On the Northward Expansion of Scallops (Pecten maximus) Along the Norwegian Coastline

The Norwegian coastline spans from 58° N to 71° N and exhibits an environmental gradient with decreasing temperatures from south to north. Historically, the distribution of the great scallop (Pecten maximus) along the Norwegian coastline has been from Skagerrak in the south to Bodø (67° N) in the north. The northernmost distribution was documented in 2001 by scientific diving. Since 2011, monitoring has revealed a northward shift, where small populations of P. maximus have established themselves beyond the previous distribution boundaries along the coastline and in the mid-western part of the Lofoten Islands at 68° N. The northward expansion of the great scallops' distribution is believed to be limited by low temperatures. However, over the past 15 years, coastal water temperatures have increased by approximately 1°C. Although a 1° temperature increase may seem modest, it reduced the time during winter with temperatures below 4°C from three to one month at the coastal station Skrova at 68° N. As the great scallop's preference for warm water is widely documented, and the timing of the northward shift seems to occur at the same time as the warmer waters are observed, we believe that the temperature increase is the main explanation for the observed northward shift in established populations of P. maximus.