Effects of Body Condition on Buoyancy in Endangered North Atlantic Right Whales

Abstract

Buoyancy is an important consideration for diving marine animals, resulting in specific ecologically relevant adaptations. Marine mammals use blubber as an energy reserve, but because this tissue is also positively buoyant, nutritional demands have the potential to cause considerable variation in buoyancy. North Atlantic right whales Eubalaena glacialis are known to be positively buoyant as a result of their blubber, and the thickness of this layer varies considerably, but the effect of this variation on buoyancy has not been explored. This study compared the duration and rate of ascending and descending glides, recorded with an archival tag, with blubber thickness, measured with an ultrasound device, in free-swimming right whales. Ascending whales with thicker blubber had shorter portions of active propulsion and longer passive glides than whales with thinner blubber, suggesting that blubber thickness influences buoyancy because the buoyant force is acting in the same direction as the animal's movement during this phase. Whales with thinner layers also used similar body angles and velocities when traveling to and from depth, while those with thicker layers used shallower ascent angles but achieved higher ascent velocities. Such alterations in body angle may help to reduce the cost of transport when swimming against the force of buoyancy in a state of augmented positive buoyancy, which represents a dynamic response to reduce the energetic consequences of physiological changes. These results have considerable implications for any diving marine animal during periods of nutritional stress, such as during seasonal migrations and annual variations in prey availability.

Swimming kinematics and efficiency of entangled North Atlantic right whales

“…experienced vs. inexperienced high-heel wearers (Opila-Correia, 1990) or previous study subjects (Selinger et al 2015) enhances the speed and plasticity of individual responses. Whales and other marine mammals make small gait changes in response to repetitive and transient actions, such as reducing thrust production in the buoyancy-aided direction on dives (Nowacek et al 2001, Aoki et al 2011, Miller et al 2012, Adachi et al 2014, Nousek-McGregor et al 2014. These case studies show yet another condition in which animals perceive changes in movement constraints over the course of seconds to minutes, here resulting from disentanglement response, and alter their movement accordingly (Duysens et al 2000).…”

“…To overcome additional drag forces, individuals increase thrust by altering fluke stroke frequency, amplitude, or a combination of both (Skrovan et altend to respond to natural (e.g. lipid loss or gain, lung compression; Biuw et al 2003, Nousek-McGregor et al 2014 or experimental (Skrovan et al 1999, Aoki et al 2011) changes in buoyancy by reducing thrust output or by gliding in the buoyancy-aided direction to maximize swimming efficiency when possible. Swimming animals therefore show plasticity in their ability to adjust their fine-scale movement patterns in response to changes in drag and buoyant forces and moments to optimize propulsive efficiency within constraints (Fish 1999).…”

“…An additional consideration is buoyancy. Right whale buoyancy likely ranges ±1000 N depending on nutritional condition and life stage; most right whales are positively buoyant (Miller et al 2004, Nowacek et al 2001, Nousek-McGregor et al 2014. Some entangling sets of fishing gear include floats and buoys that can add 26 to 665 N of buoyancy.…”

Effects of added drag on cetaceans : fishing gear entanglement and external tag attachment

“…Marine mammals tend to respond to natural (e.g. lipid loss or gain, lung compression; Biuw et al, 2003;Beck et al, 2000;Nousek-McGregor et al, 2013) or experimental (Webb et al, 1998;Skrovan et al, 1999;Aoki et al, 2011) changes in buoyancy by reducing thrust output or by gliding in the buoyancy-aided direction to maximize swimming efficiency when possible.…”

“…Right whale buoyancy likely ranges ±1000 N depending on nutritional condition and life stage; most right whales are positively buoyant Nowacek et al, 2001;Nousek-McGregor et al, 2013). Some entangling sets of fishing gear include floats and buoys that can add 26 -665 N of buoyancy.…”