After listening to Capt. Hillman’s podcast, I wanted to share a graphic I created for the Sportfish Center's social media. I hope this information helps you learn about slicks and how they're formed by fish. Yes, even Gafftops...
If you have ever gone fishing on the coast, you have probably seen a fish slick on the water's surface. It has a sweet and pungent smell akin to watermelon or fresh-cut grass. This slick is a telltale sign that helps anglers locate fish in the area. Spotted seatrout are often the cause of these slicks. However, they can also form from other fish such as red drum, black drum, southern flounder, ladyfish, hardhead catfish, and gafftopsail catfish.
Two scenarios can explain how slicks form. First, a predatory fish will consume a prey item such as mullet, menhaden, pinfish, bay anchovy, or shrimp. During consumption, the prey can be torn apart and release oily body fluids. Second, after a heavy meal, a predator will regurgitate its food (e.g., like a person burping), releasing fragments of its prey and even digestive juices. In either case, the contents are less dense than the surrounding saltwater environment and float to the surface, creating an oily slick. These slicks start small and can quickly expand in size, depending on the strength of the wind or current. If you come across a sizable one, it has had time to spread out, and you should position yourself upwind where the fish that created it will be holding.
