Why did countries declare exclusive economic zones of up to 200 nautical miles?

Question

Today coastal countries own the surrounding marine waters stretching 200 nautical miles from their shoreline in terms of exclusive rights of exploiting its resources (their exclusive economic zone (EEZ)).

The previous limits of 3 and later 12 nautical miles (nm) are said to resemble the range of a cannon shot and therefore may have been established because of strategic reasons.

Iceland extended its EEZ to 200 nm during the cod wars to secure and be able to defend fishing rights against the British. Chile and Peru already claimed in 1947 EEZs of 200 nm but I'm not aware of the reasoning.

I have not much doubt that, unlike the case with Iceland, the main motivation behind establishing large EEZs is to extend territories out of strategic reasons and to secure marine resources (especially energy resources).

But, are there other cases where reasons of seemingly minor importance (well, fishing was certainly so much of importance to Iceland to have a big quarrel with the British) were the motivation of countries to extend their EEZ beyond the previous limits of 12 nm? What was the reasoning of Chile and Peru? I'm especially interested in cases where fishing was found to be the single most important factor.

Answer 1

The ideas behind the EEZ are put forward quite well in this proclamation by President Harry S Truman in 1947

Formally: Proclamation 2667 - Policy of the United States With Respect to the Natural Resources of the Subsoil and Sea Bed of the Continental Shelf

"Having concern for the urgency of conserving and prudently utilizing its natural resources, the Government of the United States regards the natural resources of the subsoil and sea bed of the continental shelf beneath the high seas but contiguous to the coasts of the United States as appertaining to the United States, subject to its jurisdiction and control."

The apparent reasons stated - the conservation and prudent use of natural resources.

EDIT:

Since this relates to resources of the subsoil and sea bed of the continental shelf, it is reasonable to assume that one such resource could be oil.

HT to @T.E.D. for pointing this out!

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@LennartRegebro @Kobunite Correct! The proclamation by Truman does not specify any particular resource, which also means it does not exclude any particular resource. More importantly, the proclamation asserts the right of the US to conserve/use some areas. This assertion may have been important for Chile/Peru when they made their claims.

Answer 2

Read about the Tragedy of the Commons. It is a well understood economic observation that any resource left unmanaged will tend to be over exploited, as it is in the individual self-interest to do so. When combined with the basic observation, in life as well as child-rearing, that one should never make idle threats, the question you ask really should be turned on its head.

Given the imperative to attempt management of resources, to protect their economic value, why did it take so long for EEZ's to expand to the (new) limit to which they could reasonably enforced?