Fish Extenders are handled on your Cruise Meet thread (if there is one). it's not rare that the shorter cruises do not have either a Cruise Meet thread, or if there is one, no one has organized a FE group.
Yes, fish extenders are always happening on every cruise. They're so fun for us! They're not organized by Disney, but instead are independently organized on various social media sites. The Dis is one of the social media sites that sometimes offers them. You'd find the one for your cruise on the Cruise Meets page, if there is a Fish Extender group on the Dis currently available.
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