Metabolism and hypoxia tolerance of the endangered white abalone (Haliotis sorenseni): Implications for conservation and restoration efforts

The white abalone, Haliotis sorenseni, is an endangered marine gastropod that has shown no signs of population recovery despite fishery closure and protective status. To better understand the energetic demands, hypoxia tolerance, and critical habitat of this species, we measured oxygen consumption rates over a size range of captive-reared H. sorenseni at different environmental oxygen concentrations and temperatures in comparison to the more common red abalone, H. rufescens. We found that H. sorenseni has a relatively low metabolic rate that likely contributes to generally slow growth that can hamper recovery efforts. We also discovered that both H. sorenseni and H. rufescens appear to partially conform to ambient oxygen conditions by lowering their metabolism to deal with increasing hypoxia while still retaining an aerobic scope until reaching a critical oxygen concentration (P_crit), at which point they become oxylimited. For species exhibiting such relationships, determining the P₉₀, P₇₅, P₅₀, and P₂₅ (dissolved oxygen value at which oxygen consumption is 90 %, 75 %, 50 %, and 25 % of resting metabolic rate), as well as the P_crit and oxygen supply capacity, can provide useful metrics to compare hypoxia sensitivities among species and individuals. Variability in these metrics suggest potential fitness differences for H. sorenseni individuals spawned and raised in captivity for restoration outplanting. Higher temperatures also led to an increase in P₉₀, P₇₅, P₅₀, P₂₅, and P_crit and decrease in factorial aerobic scope for H. sorenseni, revealing the potential compounding effects of high temperature and low oxygen. Our results thus provide a suite of physiological metrics on which to test the health and fitness of captive-reared abalone and can help inform selection of appropriate outplanting sites for endangered H. sorenseni.