TROUBLED WATERS - Whale and Dolphin Conservation Society

and the type of source used depends on the situation. The International Law Commission itemised a
non-exhaustive list of the classic forms of evidence of customary international law. This list includes:
treaties, decisions of national and international courts, national legislation, diplomatic
correspondence opinion of national legal scholars, and practice of international organizations.
The interplay between treaty law and international customary law
Treaty and international customary law exist side by side. Most legal scholars posit that treaty and
international customary law are of legally equivalent weight, rather than hierarchical, in the sense
that one source supersedes the other (Kontou 1994). Since treaty and customary law are of equal
status legally, it is irrelevant whether a practice or a norm is clad in customary rule or in a treaty
rule, since in either case, the rule of law is binding (Villiger 1985).
A treaty and an international custom can be complementary. Despite the widespread use of treaties,
a treaty obligation is often more limited than a customary obligation. For instance, a treaty is
binding only on those countries that are signatories (unless the treaty requirements have become
customary law). Treaties are difficult to modify and often require consensus to change treaty
language. Consensus in this case may be hampered by political considerations, or by substantive
discord or disagreement by one or a small minority of countries 30 . Because of the cumbersome
nature of treaty law, international law more often may be modified by custom than by treaty
negotiation; in this sense, treaties are inevitably, inextricably, linked to the evolution of customary
international law (Janis 1999).
The basic principle of treaty law, pacta sunt servanda (treaties are legally binding) is itself a rule
drawn from the customary practice of states. Thus, the concept that a treaty is binding is itself
legitimate only due to a customary international norm obligating state action. Moreover, treaties
must often be interpreted in light of the rules of customary international law. Like statutes in a
common law context, treaties often presume and rely upon pre-existing or subsequently established
set of legal rules.
As a contemporary form of international law, international custom can modify or abrogate a treaty.
Likewise, states can modify a customary rule by concluding a treaty. Moreover, a treaty that
modified customary law can then be modified once more by customary international law and so
on. 31
A treaty, however wide its membership, cannot prevent the formation of new law. State practice may
continue evolving outside the treaty in response to changing conditions or perceptions of interest,
and new custom emerges as a result (Kontou 1994). This new custom may then either modify,
reinterpret or even terminate an existing treaty. When a new customary rule has developed on the
same subject matter as a pre-existing treaty rule, the latter will either be reinterpreted or modified. A
treaty will be clarified or exemplified in light of the new international custom if the rules are
analogous. However, a treaty will be modified if the emerging rules are not identical. Generally,
modification of a treaty occurs when growing state consensus or practice reveals that the treaty rules
are out of date or inappropriate. Changing structure or shifting values or principles of the
international community can also inspire arguments for modification (Villiger 1985). In such
circumstances, a new customary law overrides the original law, and the new law will be binding not
only on the parties, but possibly non-parties as well.
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