This knowledge and it's related guides are ALL out of date. In order to acquire this knowledge you have to complete the short questline starting at Igor Bartali: Abelin, the Fishing Boy, culminating in handing in a mackerel to Curio. Additionally, you must complete the quest from Abelin [Fishing] Hauling in Discarded Glass Bottles. This quest was previously for seaweed, but was updated to the bottles. After that you have to complete the quest [Fishing] Catching Mackerel from Abelin. Once you've completed the mackerel turn in to Curio as well as the glass bottle quest (from Abelin, not the daily from Curio) and the follow-on Mackerel quest from Abelin, you can then talk to Abelin again and acquire the knowledge for 5 Energy.