Honey, We Shrunk The Cod

By Will Parker on August 27, 2008 in News

Analysis of ancient fish unearthed on an island in the Baltic Sea suggests overfishing by humans is causing fish populations to evolve and driving commercially valuable species like cod to the brink of economic extinction. In a report published in Proceedings of the Royal Society B, an international team of scientists reports that the remains of 4,500-year-old Baltic cod found in a pre-Viking settlement on the Swedish island of Gotland indicate the fish harvested by Neolithic fishers were significantly larger than those caught by 21st century trawlers.

“It’s such an overfished system,” said Karin Limburg, a fisheries ecologist at the SUNY College of Environmental Science and Forestry and the study’s lead author. “The big concern is that overexploitation is causing the fish to evolve. The finding that humans can actually cause evolution of fish populations, which in turn can drive their degradation, is relatively new and is drawing a lot of attention. Some fisheries, including that for cod, are now known to cause ‘juvenescence,’ or the evolution of younger, smaller adult fish. The ecological and economic consequences both appear to be negative.”

Extrapolating size, age, growth and mortality rates based on examination of the fish remains, the researchers concluded that Stone Age fisheries produced cod that were harvested at an average length of 22 inches. By contrast, today’s catch averages 18 inches.

The difference becomes more significant, Limburg said, when comparing the difference in fishing practices between Neolithic times and the modern era. Neolithic fishers worked from the shore and in simple boats that did not stray far out to sea. Today’s trawlers, by contrast, operate in the deepest waters, where the largest and oldest cod would typically live. These modern fishing techniques remove the older females which produce the greatest number of viable eggs. Additionally, these older fish have the most knowledge about how to find productive spawning grounds. In their absence, the fish left in the ecosystem are less likely to spawn productively.

Limburg said the research will provide baseline information so modern managers can better assess the fishery and plan for its future. “With cod populations at historic lows and challenges to rebuild them mounting, it is important to explore the past for clues about characteristics such as stock sizes, exploitation rates, age and size structure, etc. and what ecological roles cod would have played when their populations were larger,” she concluded.