Altered swimming gait and performance of dolphin mothers: implications for interactions with tuna purse-seine fisheries

Physical constraints while carrying an infant represent one of many reproductive costs. For bottlenose dolphins Tursiops truncatus, near-term pregnancy and ‘carrying’ a calf in echelon position (calf alongside mother’s mid-lateral flank) alter maternal swimming gait and performance. As calves mature an alternate form of ‘carrying’, infant position (calf underneath mother’s tailstock) dominates. To complete our understanding of locomotion during motherhood in dolphins, kinematics (peak-to-peak stroke amplitude, A, and tailbeat oscillation frequency, f) and performance (swim speed, V) while ‘carrying’ a calf in infant position were quantified. The relationship (slope) for A and V differed between solitary swimming and swimming with a calf in infant position (Z = −1.706, p = 0.088) as did the relationship (slope) for f and V (Z = −3.699, p < 0.001). Compared to solitary swimming (2.17 ± 0.02 m stroke−1; n = 166), mothers ‘carrying’ a calf in infant position had diminished distance per stroke (1.82 ± 0.07 m stroke−1; n = 27, t = 5.209, df = 191, p < 0.001; means ± SE) concomitant with a significant reduction (54%) in average swim speed (n = 27, 166, T = 629.00, p < 0.001). These results have implications for dolphins that interact with tuna purse-seine fisheries in the Eastern Tropical Pacific (ETP). Dolphins ‘carrying’ a calf may be unable to achieve speeds sustained by groups evading fishermen. To maintain proximity with the group, mothers may become separated from their calf. Permanently separated dependent calves represent unobserved mortality events, which may partially explain the non-recovery of depleted ETP dolphin populations.