Effects of landscape-scale hypoxia on Salish sucker and salmonid habitat associations: implications for endangered species recovery and management.
To understand the effects of widespread urbanization and agricultural impacts on recovery of Salish sucker (Catostomus sp. cf. catostomus), a federally threatened catostomid endemic to the lower Fraser Valley of British Columbia, we assessed (i) the current extent and effects of hypoxia on the distribution of Salish sucker and juvenile salmonids, (ii) potential drivers of hypoxia, and (iii) management options for hypoxia mitigation. Over 40% of sucker critical habitat experiences hypoxia (dissolved oxygen (DO) < 4 mg.L- 1) by late summer, indicating widespread noncompliance with water quality guidelines. The strong positive relationship between seasonal hypoxia and temperature (R2 = 0.53) and negative relationship with streamflow (R2 = 0.78) indicates that hypoxia is driven by a synergy among low summer flows, elevated temperatures, and high primary production associated with nutrient enrichment (eutrophication). Sucker show strong selection against high water temperatures and weaker negative selection against low DO; juvenile salmonids show very strong selection against both high temperatures and low DO. Climate projections for declining summer flows and elevated temperatures indicate worsening trends in DO without intensive watershed-scale management to reduce nutrient loads, maintain or increase flows, and riparian restoration to lower stream temperatures, control primary production, and inhibit growth of invasive reed canary grass (Phalaris arundinacea).