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

Abstract

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 eche lon 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.

Mitogenomic differentiation in spinner (Stenella longirostris) and pantropical spotted dolphins (S. attenuata) from the eastern tropical Pacific Ocean

“…Finally, researchers and managers should focus on other possible causes for the lack of recovery in ETP pelagic dolphins, including the possibility that ongoing fishing activities may be having a negative impact on reproduction (Archer et al . , Noren , Noren and West ).…”

A History of the Tuna-Dolphin Problem: Successes, Failures, and Lessons Learned

“…A possible mechanism to explain this calf deficit is that females with dependent calves are separated during the chase prior to the set of the purse-seine net (Noren and Edwards, 2007). In bottlenose dolphins, the normal echelon swimming position of a calf is energetically beneficial to a calf, but costly to a mother (Noren, 2008(Noren, , 2013Noren et al, 2008). Because the chase is a fast-moving, chaotic environment (National Research Council, 1992), it may be difficult for mothers and calves to maintain their normal swimming positions.…”

Sustainable Fisheries Management and the Welfare of Bycaught and Entangled Cetaceans

“…Permanently separated dependent calves may then represent unobserved mortality events which are a significant welfare concern since un-weaned calves may die of starvation following orphaning. This may partially explain the lack of recovery of depleted ETP dolphin populations ( 69 ) where, in the case of mothers dying, a calf or dependent juvenile must be assumed to become a secondary victim ( 68 ). There is also some evidence that setting nets on dolphins can result in miscarriage in pregnant females ( 70 ).…”