Sounds like somebody at the shipping company thought it'd be funny to dump a can of tuna juice in your car.
Decaying animals have a pretty distinct smell...I wouldn't classify it as "fishy".
2011 E92 M3
| Melbourne Red | Black Novillo w/Carbon Leather Trim | M-DCT | Competition Package | Premium Package 2 | Convenience Package | Enhanced Premium Sound | Heated Front Seats
2004 E46 M3
| Silver Grey | - Retired