Credit: ©Istockphoto/Alexander Klemm

Climate change may render the southeastern coast of Norway ill-suited for Atlantic cod, one of the region's most renowned fish.

By studying the size distribution of more than 102,000 first-year cod netted at sites along Norway's Skagerrak coast from 1919 to 2009, fisheries ecologist Lauren Rogers of the University of Oslo and her colleagues assessed how near-shore water temperatures influenced cod growth rates1. On average, every 2 °C rise in springtime water temperature yielded a 3.1 per cent increase in fish length, but the same increase in summertime water temperature resulted in a 3.1 per cent decline in length. During the warmest years the positive effects of springtime warmth levelled off whereas the negative effects of summertime warmth increased dramatically, leading the researchers to surmise that in future the detrimental effects of warmer summers will increasingly outweigh the benefits of warmer springs.

Previous studies have shown that mortality rates among first-year cod jump once water temperatures exceed an optimum level. Near-shore waters along the Skagerrak coast already exceed that temperature regularly in summer — a sign that suitable hatchling habitat may disappear from the region as the climate warms, the researchers contend.