Rabbi J. David Bleich quotes R. Moshe on this question as follows ("SURVEY OF RECENT HALAKHIC PERIODICAL LITERATURE," Tradition 18:4):
Rabbi Feinstein addresses himself specifically to the question of
government supervision and to the contention that fear of punitive
measures may constitute an adequate substitute for the presence of a
mashgiach. Rabbi Feinstein peremptorily dismisses the notion that
government inspection might constitute an adequate safeguard.
Moreover, he argues, even a mashgiach "exiting and entering" (i.e.,
a mashgiach who comes and goes periodically but who is not constantly present) is not sufficient. Such supervision is satisfactory, declares Rabbi Feinstein, only if the activity under supervision is one of sufficient duration so that the mashgiach, on his return, might be able to apprehend a malfactor in the midst of the act. Removal of the skin of a fish, however, can be accomplished so swiftly that a person who wishes to substitute a nonkosher fish need not fear that the kashrut supervisor might return and catch him in the act. Therefore, rules Rabbi Feinstein, it is necessary for the mashgiach to be present constantly throughout the canning process and for him to examine each fish.