I assume its for the same reason as over here SS. If you catch certain fish you are not allowed to fillet them onboard. For example, In relation to coral reef fish, a recreational fisher:
must not possess, on board a boat, a fish other than in any of the following forms-whole, gilled, gutted or filleted
may possess, on board a boat, a fillet of a fish other than a Chinese footballer (blue spot) trout as long as the length of the fillet is at least 40 cm, and the skin and scales of the fillet are attached to the fillet
must not return fish that have been taken ashore from a boat and filleted, and then returned to a boat, unless the length of the fillet is at least 40 cm and skin of the fillet is attached to the fillet
There are very serious implications to those that disobey rules like this and penalties can range from on the spot fines, impounding of fishing equipment including the boat, if you are a trailer "fisherperson" (that quotes for you saucy) they can and will cease the trailer and car and for repeat offenders ... Jail