Fishing records from anglers on 59 Norwegian rivers show that more and more salmon are staying out at sea for two or more winters - instead of one - before migrating back upriver to mate.
The trend, spotted by Leif Asbjørn Vøllestad of the University of Oslo and colleagues, coincides with warming in the North Atlantic between 1991 and 2005.
Salmon need to eat enough in the autumn for their gonads to mature the following spring.
Temperature-driven changes in the food web mean fish may lack food at this critical time, forcing them to fatten up for longer before reproducing.
Vøllestad says more time in predator-rich open waters may help explain their overall decline. On the upside, fish should be bigger when they swim upriver.
