FISH is the acronym for First-in, Still-here and is basically an alternative name for Last-in, First-out (LIFO) method.
Usually authors and article writers come up with novel terms sometimes to make things even more understandable or sometimes just to make fun. So FISH terminology for LIFO system might just be the similar kind of case.
However, some accountants are of the view that FISH method is merely a reference in a sense that inventory was bought way back in time and is still lying in the store. So, in their opinion, FISH is used to refer to obsolete inventory in a funny way that these goods were among the first to come in and are still here! So basically they take FISH method not to be connected with any of the existing popular inventory valuation method LIFO which is based on particular cost flow assumptions.
Just another similar term for FIFO is LISH which stands for Last-in, Still-here.
I personally believe that these are just another names given to existing FIFO and LIFO methods. However, it is always a good idea to stick with the terms that are widely used and not to use alternative terms as they can cause confusions. But certainly these can be an addition to our general knowledge.