I don’t waste time filleting fish at the beach – unless I am going to be there all day or longer. Instead I head and gut them as I find it easier to fillet them later at home over the kitchen bench. After dispatching with a blow to the head, take the spiny dogfish by each pectoral fin and make a deep angled cut from the base of the pectoral fin back towards the head. Then slide the knife up the rear vent to the head while pulling a pectoral fin. With a bit of practice as you slice the head off the guts and both pectoral fins will be removed in one go. This also bleeds the shark and leaves you with a smaller tidy package.
Back at home I place the shark on the bench top with one side facing upwards. Run a sharp fillet knife along the top of the back bone from the head end to the tail. Flip it over and take the other side off in the same manner. Finally lay the fillet skin side down on the bench. Make a cut an inch or so downwards on an angle from the tail end. Grasp this piece of the skin at the tail end and slide your knife along the skin to the other end to remove the clean fillet. As you do so keep the blade flat against the skin pressing it down onto the bench pulling the skin at the same time. This process can be performed very quickly with a bit of practice.