Fig. 1. (A) Rainbow trout, Oncorhynchus mykiss, shown with the three laser plane positions used to image flow in the wake of the dorsal and caudal fins. Note the small adipose fin, common to salmoniform fishes, located in the midline between the dorsal fin and the tail. Unlike other median fins, the adipose fin does not possess intrinsic musculature or skeletal supports. Plane 1, located at mid-dorsal fin height, was used to image wake flow patterns produced by the dorsal fin alone. At this position, the light sheet was sufficiently distant from the dorsal surface of the trout's body that dorsal-fin wake flow patterns could be calculated without interference from body flows. Plane 2 intersected both the trailing edge of the dorsal fin and the dorsal lobe of the tail. This plane was used to quantify wake patterns from the lower portion of the dorsal fin and to quantify movement of the tail through the dorsal fin wake. This position also permitted occasional observation of adipose-fin wake flow patterns when trout moved slightly upward, bringing the adipose fin within the light sheet. Plane 3, located at the tail mid-fork position, was used to image the wake shed by the body and caudal fin alone. Laser planes 1–3 were similar in relative position to those used by Drucker and Lauder (2001a) in their study of sunfish dorsal fin function. (B) Sunfish, Lepomis macrochirus, scaled to the same body length as the trout in A, showing differences in dorsal fin morphology, placement and relative size. The spiny dorsal fin (absent in trout) is anterior to the soft dorsal fin, which is shaded gray in both species. Both the relative area of the soft dorsal fin and the portion of the fin's trailing edge that extends posteriorly free from the body (marked by asterisks) are smaller in trout than in sunfish. As a result, the trailing edge of the soft dorsal fin is considerably closer to the leading edge of the tail in sunfish than in trout.