OceanAdapt

Climate Change Doesn’t Only Change Temperature

By Rachel Berman

Temperature change associated with global warming has been presented as a main issue of the future, but it can be confusing why a few degrees warmer would matter. In the northeastern U.S., temperature ranges from 0-100°F and humans can casually adjust the thermostats and layer on socks to remain comfortable. However, fish don’t have thermostats and wool hats. What does temperature change mean for a water-breathing, ectothermic bony fish?
Warmer water is serious business for fish: higher temperatures means less dissolved oxygen in the water. Just as for humans, oxygen fuels the energy-producing pathways inside fish and is an absolute necessity for life. At the same time, warmer water temperatures increase a fish’s demand for oxygen. Something called the “resting metabolic rate” of fish increases as temperatures warm, and fish require more oxygen at warmer than at cooler temperatures. Even when fishes, or humans, are not externally moving there’s a lot going on inside of bodies at rest. This is what is meant by the resting metabolic rate: it’s the energy required for the body to physically do nothing except sustain itself. At warmer temperatures the resting metabolic rate increases and demand for oxygen increases.
For fish to survive in an environment, the oxygen supply must exceed the fish’s need for oxygen. At somewhat lower levels of oxygen in the water, fish might have enough to survive at rest, but not to be more active, like feeding, evading predators, or reproducing. Larger fish in particular may be affected by low oxygen because their large size leads to a large demand for oxygen. If warmer temperature leads to less oxygen in the water at the same time that higher amounts of oxygen are needed for fish to survive…does this sound like a good situation for a fish?
Temperature also directly affects growth and reproduction. Temperature is an abiotic “on-button” for sexually maturing fish, and at warmer water temperature, fish may reach sexual maturity faster. When fish mature earlier, they may also mature at smaller sizes. Since smaller fish produce fewer eggs than larger fish, warmer temperatures may also mean populations that can’t reproduce as well.
A change in temperature can seem like a small detail in a complex ecosystem like the ocean, but its effects are dramatic. While humans can easily throw on a hat or turn on the AC if the temperature isn’t comfortable, it’s a whole different kettle of fish in the ocean. Fish are a lot more sensitive than you might expect.