The episcopal conference has the competency to determine the exact nature of the abstinence:
Can. 1253 The conference of bishops can determine more precisely the observance of fast and abstinence as well as substitute other forms of penance, especially works of charity and exercises of piety, in whole or in part, for abstinence and fast.
For example, here are the restrictions placed by the
Antilles Bishops Conference (covers the Caribbean nations that don’t have their own conferences):
Because of the dire poverty of so many of our people and the limited variety in the quantity and quality of their diet, it is difficult to establish common norms, even within one diocese. The Antilles Episcopal Conference, therefore, decrees that our faithful may fulfill the obligation of penance by either fast or abstinence on the designated days and seasons by one or more of the following forms of penance or prayer or acts of charity:
*]Making a visit of about 15 minutes to the Blessed Sacrament;
*]Reading the Scriptures for 15 minutes;
*]Skipping any one meal and not eating between meals;
*]Eating only vegetables that day (avoiding fish and eggs as well);
*]Abstaining from alcohol, for those accustomed to such drink;
*]Abstaining from smoking, for those who smoke;
*]Abstaining from all beverages except water between meals;
*]Giving 10% of that day’s earning to charity;
*]Abstaining from meat for those for whom meat is a regular part of their diet.
It wouldn’t surprise me if they restricted people from eating fish in Polynesia on days of abstinence (think about why we fast…)