Oncorhynchus sp. nov. 'Sinaloa Golden Trout'

TAXONOMIC NOTES The Sinaloa trout is phenotypically and meristically distinct from Oncorhynchus chrysogaster specimens in the rios Fuerte and Culiacán, and is therefore treated separately from those taxa. JUSTIFICATION This species is restricted to headwater tributaries in the Río Sinaloa basin between 2,600-2,800 m asl, exhibits a restricted extent of occurrence (EOO) and area of occupancy (AOO), and occurs at less than 10 locations. Total population size is unknown but suspected to be relatively small. Population trend is unknown. Subpopulations are inferred to be severely fragmented, given specific habitat requirements for cold, clear water, and an inability to disperse across mainstem river thermal barriers. Localized threats include siltation due to livestock overgrazing, deforestation, and introgression with hatchery raised rainbow trout, which has resulted in a continued decline in the area, extent, and quality of available habitat, and a projected continued decline in EOO and AOO. Therefore, this species is assessed as Endangered (EN) under criterion B1ab(i,ii,iii)+2ab(i,ii,iii). GEOGRAPHIC RANGE INFORMATION This species is restricted to headwater tributaries in the Río Sinaloa basin between 2600-2800 masl. Arroyos El Soldado, Casa Quemada, and El Potrero appear to have the most broadly distributed populations. Extent of occurrence (EOO) for this species is estimated to be 368 km² and area of occupancy (AOO) is estimated to be 18 km². This species is known from nine localities and major threats are expected to impact each locality separately. Therefore, the number of locations where this species occurs is nine. Both EOO and AOO are projected to undergo continuing decline if threats are not mitigated. Edward William Nelson saw trout in streams on the slopes of Mt. Mohinora in 1898, and we suspect that he was in the Sinaloa drainage at the time. POPULATION INFORMATION Information on population size and trends is limited, as most of these streams have been collected once in the last 50 years. This species is localized to tiny headwater streams, and therefore population size is suspected to be less than 1,000. Observation of degraded habitat and the difficulty of finding viable populations suggests that this species was more widespread in historical times. It is suspected that more intensive collecting would produce more sites at lower altitudes for Sinaloa Trout. Subpopulations are considered severely fragmented, given habitat requirements and the presence of thermal barriers in mainstem rivers that prevent dispersal. HABITAT AND ECOLOGY INFORMATION This species requires cold, clear water in headwater streams, with gravel for spawning. The specific habitat uses, basic life history, and basic ecology of this species are uncertain and require additional research. THREATS INFORMATION Overgrazing by livestock has caused siltation in Sinaloa tributaries and remains an ongoing problem. The watershed is logged, but does not appear to be heavily so. One locality (Arroyo Rancho en Medio) contained hybrids between Sinaloa Trout and hatchery Rainbow Trout. The hybrids were localized near a grow-out facility that contained hatchery trout. A waterfall prevented the movement of rainbow trout upstream, but no such barriers exist to downstream dispersion. 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 trade in the taxon. CONSERVATION ACTIONS INFORMATION There are currently no species-specific conservation measures in place. Education of the local population to prevent livestock from degrading riparian areas, and encouraging the production of native trout in grow-out facilities to eliminate hybridization with rainbow trout are priority conservation actions. More research regarding distribution, population size, population trend, and the impacts of localized threats would be useful in guiding future conservation action.