Now best known as a magus, Dee also took a worldly interest in politics, and was an early advocate of British imperialism. Beyond imparting nautical wisdom, his treatise on navigation makes a staunch defence of England's fishing rights in its territorial waters, sounding the alarm that incursions by the Dutch might deplete fish stocks in the Atlantic and North Sea – a topic he returned to twenty years later in his essay British Sea-Sovereignty. The excerpt below voices outrage over commercial fishing in the Thames, as Dee employs his considerable mathematical expertise to present one of the earliest arguments for sustainable fishing based on quantitative analysis.

Source: The Perfect Art of Navigation (1577), 43–6.

You may understand that in the River of Thames … every year by the fishermen belonging to some one small town … there is destroyed above 1,000 bushels of the young fry of diverse kinds of good fish, which in due time of their growth and by lawful order being taken would have been able to satisfy 200,000 one day … or 4,000 men fifty days, etc. [44] What is then, I pray you, to be reckoned of the public damage of 500 cartloads of fish yearly thus destroyed? How may it (reasonably) any longer remain unreformed? °

[These] 500 cartloads … do amount [to] only 2,400 bushels of fry yearly destroyed. But if you suppose … thirty trink-boats ° … every day for 300 days only in every whole year, [and each] destroy but one bushel of very small fry, the sum thereof doth amount to 9,000 bushels of fry so destroyed yearly: which fry, when it should be marketable, would be … of ten times more meat and fish than when it is so destroyed. Therefore, the craftsmen of the trink-boats on the Thames may very probably be accused, convicted, judged, and condemned as the most abominable yearly destructioners of 90,000 bushels of marketable fish. So is it now also probable that in all England by the manifold disorder used about fry and spawn destroying, there is yearly spoiled or hindered the brood of 2,000 cartloads of fresh fish. The value of the foresaid 90,000 bushels of fresh fish … (being rated at five shillings a bushel) is ￡220,500 ° …