Unlike fish, which are non mammalian animals, dolphins do not possess gills (gills allow fish to extract oxygen from the water) so they must come to the surface of the water to breathe.
While dolphins breathe air like other mammals they also breathe in a way that is unique to most mammal species.
In order for dolphins to inhale and exhale they must breathe through their blowhole, not their mouth.
The reason for this is that there is a major difference between dolphins and land mammals which is the fact that dolphins live exclusively in the ocean, so in order to maximize their survival and make life easier they have developed separate holes for breathing and eating.
So while humans (and most land mammals) can both breathe and eat through their mouth dolphins can only breathe through their blowhole and are incapable of breathing through their mouth; in fact dolphins don’t even possess vocal cords.
This is very important for a number of reasons.
First, when a dolphin dives underwater it is actually holding its breath.
In order to prevent the dolphin from accidentally sucking up water when hunting for and consuming prey the dolphins esophagus (food passage) remains separate from its nostrils and lungs (air passage) which is connected to its blowhole.
If a dolphins air passage was connected to its mouth and it accidentally inhaled water while hunting for food underwater there is a good chance the dolphin would get water in its lungs and possibly drown, so in theory it is extremely important that the dolphins air passage isn’t connected to the dolphins mouth.
Second by having a blowhole on the top of its head a dolphin can rest at the surface of the water without having to lift its head all the way up to breathe.
This can be extremely important when dolphins are at rest near the surface of the water as it makes it much easier for these marine mammals to acquire the oxygen they need with as little effort as possible.
Dolphins are known as conscious breathers, meaning they never fall completely asleep because if they did they might drown, so when they go into a resting state half of their brain remains alert in order to allow them to easily obtain oxygen when necessary.
Note: Remaining semi-awake can also help the dolphin react quickly to potential threats or predators by maintaining some awareness of its surroundings.
When a dolphin is seen spouting water from its blowhole it is actually water that has collected around the blowhole when the dolphin was submerged; it is not water that has entered the dolphins blowhole and/or lungs which could cause the dolphin to drown or panic.
In order for a dolphin to dive into the water without worrying about inhaling water when it is submerged the muscles surrounding the dolphins blowhole contract to prevent the dolphin from accidentally taking in any unwanted water.
As far as how long a dolphin can hold its breath varies depending on the species.
Some dolphins can only hold their breath for a few minutes (3 - 7 minutes) while underwater, while other species have been known to hold their breath for over 20 minutes during deep dives.
Most of what is known about a dolphins ability to hold its breath is from observations of their dives.
It is possible the their ability to hold their breath is a result of their physiology.
Some species of dolphin may simply possess better lungs, organ systems and breathing control than other species or it could be the fact that some dolphins hunt for food at deeper depths, so they generally take longer dives than other species.
Threats that can cause drowning
For dolphins being able to hold their breath for extended lengths of time has been an evolutionary transition.
Prior to inhibiting the ocean the ancestors of these marine mammals were believed to once be land dwelling animals.
Over the course of millions of years they evolved into fully aquatic ocean living dolphins.
Despite huge advances in evolution dolphins are still very limited in their ability to dive for extended periods of time.
When compared to other land mammals such as humans the length of time a dolphin can hold its breath for seems quite amazing, however because they are mammals they face the same threats all mammals do when it comes to long dives and/or an inability to rise for air.
The first major issue dolphins face underwater is drowning.
If a dolphin gets caught underwater and is unable to resurface it can easily drown and die.
This can occur if a dolphin gets caught in a fishing net or other fishing hazard, gets harmed or stuck in an area where constriction is being performed, becomes injured or unconscious from a hard impact or gets trapped underwater by an object.
In many cases dolphins are excellent swimmers and can avoid getting trapped by most common underwater objects, however they can still be at risk of becoming trapped in fishing equipment that’s intended to capture fish as this is one of the most common ways dolphins are known to drown.
In these instances a dolphin may lunge at a condensed group of fish thinking that they are easy prey only to get trapped by the net or they can simply swim into the net as it may appear fairly transparent and invisible.
Being unable to rise back to the surface they drown as all mammals would.
The second major but less common threat dolphins face when diving underwater is decompression sickness.
Decompression sickness occurs when a mammal (particularly humans) dives deep underwater and then rise to the surface too quickly.
The quick rise and release of pressure on the body and cause bubbles to develop in the body and blood stream which can lead to significant medical conditions and even death.
Although dolphins have developed ways to assist in preventing decompression sickness from happening under extreme cases it has been observed that dolphins may also get decompression sickness (aka the bends).
Observations of beached dolphins have found occurrences of developing gases and bubbles in the bloodstreams of a number of dolphins that have been beached.
It is believed that dolphins may rise to the surface too quickly due to loud ambient noises that can interfere with their echolocation or sense of direction.
Loud explosions, jet engines and sonar have all been thought of as possible causes of dolphin decompression.
Solving the problem
In order to solve the problems of fishing hazards and noise pollution organizations and governments may set up policies to minimize the usage of certain objects, materials or noises that can potentially harm dolphins.
Depending on where these marine mammals live there have already been a number of policies put into place to minimize the chances of whales and/or dolphins being harmed.
Individuals and companies operating boats may be required to maintain a certain distance from known cetacean habitats, especially boats/ships that perform regular whale watching activities.
Commercial fishing boats may also be required to follow strict policies regarding the use of their nets, where they fish and how they monitor and/or dispose of their materials.
In addition to setting policies research is also being performed on the activities that are thought to be the cause of unnatural dolphin deaths in order to determine if such activities are causing an issue and if so how to best resolve the issue.
Issues that are found to harm marine mammals such as dolphins may be sent to specific governing bodies or organizations such as the International Whaling Commission to be reviewed and weighed regarding possible interventions.