Larger Nile tilapia struggle in hot water

Abstract

When your metabolism depends on the temperature of your surroundings, climate change represents a unique and alarming challenge. As temperatures increase, your metabolic rate rises too, requiring that you consume more oxygen to maintain your body at a faster pace. ‘At warmer temperatures, larger individuals may be prone to limitations in their capacity for oxygen uptake’, says David McKenzie, at the CNRS's Centre for Marine Biodiversity Exploitation and Conservation, Montpellier, France, with the consequence that larger fish may struggle to survive at higher temperatures. ‘If true, this would be important in projecting how global warming will affect fish populations’, says McKenzie; the size distribution of fish populations could become smaller as temperatures rise. ‘But evidence is lacking’, McKenzie admits. So, he and colleagues from the Federal University of São Carlos, Brazil, decided to find out how well Nile tilapia (Oreochromis niloticus), ranging in size from 21 to 313 g, managed to keep swimming at high speed as the temperature rose. First, the team tested how resilient the resting fish were as the water temperature rose. Initially, the fish began behaving erratically but eventually they toppled over when the temperature was too high. However, the largest fish were as resilient as the smallest; getting enough oxygen was not an issue when the fish were resting. ‘They may lose their balance for another reason when they are warmed up while resting in a tank’, McKenzie says.He and his colleagues then set the fish a challenge, encouraging them to swim at 85% of their top speed while the researchers gently increased the water temperature, and they found that the largest animals struggled most as the temperature rose. They gave up swimming at lower temperatures (around 37°C), while the smaller fish managed to keep going at temperatures up to 39.5°C. And when the team investigated the fish's oxygen consumption as the temperature increased, they found the largest fish got out of breath at far lower oxygen consumption rates (∼18 mmol O2 kg−1 h−1) than their smaller counterparts (41 mmol O2 kg−1 h−1).So, larger tilapia struggle to keep swimming as the temperature rises and they seem to find it difficult to get enough oxygen, which could have implications for the size of fish in future populations. McKenzie suspects that ‘Global warming may favour the small’, as smaller individuals may be able to take the heat while larger fish may have more trouble surviving in hot water.