Important note: It would be impossible to try to list all of the possible data sources for the various treaties you might be researching. Generally, if you do a web search for the name of the treaty or convention, it can lead you to sites that have data, e.g., on catches for a particular fishery. I am here to help you track this stuff down, if you need help! However, be aware that sometimes the responsible organizations do not provide much accessible data.
You can often find at least mention and description of data used for research in the scholarly literature (see 'Finding articles'), and sometimes that may be your best source. If the author(s) of an article appropriately cited their data sources (unfortunately not that common in older papers), that may help lead you to the actual data.