The adipose fin is a small, fleshy fin located on the back of certain fish, positioned between the larger, rayed dorsal fin and the tail, also known as the caudal fin. Unlike other fins supported by bony rays, the adipose fin is distinct for its soft, rayless structure. Its name is derived from an early, and since disproven, hypothesis that it was a site for fat storage. Its purpose has evolved from being considered a non-functional remnant to a recognized sensory tool.
Identifying Fish with an Adipose Fin
The adipose fin is a distinguishing characteristic for a few groups of fish and a useful feature for identification. The most prominent groups possessing this fin are the salmonids (family Salmonidae) and catfishes (order Siluriformes). Within the salmonid family, well-known examples include salmon, trout, and char. For catfish, this fin is a common feature across thousands of species that inhabit diverse environments, and other less common groups, such as the lanternfish found in deep-sea environments, also exhibit an adipose fin.
The Sensory Role of the Adipose Fin
For many years, the adipose fin was dismissed by biologists as a vestigial organ with no modern function. This belief was so widespread that the fin was routinely removed, but this perspective has been overturned by modern research revealing a complex and functional structure. This neural network connects to the fish’s sensory system, allowing the fin to function as a pre-caudal flow sensor. It helps the fish detect subtle changes in water movement and turbulence before the water reaches the caudal fin, which is particularly useful for maneuvering in the turbulent currents of rivers and streams.
Studies on juvenile steelhead trout, for instance, have shown that fish with their adipose fins removed exhibited an average 8% increase in tail fin movement to compensate, suggesting the fin plays a role in swimming efficiency. In catfish, the fin is sensitive enough to detect deflections as small as 0.12 mm, providing detailed information about its position and the surrounding water flow.
Adipose Fin Clipping in Fish Management
The historical view of the fin as non-functional led to the practice of fin clipping in fisheries management. This procedure involves the removal of the small fin, which provides a permanent and easily recognizable mark. The primary purpose of this practice is to distinguish between fish raised in hatcheries and those born in the wild.
By clipping the adipose fin of hatchery-raised fish before they are released, wildlife managers can effectively track their populations. When these fish are later caught by anglers or recaptured by researchers, the absence of the fin immediately identifies them as stocked fish. This data helps managers assess the survival rates of hatchery programs, monitor the health of wild populations, and implement targeted fishing regulations.
While research now shows the fin has a sensory function, the practice of clipping continues because it is an effective and established management tool. This information allows for informed decisions that support both wild and stocked populations.