Warm water closes in first. At the gill surface, oxygen solubility falls while aerobic metabolism accelerates, turning each fish into a processor whose power draw rises as its oxygen supply shrinks. Fish pay first. This is a physiological squeeze, not a passing inconvenience: body size can hit a ceiling before food does. Warmth creates a double bind.
The arithmetic bites. Fish ventilate water across gill lamellae, but higher temperatures lift metabolic rate and oxygen demand; when dissolved oxygen drops, aerobic scope for swimming, digestion, growth, and reproduction contracts. The margin vanishes. Oxygen transport also relies on ventilation and circulation, processes that consume energy while serving the body. As fish enlarge, body volume can outpace respiratory surface, adding maintenance costs and making growth a zero-sum wager against water chemistry.
That should unsettle fisheries planners. A warmer sea may still hold fish, yet fewer may reach sizes that sustain catches, predators, or breeding output where heat overlaps with hypoxia. Food cannot solve the pinch. Processing more prey requires more oxygen, so abundance may not translate into size. Aerobic scope is the scientific term behind the device-battery analogy: background metabolism drains capacity, leaving less for every visible task. If heat and hypoxia compound, the failure can cascade from gills to food webs, recoding which bodies endure.
