Researchers have observed that the Fish are declining in size. This observation was first confirmed three years back in a research study which revealed that the body size of some fish stocks in the North Sea, such as haddock, herring and sole, had decreased significantly in past four decades. Some scientists suggested Global warming as the culprit behind this new finding, but there was no concrete theory to explain how climate change could result in the shrinking of fish sizes.

Now a new study carried out by researchers from the University of British Columbia (UBC) in Canada suggests this is true, and climate change is playing a major role in shrinking the size of fish.

The study also claims that fish size may shrink by up to 30 percent if ocean temperatures continue to increase due to climate change.

Fish can’t regulate their body temperature

Fish are cold-blooded animals, meaning they can’t regulate the temperature of their own body. Their body temperature depends completely on environmental temperature. William Cheung, director of science for the Nippon Foundation Nereus Program at UBC and the co-author of the study, reveals that as oceans warm up, the body temperature of fish also increases. This results in speeding up of their metabolism, and therefore they need more oxygen to carry out their body functions. Fish get oxygen through gills that have their own limits to supplying oxygen to the body.

Less supply of oxygen in the body eventually results in stopping the growth of fish body size.

Daniel Pauly, a principal investigator with the Sea Around Us project at UBC and the lead author of the study, suggests that as fish move into adulthood, they become larger in size and so their demand for oxygen also increases. However, their gills don’t grow with same speed.

For example, gills of a cod grow only 80 percent or less for a 100 percent increase in its body weight.

Increase in water temperature increases fish's need for oxygen

Pauly further explains that an increase in water temperature increases the need for oxygen by fish, but at the same time, it decreases the amount of oxygen available for fish in the oceans.

This means there is less oxygen for gills to supply to a body that has already grown faster than them. The long-term effect of this oxygen shortage is that fish stop growing to fulfill their oxygen needs. Pauly names this principle as "gill-oxygen limitation theory."

According to researchers, different fish species may be affected differently due to a combination of factors. For example, fast-moving Tuna that require more energy may shrink more with warming ocean waters. The study also suggests that the body size of the Atlantic bluefin tuna is likely to decrease by 30 percent with a 2°C (3.6°F) increase in water temperature.

Detailed findings of the study have been published in the journal Global Change Biology.