The critical oxygen partial pressure (Pcrit), typically defined as that below which an animal's metabolic rate (MR) is unsustainable, is widely interpreted as a measure of hypoxia tolerance. Here, Pcrit is defined as the PO2 at which physiological oxygen supply (α0) reaches its maximum capacity (α; µmol O2 g−1h−1kPa−1). The α is a species- and temperature-specific constant describing the oxygen dependency of the maximum metabolic rate (MMR= PO2*α) or, equivalently, the metabolic rate-dependence of Pcrit (Pcrit=MR/α). We describe the α-method, in which the MR is monitored as oxygen declines and, for each measurement period, is divided by the corresponding PO2 to provide the concurrent oxygen supply (α0=MR/PO2). The highest α0 value (or, more conservatively, the mean of the three highest values) is designated as α. The same value of α is reached at Pcrit for any MR regardless of previous or subsequent metabolic activity. The metabolic rate needn't be constant (regulated), standardized, nor exhibit a clear-breakpoint at Pcrit for accurate determination of α. The α-method has several advantages over Pcrit determination and non-linear analyses, including: 1) less ambiguity and greater accuracy, 2) fewer constraints in respirometry methodology and analysis, and 3) greater predictive power and ecological and physiological insight. Across the species evaluated here, α values are correlated with MR, but not Pcrit. Rather than an index of hypoxia tolerance, Pcrit is a reflection of α, which evolves to support maximum energy demands and aerobic scope at the prevailing temperature and oxygen level.

Over the last decade, ocean temperature on the U.S. Northeast Continental Shelf (U.S. NES) has warmed faster than the global average and is associated with observed distribution changes of the northern stock of black sea bass ( Centropristis striata ). Mechanistic models based on physiological responses to environmental conditions can improve future habitat suitability projections. We measured maximum, standard metabolic rate, and hypoxia tolerance (S crit ) of the northern adult black sea bass stock to assess performance across the known temperature range of the species. Two methods, chase and swim-flume, were employed to obtain maximum metabolic rate to examine whether the methods varied, and if so, the impact on absolute aerobic scope. A subset of individuals was held at 30°C for one month (30 chronic °C) prior to experiments to test acclimation potential. Absolute aerobic scope (maximum–standard metabolic rate) reached a maximum of 367.21 mgO 2 kg -1 hr -1 at 24.4°C while S crit continued to increase in proportion to standard metabolic rate up to 30°C. The 30 chronic °C group exhibited a significantly lower maximum metabolic rate and absolute aerobic scope in relation to the short-term acclimated group, but standard metabolic rate or S crit were not affected. This suggests a decline in performance of oxygen demand processes (e.g. muscle contraction) beyond 24°C despite maintenance of oxygen supply. The Metabolic Index, calculated from S crit as an estimate of potential aerobic scope, closely matched the measured factorial aerobic scope (maximum / standard metabolic rate) and declined with increasing temperature to a minimum below 3. This may represent a critical threshold value for the species. With temperatures on the U.S. NES projected to increase above 24°C in the next 80-years in the southern portion of the northern stock’s range, it is likely black sea bass range will continue to shift poleward as the ocean continues to warm.

46Over the last decade, ocean temperature in the U.S. Northeast Continental Shelf (U.S. NES) has 47 warmed faster than the global average and is associated with observed distribution changes of the 48 northern stock of black sea bass (Centropristis striata). Mechanistic models based on 49 physiological responses to environmental conditions can improve future habitat suitability 50 projections. We measured maximum, resting metabolic rate, and hypoxia tolerance (S crit ) of the 51 northern adult black sea bass stock to assess performance across the known temperature range of 52 the species. A subset of individuals was held at 30˚C for one month (30 chronic°C ) prior to 53 experiments to test acclimation potential. Absolute aerobic scope (maximum -resting metabolic 54 rate) reached a maximum of 367.21 mgO 2 kg -1 hr -1 at 24.4°C while S crit continued to increase in 55 proportion to resting metabolic rate up to 30°C. The 30 chronic°C group had a significant decrease 56 in maximum metabolic rate and absolute aerobic scope but resting metabolic rate or S crit were not 57 affected. This suggests a decline in performance of oxygen demand processes (e.g. muscle 58 contraction) beyond 24°C despite maintenance of oxygen supply. The Metabolic Index, 59 calculated from S crit as an estimate of potential aerobic scope, closely matched the measured 60 factorial aerobic scope (maximum / resting metabolic rate) and declined with increasing 61 temperature to a minimum below 3. This may represent a critical value for the species. 62Temperature in the U.S. NES is projected to increase above 24°C in the southern portion of the 63 northern stock's range. Therefore, these black sea bass will likely continue to shift north as the 64 ocean continues to warm. 65 66 67 likely continue to expand northward as the U.S. NES continues to warm. 132 133 Materials and Methods 134 135