Thermal windows, or the tolerable temperature ranges of some marine organisms, have evolved to be very narrow, possibly as a means to reduce the amount of energy and time spent on certain biological activities, such as feeding, growth, or reproduction. Scientists studying the impacts of climate change on North Sea Atlantic cod, a large fish with a narrow thermal range, attribute its recent northerly expansion into colder waters to climate change.
Climate change is causing warmer sea surface temperatures and declining oxygen, along with a reduction in the size of cod's primary food source, zooplankton. Climate change, by way of hypoxia, warmer waters, and changes in salinity, is favoring smaller zooplankton, which have been found to have a greater tolerance for warmer, low-oxygen conditions.
With greater numbers of small sized zooplankton present, juvenile cod, which favor eating larger zooplankton, are exerting more energy feeding, which is resulting in their growth rates slowing. As a consequence of the lack of adequate prey and warming waters, cold-loving cod are moving north in search of larger prey and colder temperatures.
The scientists said that their findings suggest that climate change will favor species, both marine and terrestrial, with broad thermal ranges, capable of tolerating warmer conditions, and shorter generations, since long-lived species may not be capable of such quick adaptations as will be required in a changing climate.