The Danish fish fauna during the warm Atlantic period (ca. 7000–3900bc): Forerunner of future changes?

Environmental Impacts—Marine Ecosystems

“…Cold-water species such as Atlantic cod Gadus morhua and Atlantic herring Clupea harengus occur in the North Sea close to the warm end of their range and southern, warm-water species such as common sole Solea solea and sardine Sardina pilchardus close to the cold end of their range. Abundances of each species range from rare to common, but with considerable variability in relative numbers over time, as the North Sea has undergone warmer and cooler periods since the last ice age (Enghoff et al 2007). The post-glacial inundation of the area south of 55°N occurred about 8000 BP, so much of the North Sea has been invaded by fish fairly recently.…”

“…Fish identified from archaeological remains of Mesolithic settlements in Denmark include at least 49 species, most of which are common today, but also several that occur mainly in warm-temperate water, including smoothhound Mustelus sp., common stingray Dasyatis pastinaca, European anchovy Engraulis encrasicolus, European seabass Dicentrarchus labrax, black sea bream Spondyliosoma cantharus and swordfish Xiphias gladius (Enghoff et al 2007). These warmer water species have all reappeared or increased in abundance over the past thirty years.…”

“…In addition to these well maintained observational time-series data are accumulating from historic reconstructions (e.g. Poulsen et al 2007), archaeology (Enghoff et al 2007) and other sources. The wealth of sampling and scientific analysis that exists for the North Sea shows the need to look deeper than simple, direct effects and linear responses to one or two variables and to be wary of general conclusions from incomplete models (Heath et al 2012).…”

“…Increases in species richness seem to have been due to the addition of warm-temperate species without the loss of cool-temperate species. The warm period since the late 1980s is still too short to determine whether the present ecosystem state is transitional and whether the cool-temperate species will gradually disappear, but evidence from archaeological material laid down during the Mesolithic warm period (4000-7500 y BP) indicates that cool-temperate species may remain (Enghoff et al 2007) at least until temperatures rise by considerably more. The time taken for this to happen depends on the rate of global warming but also on decadal regional variability, which could maintain a cooler state over the next few decades.…”

“…8.2). Written records go back 500 years in some cases and archaeological records go back many thousands of years (Enghoff et al 2007), covering a wide range of temperature conditions and providing a basis for establishing the response of the ecosystem to natural climate variability and long-term change, but confounded by the effects of increasing fishing pressure and other anthropogenic drivers. An introductory account of the ecosystems of the North Sea is included in Chap.…”