Hooded seals have a remarkable diving capacity, often diving to depths in excess of 1,000m and for periods of up to 1 hour (Folkow et al., 2010), however they seem to Mainly preform meso/bathypelagic dives of 5 to 25 minutes down to 100 to 600m (Blix, 2005).
In the Newfoundland area, pups leave the whelping areas and become pelagic when they are only around 8 days old and can reach depths in excess of 100m for durations of up to, and over 15 minutes at this age. They are born with oxygen stores that are comparable to those of the adults but their skeletal muscle stores are much less developed (Folkow et al., 2010). This impressive diving ability is partly explained by the highest capacity to store oxygen in blood and skeletal muscles of any pinniped (Burns et al., 2007). This is combined with specialized physiological adjustments which are aimed at minimizing oxygen use during dives. In addition to this, they also display a very high tolerance to hypoxia in vital tissues such as the brain (Folkow et al., 2010). While diving they must expend oxygen for locomotion, food processing, and body maintenance (Crocker at al., 1997).
Odden et al (1999) preformed a study on a Harp and Hooded seals to measure some of the affects of diving. They found that diving results in a drop in heart rate and a decrease in brain temperature. By decreasing the brain temperature, the demand for oxygen to the seals brain is also decreased, and provides neuroprotection against hypoxic injury. They found that this could be an important factor in how some seals repeatedly exceed their predicted aerobic dive limits. It also appears that most of the blood oxygen is sent to the brain while the rest of the body has to rely on local stores of oxygen and anaerobic metabolism. Odden et al (1999) also found that seals have the ability to cool their body core during prolonged diving and that the ability of seals to perform extended dives seems to depend on the lowering of body temperature before a dive, and even before the seal is submerged.
Hooded seals perform drift dives at regular intervals throughout their annual migration across the Northwest Atlantic Ocean (Andersen et al., 2014). Andersen et al (2014) defines drift dives as a dive type with a direct descent to a depth at which point the descent rate decreases dramatically until the bottom of the dive, followed by a direct ascent to the surface. Andersen et al (2014) also describes how a positive drift dive may occur in blubber rich seals, which is where the seal is positively buoyant and drifts upward during the drift phase, but this is rare. During these dives the seals drift passively thought he water column and it is hypothesized that they represent periods of physiological processing such as digesting recently eaten food or for rest/sleep (Crocker at al., 1997). These dives also make the seals less susceptible to predators as the drift phase starts once they reach the “safe zone” which is below possible predator depth range (Mitani, 2010). Figure two shows an example of a typical drift dive.
Andersen et al. (2014) found that regular dives in Hooded seals were longer and deeper during the day than at night but the frequency of dives was similar during the night and day. Folkow and Blix (1999) suggested that this was because the Hooded seals are feeding on diurnally migrating prey which are found at depth during the day but migrate to the surface at night. The Hooded seals were found to preform the most drift dives in the day when dive durations were lowest and most shallow (Andersen et al., 2014).