A direct link between rising sea temperatures and a declining fish population has been established for the first time.

The decade-long study of a North Sea fish species shows how small ocean temperature changes can drastically affect a marine organism’s oxygen supply.

Previous research on the effect of warming on fish stocks have used statistical correlations only, says Hans Pörtner at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. This study is the first evidence to link climate changes to physiological effects in a fish species, he claims.

For over a decade, Pörtner and colleagues followed the declining fortunes of the eelpout fish (Zoarces viviparous), which lives on the bed of the North Sea and Baltic Sea. Used to a chilly environment, the fish exist within a narrow band of sea temperatures.

As well as counting population numbers, the researchers also studied the physiological effect of temperature change on the fish in the laboratory.
Difficult delivery

Fish are thought to undergo a number of physiological changes when sea temperatures change. The researchers’ laboratory studies of the fish’s biology showed the first thing to suffer as temperatures rise beyond a certain threshold – 17°C - is the eelpout’s oxygen supply.

In higher temperatures during the summer, the fish needs more oxygen to its tissues. But the rise in temperature also reduces the animal's ability to deliver that oxygen, because its respiratory and circulatory systems are performing outside of their optimum temperature range.

In their population studies in the North Sea, the researchers noted that when sea temperatures reached the same level that had caused this drop in oxygen supply in the laboratory, the abundance of eelpout dropped. For example, in the summer of 1997 sea temperatures reached 23°C, and the population dropped from four fish per kilometre squared to below one.

The oxygen deficiency first affects the swimming performance of the fish, which exposes it to predatory attack. Second, its growth and reproduction is reduced. Ultimately, the fish's oxygen supply fails altogether and it dies, with the larger fish succumbing first.
Useful indicator

Over the past four decades, sea temperatures in the part of the North Sea studied have risen by about 1°C. In the next century, they are expected to rise by a further 4°C.

The researchers were able to rule out other obvious explanations for a falling population during hot summers because the eelpout is non-migratory, so could not have moved elsewhere, and is also rarely fished.

This makes the species a useful indicator of what is happening to other marine species due to sea temperature change in the region. While some organisms are better than others at adapting, all marine organisms live in a certain temperature band and are vulnerable to change, Pörtner says. This is especially true for polar species, which have adapted to live at a constant low temperature.

“Everybody’s talking about cod overfishing in the North Sea, but that’s not the key point,” Pörtner argues, adding that even if fishing is reduced, then the additional effect of climate change on populations needs to be taken into account.