What I was curious about today: why does "moose" not have a plural form & what is that called? Etymology background at http://t.co/4apWoOZr
August 18, 2012
Why don't ''fish'', ''sheep'', ''deer'', ''cattle'', | Antimoon Forum
I've never heard anyone say "salmons" but it can be used as well as the plural form "salmon", so Paul is quite correct. The same applies to "fish/fishes". Also "trout/trouts"; "mackerel/mackerels"; "cod/cods"; "plaice/plaices". However, it seems to be more usual to say "there is a shortage of cod in the North Sea" and "salmon is plentiful in the River Dee".

