Finlets and the steady swimming performance of Thunnus albacares

The functional significance of finlets on the steady swimming performance of yellowfin tuna Thunnus albacares was evaluated by measuring the speed and tail‐beat frequency of the fish with and without them. It was hypothesized that if finlets do improve swimming performance, fish without finlets would have to work harder to maintain the same swimming speed as fish with them and that this would be reflected in kinematic differences. Two‐way ANOVA showed significant effects between individuals on speed (d.f. = 5 and 228, P < 0·001) and tail‐beat frequency (d.f. = 5 and 48, P < 0·001), but no significant effects of treatment on speed (d.f. = 1 and 228, P = 0·25) and tail‐beat frequency (d.f. = 1 and 48, P > 0·1). No interaction effects on speed (d.f. = 5 and 228, P > 0·1) and tail‐beat frequency (d.f. = 5 and 48, P > 0·25) were found. This suggested that finlets were unlikely to function as significant drag reduction and thrust enhancing devices in routine steady swimming. Though not statistically significant, small percentage differences between the mean swimming speeds and tail‐beat frequency of the untreated and treated groups (fish with and without finlets respectively) of the order of 0·5% may be meaningful over the life of a fish. Also, finlets may improve performance at high sustained speeds in rapid accelerations and turns.