Depends where they come from... Every now and then there are disruptions in local supply - most of the time due to political upheaval. Red Sea fish are currently hard to get - due to terrorism concerns, the situation in Egypt, and the situation in Somalia. Have you seen how expensive Purple Tangs are currently? For a long time Sri Lanka was a source for carpet anemones, but the export trade was almost shut down due to rebellion violence. You can also have disruptions due to environmental concerns or environmental activist governments like what has happened in Fiji, Palau, or Tonga.
These disruptions sometimes shut down exporters entirely - or require them to move to another island - so you can sometimes see a 6 month or 1 year disruption. In one case an exporter that used to export anemones and clams, was forced to move due to government regulation, and ended up on another island where they switched to designer "combo" rock instead of clam farming. Additionally the new island had fewer anemones, so it was not commercially viable for them to export.
Exports that come from south of the Equator tend to skew towards winter months (in the North) - because their summer weather is normally much calmer than winter (which is their rain/storm season). I spoke to a collector in Australia that explained to me that during their Winter (our Summer) he could only collect larger fish because his time in the water was shorter and seas were rougher. Large fish are sometimes collected with set or drift nets that don't even require dive time - versus small reef fish and inverts that almost always need to be collected by hand.