Sitting on the back of many fishes, in between the dorsal fin and the tail, is an enigmatic little fatty flap of skin called the adipose fin. It looks a bit like an extra dorsal fin, and though it's present in eight large groups of fishes, no one knows why it's there. It might help prevent flow from wrapping over the top of the fish; it might help counteract forces from the anal fin, which is in about the same place, but on the ventral side; it might be a flow sensor; or it might not do anything, persisting due to developmental constraints. Whatever the fin's function, most fisheries scientists think it's not terribly important, because they regularly snip it off to mark millions of hatchery fish released into the wild each year.

Thomas Reimchen and Nicola Temple at the University of Victoria in Canada devised a simple test to find out how important the adipose fin really is. They swam steelhead trout at speeds between about one and three body lengths per second, measured the tail beat frequency and amplitude, then clipped the adipose fin off and made the same measurements again. They expected that the standard fisheries wisdom would be right, and they'd see no difference between the clipped and unclipped fish.

But, in fact, the fish with clipped fins tended to use a higher tail beat amplitude at all swimming speeds. It wasn't a lot higher - only about 8% on average - but it was usually a significant difference, except in the smallest fish. Reimchen and Temple worried, though, that the effect might not represent any intrinsic function of the adipose fin, but just the trauma of having a fin snipped off. So they tested another batch of fish in which they made a scratch along the base of the adipose fin, without actually cutting the fin off. The scratched fish swam the same as the unscratched ones, eliminating the trauma as a possible cause of the increase in tail beat amplitude.

Why would snipping off the adipose fin lead to a higher amplitude? Reimchen and Temple can only speculate, but they raise some important questions. Perhaps the fin generates some thrust on its own, or makes vortices that increase the thrust of the tail fin. Without the extra thrust, trout would have to compensate by swimming harder. Or the adipose fin might help the fish swim more efficiently by sensing vortices upstream of the tail fin. Trout might counteract the lower efficiency after their adipose fin is clipped by using a higher tail beat amplitude. The suggestion that the adipose fin functions as a flow sensor seems plausible, since Reimchen found some small nerves running to the base of the fin.

Whatever the mechanism, it appears that trout with clipped adipose fins must swim harder. It would be useful to compare the oxygen consumption of clipped and unclipped fish, to verify that swimming without an adipose fin is truly more difficult. But Reimchen and Temple's results should give fisheries scientists pause for thought, because they could have serious consequences for the millions of fish with clipped adipose fins released from hatcheries each year.