Pectoral fins are one of the major features of locomotor design in ray-finn
ed fishes and exhibit a well-documented phylogenetic transition from basal
to derived clades. In percomorph fishes, the pectoral fins are often used t
o generate propulsive force via oscillatory movements, and pectoral fin pro
pulsion in this relatively derived clade has been analyzed extensively. How
ever, in the plesiomorphic pectoral fin condition, exemplified by sturgeon,
pectoral fins extend laterally from the body in a generally horizontal ori
entation, have been assumed to generate lift to balance lift forces and mom
ents produced by the heterocercal tail, and are not oscillated to generate
propulsive force. The proposal that pectoral fins in fishes such as sturgeo
n generate lift during horizontal locomotion has never been tested experime
ntally in freely swimming fishes. In this paper, we examine the function of
pectoral fins in sturgeon swimming at speeds from 0.5-3.0 L s(-1), where L
is total body length. Sturgeon were studied during steady horizontal locom
otion as well as while sinking and rising in the water column. Pectoral fin
function was quantified using three-dimensional kinematics to measure the
orientation of the fin surface, digital particle image velocimetry (DPIV) w
as used to describe flow in the wake of the fin and to estimate force exert
ed on the water, and electromyography was used to assess pectoral fin muscl
e function. Sturgeon (size range 25-32 cm total length) swam horizontally u
sing continuous undulations of the body with a positive body angle that dec
reased from a mean of 20 degrees at 0.5 L s(-1) to 0 degrees at 3.0 L s(-1)
. Both the angle of the body and the pectoral fin surface angle changed sig
nificantly when sturgeon moved vertically in the water column. Three-dimens
ional kinematic analysis showed that during steady horizontal swimming the
pectoral fins are oriented with a negative angle of attack predicted to gen
erate no significant lift, This result was confirmed by DPIV analysis of th
e pectoral fin wake, which only revealed fin vortices, and hence force gene
ration, during maneuvering, The orientation of the pectoral fins estimated
by a two-dimensional analysis alone is greatly in error and may have contri
buted to previous suggestions that the pectoral fins are oriented to genera
te lift. Combined electromyographic and kinematic data showed that the post
erior half of the pectoral fin is actively moved as a flap to reorient the
head and body to initiate rising and sinking movements. A new force balance
for swimming sturgeon is proposed for steady swimming and vertical maneuve
ring. During steady locomotion, the pectoral fins generate no lift and the
positive body angle to the flow is used both to generate lift and to balanc
e moments around the center of mass. To initiate rising or sinking, the pos
terior portion of the pectoral fins is actively moved ventrally or dorsally
, respectively, initiating a starting vortex that, in turn, induces a pitch
ing moment reorienting the body in the flow. Adjustments to body angle init
iated by the pectoral fins serve as the primary means by which moments are
balanced.