There is evidence for cod consumption in Europe from the Neolithic onwards; many of these sites are in Scandinavia, like Denmark and Sweden. They indicate that fisheries, even hook and line ones, were important. Here are two articles, a [pdf] (http://www.britac.ac.uk/pubs/proc/files/99p125.pdf), and [this one] (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2605816/). Fishing uses far less time and energy than farming, or even hunting. As a potential food source inshore waters, waterways, and the shoreline provided a rich and easily exploited larder for anyone willing to eat fish, shellfish, and seaweed.

Looking at fisheries and fish in terms of how they have changed over time, especially on a big time scale, is of interest to both historians and scientists. Which is why there are quite a few articles on identifying not only species when looking at fish bones, but also their origin. Up until the medieval period it looks like cod came from the [Baltic] (https://www.ncbi.nlm.nih.gov/pubmed/22110675), but later on it becomes evident that the European fisheries are going much further afield, as far as Iceland and North America to catch their [cod] (https://www.ncbi.nlm.nih.gov/pubmed/22110675). Large population centres like London would have created demand for preserved fish like cod, which can be seen in archaeological sites from the 13th century, this [article] (https://www.cambridge.org/core/journals/antiquity/article/fish-for-the-city-metaanalysis-of-archaeological-cod-remains-and-the-growth-of-londons-northern-trade/EDF4C052D0A077AB4913035FE57D9DEC) explores this.

This demand for fish can be traced back to the meatless days (sometimes several days of the week) as demanded by the Catholic Church. Fish as a preserved food source is also good for transporting inland, or on board ships - cod bones have been found on the [Mary Rose] (http://www.maryrose.org/cod-bones/).

As for Romans... there is certainly some evidence for consumption in Roman Britain [pdf] (https://www.cambridge.org/core/services/aop-cambridge-core/content/view/45B348383C72A2476788B56BAA222AD5/S0068113X00001355a.pdf/div-class-title-span-class-italic-in-piscibus-diversis-span-the-bone-evidence-for-fish-consumption-in-roman-britain-div.pdf), however Rome itself does not appear to have had an interest in cod, as this is/was not a species found in the Mediterranean. There were a large variety of fish species in the Mediterranean, so to resort to preserved cod from far away would have been a bit pointless. Apicius, a Roman writer wrote a [recipe book] (http://www.gutenberg.org/files/29728/29728-h/29728-h.htm) and in it there is an entire chapter devoted to fish - but not a cod in sight.