has a peculiar morphotype above the waterfall near Vencedores that is genetically unique. The San Gregorio arm of the San Lorenzo is complimentary in size to the Remedios, but is unexplored for trout. Extent of occurrence (EOO) for this species is estimated to be 4,662 km² and area of occupancy (AOO) is estimated to be 18 km², based on known collection localities. Given the scope of potential threats, the number of locations is estimated to be six. POPULATION INFORMATION Total population size is uncertain, but unlikely to exceed 1,000. Population trend is uncertain but likely to be declining at a rate that precludes listing under a threatened category under criterion A. San Lorenzo trout are common in the Río Truchas. Trout in Arroyo la Sidra above the falls are not common, and are in danger of introgression due to the release of rainbow trout from a nearby grow-out facility. Other disjunct populations occur in small headwater tributaries of the Remedios. Subpopulations are considered severely fragmented, given specific habitat requirements and an inability to migrate across thermal barriers in river mainstreams. HABITAT AND ECOLOGY INFORMATION This species occurs in cold and clear high mountain streams, headwaters and larger arroyos at elevations between 2,300-2,700 m asl. Biological habits, niche, and reproductive characteristics have not been studied for San Lorenzo trout. THREATS INFORMATION Hybridization with rainbow trout is rampant in Arroyo la Sidra below the main falls. Escapes from the grow-out facility are frequent. Hybridization may pose a threat to other populations in the San Lorenzo where populations are not fragmented by thermal barriers. Logging and livestock are present in adjacent areas and may create runoff problems in the watershed. 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 in the taxon. CONSERVATION ACTIONS INFORMATION There are currently no species-specific conservation measures in place. Recommended conservation measures include encouraging ejidos to limit access of livestock to streams, and prevention of future rainbow trout releases to reduce future introgression outside of Arroyo la Sidra. More research regarding distribution, population size, population trend, and the magnitude of potential threats would be useful in guiding future conservation action.