Atlantic cod is yet another species being threatened by climate change.

According to a study published Monday in the Proceedings of the National Academy of Sciences, warmer summer waters off the Norwegian coast are dwarfing the growth of the fish.

The researchers culled data from surveys dating back to 1919 along the Norwegian coast of the Skagerrak, a triangle of water between Norway, Sweden and Denmark.

Over 91 years, ecologists had carefully measured and recorded the sizes of Atlantic cod (Gadus morhua) in the fall. Lauren Rogers, a researcher at the University of Washington and lead researcher for the study, compared these records with data on seasonal temperatures, year by year.

In times when summer temperatures peaked, researchers found juvenile cod -- born in late winter -- to be smaller than usual. The research team found a 3.1 percent decrease in length for every 2-degree-Celsius increase in summer temperatures.

The study also showed that warmer springs actually encouraged the growth of juvenile cod, showing an equivalent increase in size for warm springs to the decrease in size for warm summers, given the same 2-degree jump.

However, the researchers concluded that the negative effects in size due to hotter Scandinavian summers will outweigh the growth in warmer springtime years, if temperatures continue to rise as predicted.

This paper challenges three past studies that found that warming summer water temperatures increased cod size in other regions. In general, biologists believe that warmer temperatures at higher latitudes are beneficial to species growth and population size. Populations increase along a bell-shaped curve, said Rogers, peaking at an optimal high temperature -- 12 to 15 degrees Celsius for Atlantic cod.

The cod in this study, however, were swimming in waters warmer than the optimal range -- 15.9 degrees on average -- which is why the results don't fit in with previous findings.

Smaller fish mean less fish
"In general, body size is a very important trait, often linked to a change in survival rates, reproduction and population growth rate," said Rogers. Big fish off a small coast protect themselves better from predators.

While most of the Norwegian large-scale fishing industry lies in the north of the country, stocks of Atlantic cod in the Skagerrak region have been noticeably dwindling.

A document released by the Helsinki Commission, an intergovernmental effort to protect the Baltic Sea, stated that the Atlantic cod population has "severely declined" in the Skagerrak region, adding "we know that cod stocks do not recover after a massive decline, even under a long-term moratorium preventing [fishing] of this species."

The Atlantic cod is listed as "vulnerable" on the International Union for Conservation of Nature and Natural Resources' Red List, meaning that, globally, it is not endangered, but is facing a "high risk" of extinction in the medium-term future.

"I think that [population size] is going to vary so much geographically," she added. "The effects of climate will vary geographically, and in this region, it appears to have a negative effect relative to other regions."

More than 100,000 Atlantic cod were counted in this nine-decade-long study, making it the most comprehensive study to date for the Skagerrak coast. "Sampling technologies have remained remarkably consistent, so standardized that there has been one leader [of the research team] at any given time," said Rogers.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500