how successful this is depends on market concentration and how vigorously these measures are pursued by the countries where the major markets are to the extent that fish migrations into the high seas increase fish stock management is bound to become more difficult that difficulty is due to the fact that it is more difficult to reach agreement the more parties that must agree and on the high seas there are more parties to be reckoned with than there are for stocks that stay within the eezs this problem is aggravated to the extent that the number of parties with an interest in a high seas stock is indeterminate while the number of countries with an interest in stocks that stay within eezs is either just one or at any rate defined by the migratory habits of the stock in question and which may change as already argued