What was done
To obtain an idea of the true magnitude of historical over-fishing of the seas and what it has done to marine fish stocks requires, in the words of the authors, "comparison of current observations with baseline records of marine ecosystems in their 'pristine' state."  Hence, they set about to "determine the origin of intensive, probably commercial, cod and herring fishing by assessing the relative abundance (by number of identified specimens) of these taxa in 127 English archaeological fish bone assemblages that date from the seventh to the sixteenth centuries AD."

What was learned
Barrett et al. report that "zooarchaeological evidence shows that the clearest changes in marine fishing in England between AD 600 and 1600 occurred rapidly around AD 1000."  Surprisingly, however, they say this revolution in marine fishing "coincided with the Medieval Warm Period - when natural herring and cod productivity was probably low in the North Sea," according to what is known about "climatically determined patterns in fish abundance."

In explaining this conundrum, they say "the counterintuitive discovery can be explained by the concurrent rise of urbanism and human impacts on freshwater ecosystems," and that "rapid population growth may also have increased the demand for marine fish (Dyer, 2002; Hoffmann, 2002)."  In addition, they note that "the most important variable ... may have been declining freshwater fish stocks - owing to siltation from more intensive agriculture."

What it means
It is interesting to note that this "revolutionary expansion of marine fishing in England within a few decades of AD 1000" was likely driven by the growth of human society and improved and expanded agricultural production associated with the higher temperatures of the Medieval Warm Period.  Not only do these observations demonstrate the positive value of the warmth of this period, as compared to the lower temperatures of the prior Dark Ages Cold Period and subsequent Little Ice Age, they also speak volumes about the reality of the non-CO2-induced millennial-scale oscillation of climate that alternately produces these multi-century warm and cold periods, and which has most recently led to the development of the Modern Warm Period, all without any help from the concurrent historical increase in the air's CO2 content.

Hey, it's nice out. Let's go fishin'!

References
Dyer, C.  2002.  Making a Living in the Middle Ages.  Yale University Press, London, UK.

Hoffmann, R.  2002.  Carp, cods and connections: new fisheries in the medieval European economy and environment.  In: Henninger-Voss, M.J., Ed.  Animals in Human Histories: The Mirror of Nature and Culture.  University of Rochester Press, Rochester, New York, USA.