If you are a fish, your fins are obviously critical for making a living. Fins provide a long list of essential functions, from generation of movement to stabilization, turning, stopping, and dynamic lift (to name a few). Yet only select families of fish—namely trouts and salmons and catfishes—possess a small nub-like fin behind their dorsal fin and before their caudal (tail) fin. This fin is called the adipose fin*, and while many fisheries biologist and fishermen know what it is, explanations have been few and far between when it comes to the function of the adipose fin.
|Diagram of fish fins, including the adipose fin. (Source: L.A. Walford)|
*Because the adipose fin is not rayed and appears as a unsupported finlet, one early hypothesis was that the fin stored fat, or adipose tissue. Studies have generally confirmed that the fin does not hold adipose, yet the name stuck.
Naturally, a proposed ‘non-functional’ fin was too much for some biologists to take, and experiments were designed. In 2004, two authors published a paper that took a first step in establishing the importance of the adipose fin. Reimchen and Temple removed the adipose fin from a set of juvenile steelhead and compared their swimming abilities to similar steelhead whose adipose fin remained attached. They found an 8% increase (on average) in caudal fin amplitude (i.e. work) in the fish without the adipose fin. Although they didn’t get into the mechanisms of how the fin possibly increases swimming efficiency, they did suggest that the adipose fin may create vortices that impact the caudal fin, or the fin may serve as a pre-caudal sensor to flow.
|Close-up of adipose fin on salmonid species. (Source)|
As a natural extension of their fin-clipping comparisons, Temple and Reimchen followed up in 2008 by investigating catfish, another fish group exhibiting an adipose fin. This time they consider 1906 catfish species, and compared the presence of the adipose fin to the habitat in which the species lived. They found that for species in moving waters (streams and rivers), an adipose fin was more often present than for species in low-flow environments, such as lakes. Although the specifics of how the fin might work were still unsubscribed, this work was critical toward supporting the notion of the adipose fin as a sensory organ.
|The catfish (here, flathead) is another group of fish exhibiting an adipose fin, particular for those species in moving waters. (Source: Tennessee Wildlife Resources Agency)|
And most recently, Buckland-Nicks and coauthors in 2012 began to articulate the how. This work reported that indeed the adipose fin was innervated (connected to the nervous system). The authors called the adipose fin a “precaudal flow sensor” allowing improved maneuverability in turbulent waters. And while other studies failed to demonstrate swimming performance differences in clipped and unclipped fish, the authors note that at least three of these studies took place in non-turbulent waters. Of course, this also fits with the catfish results from Temple and Reimchen.
So why the concerns with the loss of the adipose fin? Turns out that clipping this ‘non-functional’ fin has been routine for fisheries management agencies for decades. Surely, millions of adipose fins have been removed from fish! Simply snipping this small fin provides a permanent marker as to whether the fish (when caught later) is a naturally-produced individual or a stocked individual.
|Comparison of stocked and wild salmon, based on presence of adipose fin. (Source)|
Perhaps the benefit of adipose fin clipping through the information retrieved at the time of (re)capture outweighs the cost of a marginal decrease in swimming ability. And remember, the adipose fin swimming benefit is conferred only in turbulent waters—so we can’t assume that all stocked fish swim around at a constant disadvantage. (And of course, other differences between stocked and wild fish many swamp any impact of the adipose fin, but we’ll save that comparison for a later date.) So although we have made great strides in understanding the function of the adipose fin, the impacts of adipose clipping on stocked fish remain unknown.
Do you have an opinion on clipping the adipose fin? Please let us know in the comments below!