Climate change and overfishing cause fish to shrink and force many species to migrate in search of more favorable water temperatures where they have better chances of survival. They essentially become “climate refugees,” so to say.
This is the conclusion of research by honored Professor at the Fisheries Center of the University of British Columbia, Canada, Dr. Daniel Pauly. The renowned researcher is also an honorary doctorate of the Biology Department at the Aristotle University of Thessaloniki (AUTH).
Pauly is considered one of the greatest scientists of all time in the field of marine fisheries biology. He was one of the first scientists to sound the alarm about the ecological effects of overfishing on a global level.
The renowned researcher was the key speaker at the 22nd Annual FishBase Symposium, entitled “Fish in changing ecosystems,” organized by the Ichthyology Laboratory of the Biology Department of AUTH, the Athens-Macedonian News Agency reported.
According to Pauly, the most critical problem facing the world’s seas remains overfishing, but climate change is contributing to the problem, as warming oceans affect the ecosystems.
“As temperature in the seas increases, the water holds less oxygen in it,” Pauly explained.
“At the same time, the higher water temperature also requires more oxygen because the metabolism of fish and other organisms that breathe with gills is higher. The fish are actually stressed because the water holds less oxygen, but they need warmer water. They are, in other words, like animals in a fire and they cannot breathe,” the professor explained.
“A consequence of the higher water temperatures,” Pauly continued, “is that the fish remain smaller so that their gills can provide the oxygen they need in warmer waters where there is less oxygen.”
Warm waters cause fish to shrink
“Essentially there are two opposing forces pushing the fish as they want more oxygen in warmer water, but the water now holds less oxygen and this has an impact on their size,” he added. “Oxygen must pass through the gills to supply the entire body of the fish, which is easier in small fish [that] have a proportionally larger gill surface area relative to their size.”
“Large fish have less gill surface in proportion to their size, so they have more difficulty breathing. We fear, therefore, that if we do not limit the emissions of greenhouse gases, resulting in overheating, we will slowly lose the big fish,” he concluded.
As the water warms due to climate change, many marine species seek a lower temperature and go north. For example, fish from Spain move north to England, becoming “climate refugees,” so to say.
“This does not happen everywhere,” Pauly said. “In the Mediterranean, for example, where we have a geographical barrier to the north, alien species come through the Suez Canal. In the Atlantic we have some species that may become ‘climate refugees,’ looking for cooler waters to the north. But in the Mediterranean ‘climate refugees’ may be foreign species, such as harefish and lionfish.”
Explaining the situation further, Pauly’s colleague, Dr. Donna Dimarchopoulos, noted that, essentially, the fish from the Atlantic go north and from Morocco to Portugal and England.
“But where should they go in the Mediterranean?” Dimarchopoulos asked. “In this case, they either go to deeper waters, or their populations decline. ‘Climate refugees’ are alien species in the Mediterranean that invade from the Suez Canal, then grow and multiply at the expense of local species, creating problems for fishermen.”
According to Pauly, the problem can be reversed mainly by limiting overfishing. “The sea is very patient and generous with us. If we stop overfishing we will achieve greater diversity and a gene pool to have different responses and behaviors to the changing conditions in the environment.”
