Environmental flow modeling for the conservation of the endemic trout Oncorhynchus nelsoni (Evermann) in a basin located in the Mexican region of Mediterranean California

Driven by global economic and population growth, the diversion of streamflow, via pipeline networks, for agricultural, livestock, or domestic purposes, has had a significant impact on water demand. These processes have created an urgent need for sustainable water resource management. Environmental flow regimes have become a key tool in watershed-wide management plans, as they enable appropriate water allocation while maintaining the functionality of riverine ecosystems. The northwestern region of the State of Baja California, Mexico, is characterized by a mediterranean climate, an average annual precipitation lower than 400 mm, and a critical water availability situation due to the overexploitation of aquifers for agriculture and urban development. The present study proposes environmental flow modeling to support the habitat connectivity of the endemic trout Oncorhynchus nelsoni (Evermann, 1908) in a third-order stream in a basin forming part of the Mexican region of what is known as Mediterranean California, due to its specific climatic characteristics. The modeling was achieved via an adaptation of the hydrobiological Instream Flow Incremental Methodology (IFIM) that adhered to the procedures stipulated under Official Mexican Standard NMX-AA-159-SCFI-2012. Given special protection status under Official Mexican Standard NOM-059-ECOL-2010, this endemic trout is currently threatened by various anthropogenic activities and natural factors, including climate change. The present study developed a riverine habitat model in the El Potrero Stream, a representative site in the distribution range of the trout of interest, to evaluate the environmental flow required to maintain aquatic habitat connectivity. A minimum environmental flow of 0.02 m³s⁻¹ was identified as sufficient to ensure habitat connectivity, while a value of 0.01 m³s⁻¹ was determined as sufficient for the formation of a discontinuous stream with isolated pools that could serve as temporary thermic refuges for the trout during prolonged multi-year drought conditions. The present study found that levels exceeding the optimal water volume did not increase access to a suitable habitat for the endemic trout and may even become detrimental to it during high-flow events. We recommend the continued implementation of hydrological monitoring programs in the El Potrero Stream, along with the inclusion of representative sites such as the San Antonio de Murillos stream (type locality), which is currently threatened by anthropogenic activities. Monitoring these streams will enable a more robust characterization of temporal streamflow variability under seasonal climatic conditions and will contribute to a better understanding of the hydraulic connectivity between stream segments throughout the annual cycle.