The variable location of the fronts between the colder and fresher waters of Arctic origin and the warmer and more saline waters of Atlantic origin result in variable local conditions, especially on the northern part of the shelf. During the last two decades, the Atlantic water mass has been dominating, in contrast to the Arctic domination in the previous three decades.
Zooplankton biomass on the northern shelf has fluctuated in the past, cycling on a five- to ten-year periodicity, with a period of generally low biomass from the 1960s to the 1990s.
From the mid-2000s, Atlantic mackerel Scomber scombrus extended its feeding grounds from the Norwegian Sea to the Icelandic Waters ecoregion, while the summer feeding grounds of capelin Mallotus villosus moved westwards from the Icelandic Waters into Greenland waters. Norwegian spring-spawning herring Clupea harengus has, since the early 2000s, reappeared at its traditional feeding grounds east and north of Iceland. These major changes in migration patterns have been linked to prey availability, oceanographic conditions, and stock density.
Increased temperature in the lower water column on the western and northern part of the Icelandic shelf has resulted in changes in spatial distribution for a number of demersal species. Species like haddock Melanogrammus aeglefinus, anglerfish Lophius piscatorius, ling Molva molva, tusk Brosme brosme, dab Limanda limanda, and witch Glyptocephalus cynoglossus that have previously had Icelandic waters as their northern boundary of distribution and have mainly been recorded in the warm waters south and west of Iceland, are now showing a northward clockwise trend in their distribution along the shelf, and in some cases a distributional shift. Warming waters have led to a decline in the stock abundance and distribution of many cold-water species, while the previously rare occurrence of warm-water species in the ecoregion has increased in recent years.
The stocks of northern shrimp Pandalus borealis collapsed around the year 2000 and the driving factors are thought to be increased predation by gadoids, increasing temperature, and high fishing mortality.
Improved management measures for most of the major stocks (cod Gadus morhua, haddock, saithe Pollachius virens, redfish Sebastes sp., herring) have resulted in decreased fishing mortality, close to or at FMSY, and increased SSBs. This has furthermore resulted in decrease in effort and less pressure on the benthic habitats.
A recruitment failure of sandeel (Ammodytidae) was recorded in 2005 and 2006, and, with the exception of the 2007 cohort, recruitment has been at a low level since then. Fish stomach content data suggest that the decline in the sandeel population may even have started as early as around year 2000.
The abundance of minke whales Balaenoptera acutorostrata has decreased on the Icelandic shelf in recent years, following changes in prey distribution. Abundance of other species, in particular fin whales Balaenoptera physalus and humpback whales Megaptera novaeangliae, have increased over the last 20 to 30 years.
In recent decades, the breeding success of many seabird species has been poor in south and west Iceland, accompanied by declines in their breeding population sizes. These trends may be influenced by changes in density, composition, and spatial distribution of their main fish prey (i.e. sandeel).