If you're more interested in rapid SP gains over accruing gil, it may be in your best interest to go straight for a grade 10 node. Find the preferable bait for whichever area you choose and enjoy the SP. You should receive about 300 SP per caught fish from the same water source all the way from rank 1 through 50. Of course this is hardly an adventurous experience which is why SE has confirmed they plan to remove grade 6 nodes and above, possibly as early as the upcoming patch on the 21st. You may want to make use of it while can.