"We measured various aspects of oxidative stress, a condition in the cells that can lead to irreparable damage, antioxidant capacity, which protects against oxidative stress, and telomere length," says researcher and marine biologist Helen Nilsson Sköld.

Telomeres are repeated DNA sequences that protect the ends of chromosomes. The length of these telomeres and the rate at which they get shorter are closely linked to health and aging.

The researchers compared the health of cod in the Öresund, Skagerrak and Kattegat. Cod in the Öresund have been protected from trawling since 1932 and so stocks include larger and older fish, but cod in the Skagerrak and Kattegat have been seriously overfished.